by these scientific experts, non-experts and science textbooks do not provide a full .... It is a major learning tool because students form their own models in order to understand .... Complete: The model should contain all of the structural elements, ...... bonding, the molecules of solvent very weakly held together we can ...
UNIVERSITY OF READING
MODELS AND EXPLAINING DISSOLVING
DISSERTATION SUBMITTED IN PART FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN SCIENCE EDUCATION IN THE FACULTY OF EDUCATION AND COMMUNITY STUDIES
YASEMİN GÖDEK AUGUST 1997
THIS DISSERTATION IS DEDICATED ESPECIALLY TO: MY PARENTS “HACER- MUSTAFA GÖDEK” AND MY SISTER “DERYA GÖDEK”
CONTENTS Page List of Tables and Figures
ix
List of Appendices
xvi
Acknowledgements
xviii
List of Abbreviations
xx
Abstract
xxi
CHAPTERS 1.
INTRODUCTION
2.
EXPLAINING AND MODELLING
1
2.1
Introduction
3
2.2
Explaining
4
2.2.1
Why do we explain?
7
2.2.2
The teaching tools used for explaining of scientific
9
phenomena 2.3
Models
10
2.3.1
Definitions
10
2.3.2
Model, Theory and Concept in science
11
2.3.3
Analogy, Metaphor and Models
12
2.3.4
Target- Model- Source connection
12
2.3.5
The kinds of models in science education
12
2.3.6
The purposes and uses of the models
14
2.3.7
The variance of the models
16
i
2.3.8 2.4
3.
Constructing of new models in science classroom
Conclusion
17 19
PRIOR RESEARCH FINDINGS ABOUT DISSOLVING 3.1
Introduction
21
3.2
The technological use of dissolving in history
22
3.3
The conservation of mass in dissolving
23
3.4
The effects of solubility in dissolving and the solute and solvent
26
in dissolving
4.
3.5
Misconceptions in dissolving
26
3.6
Explanation of dissolving in particulate level
35
3.7
Language
35
3.8
The change of children’s ideas
36
3.9
Reversible change
37
3.10
Melting and dissolving
38
3.11
Models
38
3.12
Drawings
40
3.13
Conclusion
43
WORKING TOWARDS A FULL EXPLANATION OF DISSOLVING 4.1
Introduction
46
4.2
The definitions of the solution, solute, solvent and dissolving
46
4.3
Models of Dissolving
51
4.3.1
51
Dispersal Model (Physical Change)
ii
4.3.2
54
4.4
Chemical Change
62
4.5
Solubility
63
4.6
The Dissolving process for sodium chloride and glucose in water
67
4.7
Energy and Entropy
71
4.7.1
Hydration Energy
75
4.7.2
Dissolving Enthalpy (Enthalpy of solution)
78
4.8
5.
Chemical Model
Conclusion
81
EXPERTS’ EXPRESSED EXPLANATIONS OF DISSOLVING 5.1
Introduction
84
5.2
Ideas about dissolving in general
85
5.3
Ideas about the constituents of a solution
86
5.4
Ideas about dissolving and melting
87
5.5
Ideas about the process of dissolving
88
5.6
Examples
89
5.7
Drawings
89
5.7.1
The drawings of the lecturer A
89
5.7.2
The drawings of the lecturer B
90
5.7.3
The drawing of lecturer C
91
5.7.4
The drawing of the lecturer D
91
5.8
Conclusion
92
iii
6.
STUDENT PRIMARY TEACHERS’ VIEWS ON DISSOLVING 6.1
Introduction
93
6.2
Science specialist students’ views
93
6.2.1
Ideas about dissolving in general
93
6.2.2
Ideas about the constituents of a solution
94
6.2.3
Ideas about dissolving and melting
95
6.2.4
Ideas about the process of dissolving
96
6.2.5
Examples and drawings
97
1. The drawing of the S1
98
1.a. Non-chemical model (Solute is particle, solvent is
98
continuous- intermingled) 1.b. Chemical model, particles - detailed 2. The drawing of the S2 2.a. Non-chemical model, Solute and solvent are
98 99 99
particles - intermingled 2.b. Chemical Model, particles -general3. The drawing of the S3
99
4. The drawing of the S4
100
4.a. Chemical model, particles - detailed 6.2.6 6.3
99
Conclusion
100 100
First year primary science specialist student teachers’ views 6.3.1
Ideas about dissolving in general
102
6.3.2
Ideas about the constituents of a solution
103
iv
6.3.3
Ideas about dissolving and melting
104
6.3.4
Ideas about the process of dissolving
104
6.3.5
Examples
105
6.3.6
Drawings
105
1. S1’s drawing
105
2. S2’s drawing
106
3. S3’s drawings
106
4. S4’s drawing
107
5. S5’s drawing
107
6. S6’s drawing
108
7. S7’s drawing
108
8. S8’s drawing
109
9. S9’s drawing
109
Conclusion
110
6.3.7 6.4
Non-specialist student teachers’ views 6.4.1
Ideas about dissolving in general
112
6.4.2
Ideas about the constituents of a solution
115
6.4.3
Ideas about dissolving and melting
116
6.4.4
Ideas about the process of dissolving
118
6.4.5
Reversible Change
118
6.4.6
Examples
119
6.4.7
Drawings
120
1. Melting
121
v
2. Non-chemical model 2.1. Solute and solvent are particles
121
2.1.b. intermingled
121
2.1.c. disappear
122 123
2.2.a. breaking bits
123
2.2.b. intermingled
124
2.2.c. disappear
124
2.3. Non-chemical model, Continuous - disappear 3. Chemical model
7.
121
2.1.a. breaking bits
2.2. Solute is particle, solvent is continuous
6.4.8
121
126 126
3.1. Particles - general
126
3.1. Particles - detailed
126
Conclusion
127
SCIENCE EDUCATION TECHNICIANS 7.1
Introduction
129
7.2
Ideas about dissolving in general
129
7.3
Ideas about the constituents of a solution
130
7.4
Ideas about dissolving and melting
130
7.5
Ideas about the process of dissolving
131
7.6
Examples and drawings
131
7.7
Conclusion
132
vi
8.
OTHER FINDINGS 8.1
Introduction
133
8.2
The dissolving of iodine in heptane
133
1. Non-chemical (dispersal) model:
133
1.a. Solute and solvent are particles - breaking bits
133
1.b. Solute and solvent are particles - intermingled
134
2. Chemical model: 2.a. Solute and solvent are particles - general 8.3
137
1. Non-chemical model
137
2. Chemical model
137 138
2.a. Solute and solvent particles - general
138
The dissolving of sodium chloride in water
140
1. Non-chemical model
140
1.a. Solute and solvent particles - intermingled
140
1.b. Solute is particle, solvent is continuous - intermingled
140
2. Chemical model
8.5
137
The dissolving of iodine in ethanol
1.a. Solute and solvent particles - intermingled
8.4
137
141
2.a. Solute is particle, solvent is particle - general
141
2.b. Solute is particle, solvent is particle - detailed
142
Conclusion
144
vii
9.
CONCLUSION 9.1
EVALUATION 9.1.1 Introduction
145
9.1.2 The analysis of the responses
146
9.1.3 9.2
a. The explanations
146
b. The definitions of solute and solvent
147
c. The concepts of dissolving and melting
149
d. The process of dissolving
151
e. Reversibility
153
f. Examples
155
g. Drawings
155
Conclusion
159
IMPLICATIONS 9.2.1
Introduction
162
9.2.2
Research findings
163
9.2.3
Some suggested possible models for different age 167 group of students
9.2.4
APPENDICES
REFERENCES
Implications for teachers
168
170
214
viii
LIST OF TABLES AND FIGURES TABLES: 4.1
The comparison of the definitions of the solute and solvent (based on
47
their proportions) 4.2
The comparison of the definitions for the solute and solvent
50
4.3
Dielectric Constants of some common solvents at temperatures (K) given
59
4.4
The change of solubility depends on the H/T
74
6.1
The responses of science specialist students on dissolving
94
6.2
The ideas of science specialist students about the solvent and the solute
95
6.3
The ideas of science specialist students about dissolving and melting
96
6.4
The ideas of science specialist students about the process of dissolving
96
and the conservation of matter in this process 6.5
The examples given by science specialist students and the models that
97
has been used in their drawings 6.6
The ideas of first year primary science education students
102
about dissolving 6.7
The ideas of first year primary science education students about the
103
solute and solvent 6.8
The models that has been used in drawings by first year primary
105
science education students 6.9
The ideas of non-specialist student teachers about dissolving based
112
on the physical and chemical changes
ix
6.10
The ideas of the non-specialist student teachers about dissolving
113
6.11
The ideas of non-specialist student teachers about the solvent
115
6.12
The ideas of non-specialist student teachers about the solute
115
6.13
The ideas of non-specialist student teachers about dissolving and melting 116
6.14
The ideas of non-specialist student teachers about the words associated
117
with dissolving and melting 6.15
The ideas of non-specialist student teachers about
118
the process of dissolving 6.16
The ideas of non-specialist student teachers about
118
the reversibility of dissolving 6.17
The ideas of non-specialist student teachers about the methods
119
for retrieving of dissolved solutes 6.18
The models that have been used by the non-specialist student teachers
120
8.1
The models that have been used for explaining different phenomena
142
9.1
The explanations of different group of people about dissolving
146
9.2
The words associated with dissolving
149
9.3
The words associated with melting
150
9.4
The kinds of solutions that has been considered in examples
155
9.5
The models that have been used in drawings
155
9.6
The models suggested for different age group of students
168
x
FIGURES 2.1.
The relationship between target, source and model
10
2.2.
The kinds of models depend on the duration and public availability
13
4.1.
Dispersal Model (Physical change)
53
4.2.
Three dimensional lattice of sodium chloride
55
4.3.
The molecular structure of oil molecule
56
4.4.
The structure of the water molecule
57
4.5.
The structure of the tetrachloromethane molecule
58
4.6.
The structure of the carbon dioxide molecule
58
4.7.
Energy diagram for the formation of sodium chloride
60
4.8.
Hydration of ions in solution
66
4.9.
The structure of sodium chloride
67
4.10.
The structure of water molecule
67
4.11.
Hydrogen bond in water
68
4.12.
Dissolving of sodium chloride in water
69
4.13.
The structure of glucose molecule
70
4.14.
Dissolving of glucose molecule in water
70
4.15.
Diagrammatic representation of hydration of ions
75
4.16.
The hydrated sulphate molecule
76
4.17.
The hydrated beryllium ion
76
4.18.
The hydrated aluminium ion
77
4.19.
The dissolving of hydrogen chloride in water
77
4.20.
The change of enthalpy in the dissolving process of potassium chloride 79
xi
4.21.
The change of enthalpy in the dissolving process of sodium chloride
81
5.1.
The dissolving of sodium chloride (salt) in water Chemical model- particles -detailed
89
5.2.
The dissolving of glucose (sugar) in water Chemical model- particles- general
90
5.3.a.
The dissolving of sodium chloride in water (above picture) Chemical model-particles- detailed
90
5.3.b.
The dissolving of glucose in water (below picture) Chemical model- particles- detailed
90
5.4.
The dissolving of sodium chloride in water Chemical model- particles -detailed
91
5.5.
The dissolving of sodium chloride in water Non-chemical model- particles- intermingled
91
6.1.
Non-chemical model, Solute is particle, solvent is continuous - intermingled
98
6.2.
Chemical model, particles - detailed
98
6.3.
Non-chemical model, Solute and solvent are particles - intermingled
99
6.4.
Chemical model, Solute and solvent are particle, general
99
6.5.
Chemical model, particles - detailed
100
6.6.
Non-chemical model, solute and solvent are particles - intermingled
106
6.7.
Non-chemical model, solute and solvent are particles - intermingled
106
6.8.
Non-chemical model, solute is particle, solvent is continuous - breaking bits and Chemical model, particles - detailed
107
6.9.
Chemical model, particles - detailed
107
6.10. Non-chemical model, Solvent is continuous, solute is particle - disappear
108
6.11. Non-chemical model. Solute is particle, solvent is continuous - disappear
109
6.12. Chemical model, Particles - detailed
109
xii
6.13. Non-chemical model, solute is particle, solvent is continuous - disappear
110
6.14. Chemical model, solute and solvent particles - detailed
110
6.15. The ideas of the non-specialist student teachers about dissolving
114
6.16. Melting
121
6.17. Melting
121
6.18. Non-chemical model, solute is particle, solvent is particle - intermingled
122
6.19. Non-chemical model, solute is particle, solvent is particle - intermingled
122
6.20. Non-chemical model, solute is particle, solvent is particle - intermingled
122
6.21.
Non-chemical model, solute is particle, solvent is particle - disappear
123
6.22.
Non-chemical model, Solute is particle, solvent is continuous - breaking bits
123
6.23.
Non-chemical model, Solute is particle, solvent is continuous, breaking bits and disappearing
123
6.24. Non-chemical model, Solute is particle, solvent is continuous- disappear
124
6.25. Non-chemical model, Solute is particle, solvent is continuous- disappear
124
6.26. Non-chemical model, Solute is particle, solvent is continuous- disappear
125
6.27. Non-chemical model, Solute is particle, solvent is continuous- disappear
125
6.28. Non-chemical model, Solute is particle, solvent is continuous- disappear
125
6.29. Non-chemical model, Solute is particle, solvent is continuous- disappear
125
6.30. Chemical model (Particles, solute and solvent), general
126
6.31. Chemical model (Particles, solute and solvent), general
126
6.32. Chemical model (Particles, solute and solvent), detailed
127
8.1.
Non-chemical (dispersal) model. Solute and solvent are particles - breaking bits
134
8.2.
Non-chemical (dispersal) model. Solute and solvent are particles
134
xiii
- intermingled 8.3.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
134
8.4.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
135
8.5.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
135
8.6.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
135
8.7.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
136
8.8.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
136
8.9.
Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
136
8.10. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
136
8.11. Chemical model. Solute and the solvent are particles - general
137
8.12. Non-chemical model. Solute and solvent are particles - intermingled
138
8.13. Non-chemical model. Solute and solvent are particles - intermingled
138
8.14. Chemical model. Solute and solvent are particles - general
139
8.15. Chemical model. Solute and solvent are particles - general
139
8.16. Chemical model. Solute and solvent are particles - general
139
8.17. Chemical model. Solute and solvent are particles - general
139
8.18. Non-chemical model. Solute and solvent are particles - intermingled
140
8.19. Non-chemical model. Solute and solvent are particles - intermingled
140
xiv
8.20. Non-chemical model. Solute and solvent are particles - intermingled
140
8.21. Non-chemical model. The solute is particle, the solvent is continuous - intermingled
141
8.22. Chemical model, the solute and the solvent are particles- general
141
8.23. Chemical model, the solute and the solvent are particles- general
141
8.24. Chemical model, the solute and the solvent are particles- detailed
142
8.25. Chemical model, the solute and the solvent are particles- detailed
142
9.1.
The explanations of different group of people about dissolving
146
9.2.
The ideas of people about solvent
147
9.3.
The ideas of people about solute
148
9.4.
The ideas of people about the concepts of dissolving and melting
149
9.5.
The kinds of answers that has been given for the process of dissolving
151
9.6.
The explanations of people in dissolving
152
9.7.
The ideas of people about reversibility in dissolving process
153
9.8.
The methods for undissolving that have been suggested
154
9.9.
The models that have been used in drawings
156
9.10. The models that have been used by people in ASE conference
157
9.11. The kinds of models of dissolving that have been used by people in this research
158
xv
APPENDICES
APPENDIX 1.
QUESTIONS
170
APPENDIX 2.
THE TRANSCRIPT OF EXPERTS’ INTERVIEWS
171
2.1
Interview with the lecturer A
171
2.2
Interview with the lecturer B
174
2.3
Interview with the lecturer C
176
2.4
Interview with the lecturer D
180
APPENDIX 3.
TABLES OF EXPERTS’ INTERVIEWS
185
APPENDIX 4.
EVALUATION OF EXPERTS’ INTERVIEWS
189
4.1
Evaluation of first interview
189
4.2
Evaluation of second interview
191
4.3
Evaluation of third interview
193
4.4
Evaluation of fourth interview
195
APPENDIX 5.
THE RESPONSES OF SCIENCE SPECIALIST STUDENT TEACHERS 5.1
Ideas about dissolving in general
196
5.2
Ideas about the constituents of a solution
196
5.3
Ideas about dissolving and melting
197
5.4
Ideas about the process of dissolving
197
5.5
Examples
198
5.6
Drawings
198
xvi
APPENDIX 6.
THE RESPONSES OF FIRST YEAR SCIENCE SPECIALIST STUDENT TEACHERS
APPENDIX 7.
6.1
Ideas about dissolving in general
199
6.2
Ideas about the constituents of a solution
200
6.3
Ideas about dissolving and melting
201
6.4
Ideas about the process of dissolving
201
6.5
Examples
202
6.6
Drawings
202
THE RESPONSES OF NON-SPECIALIST STUDENT TEACHERS
APPENDIX 8.
APPENDIX 9.
7.1
Ideas about dissolving in general
203
7.2
Ideas about the constituents of a solution
205
INTERVIEWS WITH TECHNICIANS 8.1
Interview with T1
206
8.2
Interview with T2
208
8.3
Interview with T3
210
TABLES OF TECHNICIANS’ INTERVIEWS
212
xvii
ACKNOWLEDGEMENTS
Success is derived from effort, patience, guidance and, of course, study. This dissertation plays an important part in my academic life as a first professional product.
In writing this dissertation I would like to thank especially my tutor Dr. John Oversby for his priceless guidance, patience, assistance and suggestions. My greatest thanks also go to my supervisor Mr. D. David Malvern who assisted me in finishing my assignments during my course with his support. To both my tutor and my supervisor I say “thanks a lot”.
During this course and my life there were three people who always supported me. Thank you to: my father “Mustafa Gödek” my mother “Hacer Gödek” and my sister “ Derya Gödek”.
This was my first experience of being a foreign country. Special thanks to The Republic of Turkey; The Higher Education Council of Turkey and The National Education Development Project in Turkey for giving me a scholarship during my stay in England and for their attention to Education as a worthy area for study.
xviii
I wish to thank also the Turkish teachers and the Gazi University lecturers who contributed with their academic experience during my education.
I record many deepest thanks to all the experts, students and technicians who contributed to this dissertation with their explanations and drawings.
I also thank my classmate: Angelos Kavasakalis, my flatmates: Teresa-Maria Spatola; Dimitris Brikas; my dear colleagues and friends: Nese Cabaroglu, A. Lebriz Tacettin, Gulin Karabag, Sencer Bulut, for their moral support all through this year.
Finally, I would be remiss indeed if I did not express my thanks to my many other friends who also supported me.
THANKS TO EVERYBODY
xix
LIST OF ABBREVIATIONS
ASE
: The Association for Science Education
FYS
: First Year Science Specialist Student Teachers
NSS
: Non-Specialist Student Teachers
SSS
: Science Specialist Student Teachers
T1
: Technician 1
T2
: Technician 2
T3
: Technician 3
xx
ABSTRACT
This research explores models used in explanations in the example of dissolving. The variety of models discovered provide only partial explanations to describe this process. A fuller and deeper explanation is proposed and analysed in terms of its models. The variety of explanations provided by experienced, pre-service teachers and science textbooks have been sought and examined in the light of the fuller model. Models that have been offered by these scientific experts, non-experts and science textbooks do not provide a full explanation of dissolving. The major concern appears to be concentrated upon partial explanations regarding dissolving, and only minor emphasis is placed upon insolubility which is in fact a crucial part in the dissolving process. Moreover, an attempt to understand the appropriateness of explanation at different levels, regarding models of progression in explanation, has been made.
This research undertaken in this particular area, will hopefully assist teachers to transform incomplete existing ideas and make conceptional development of children with more appropriate models.
xxi
CHAPTER 1. INTRODUCTION
Explaining scientific occurrence is a crucial part of science education as it explores various ideas in order to reach suitable explanations. People from every age group, from childhood start to explore and find satisfying explanations with which they feel comfortable. Their ideas can be changed for further satisfying explanations or retained. The main purpose of science educators is to explore these personal ideas in order to provide a better understanding about science. Science educators are concerned with the models that are held by individuals. They do not consider these ideas to be ‘wrong ideas’, because historical evidence shows that often there are similar ideas which were shared by earlier scientists. As time passed the limitations of some of these ideas were altered by new ones. The models were used to explain scientific theories and phenomena that took place in the world. As a new science educator this research was conducted in order to explore and discover different ideas about dissolving.
Dissolving is one scientific phenomenon that incorporates many other phenomena in chemistry. For many people it may seem to be a very simple concept. However, after analysing people’s ideas, it is evident that a significant proportion of these people are not familiar with this concept despite the fact that it plays a significant part in their everyday life, for example through dissolving sugar in tea or coffee or dissolving salt when they are cooking. Therefore this phenomenon needs to be explained more clearly by improving new models.
1
This research aims to find the various explanations and models used by people in dissolving. It examines the importance of explaining and modelling in science education, analyses previous research undertaken in the concept of dissolving, the technological uses of dissolving in throughout history, models used to explain dissolving, the language which is used within the 7 to 17 age group, the ability to differentiate dissolving from other concepts such as melting, the importance of the solute and solvent in solubility and the process of dissolving. It also concerns explanations of dissolving and models used within degree level textbooks. The ideas of some experts, different levels of science student teachers and science education technicians are analysed and from their drawings some models were explored in order to explain dissolving. Drawings were also collected from both primary and secondary physics and chemistry teachers, PGCE students and laboratory technicians who were present at an ASE conference in 1997. It concludes by evaluating of their responses and drawings and providing implications for teachers.
2
CHAPTER 2
EXPLAINING AND MODELLING
2.1 Introduction Explaining events, objects, ideas, theories or phenomena and using models to explain them, is an important job of teachers in science classrooms. However, explaining ideas is not as easy as expected because appropriateness of the explanation is required and, in order to explain different perspectives of the phenomena, different models are usually needed.
In this chapter the main purpose is to explore the importance of explaining and modelling in science education. It consists of two main sections. In the first section the emphasis will be on explanation: firstly, the descriptions, the importance, the types and the levels of explanation will be discussed; secondly, the necessity of an appropriate explanation will be explored. In the second section of this chapter, models will be identified as teaching and learning tools. The difference between theory, model and concept, the relationship between analogy, metaphor and model, and the connection of target, model and source will be determined. The kinds, purposes and uses for models will be examined and ideas for constructing new models will be suggested.
3
2.2 EXPLAINING Explaining is a crucial part of science education. Scientific theories inform about the world and illustrate events which happen. Teachers want to explain some scientific theories in classrooms.
A definition for explanation has been given by Gilbert, et al. (1997, p.1), that it is the answer sought or provided to a specific question.
It could be assumed that explanation is just the transferring of the idea or knowledge from teachers to students. In fact, the explanation is not just the transferring of the idea. It has been pointed out by Ogborn, et al. (1996, p.15) that the explanation provides material on which to work to make an idea. So, the explanation helps to construct some further ideas.
Martin (1972), as cited in Gilbert, et al., (1997a, p.3), characterises explanation as:
a clarification of the meaning of a phrase in the scientific context. The relation between the phrase and the phenomenon is described;
a justification for some belief or action and the provision of reasons why a belief or action is reasonable;
a causal account of some state, event, or process and it is a prepositional statement stating why something is;
a citation of a theory from which a law may be deduced;
an attribution of function to an object.
Furthermore Martin (op. cit.) indicates the types of explanation which are:
4
Intentional explanation; Descriptive explanation; Interpretative explanation; Causal explanation; Predictive explanation. The questions asked by different group of people such as, scientists, science teachers, students who could be in primary, secondary school or at degree level, need appropriate explanations. In the literature the term ‘best explanation’ was suggested for different levels of students or others. However, it is impossible to identify the best explanation. The problems are that: for the best explanation what could be criteria? who could decide these criteria? how could the criteria be chosen? So, it may be logical to use the term ‘more appropriate explanation’. A more appropriate explanation should compensate for the needs of the questioner. A more appropriate explanation should be based on these questions: For what reason should be made the explanations? For whom should be made the explanation? If these points are not considered then the explanation may not be appropriate for the questioner.
According to Gilbert, et al. (1997a) in some cases even an implicit question can receive an explicit explanation. The teachers’ explicit questions were often characterised as
5
‘closed’ questions, for one explicit question it is expected to receive only one specific answer. However, Barnes, et al., (1969), as cited in Gilbert, (op. cit.), state that there is no one appropriate explanation for all cases, for all circumstances and for all questioners. The implicit questions may be due to difficulties in classrooms. Gilbert (op. cit.) claims that in classrooms some teachers provide explanations without even being aware of the question.
The explanation should meet the needs of the questioner, therefore the explanation should be provided at different levels. The levels of explanation have been described in terms of its ‘level’, ‘power’, or ‘scope’ by Gilbert (op. cit., p.7-8). They promote four criteria which define the level of explanation: Plausibility:
Whether the existing explanation can solve some problems that may arise in the future;
Parsimony:
The fewer concepts invoked to provide the better explanation;
Generalisibility: It can be applied to more contexts; Fruitfulness:
It supports a greater number of predictions.
Sometimes, the explanation given does not match the question which has been asked. Also, these criteria are based on the intentions of the questioner. For scientists, for curriculum designers, for science teachers and for science students the explanations required are different. The intentional and descriptive explanations are supplied by scientists. A range of explanations can be received.
Curriculum designers have to aware of the needs of the different students. Gilbert, (op. cit., p.10) suggests that;
6
“At the primary school and in science education, simple but scientific questions and explanations may just be implied through the requirement to study certain phenomena. At the other end of the age and experience spectrum, just before graduating with a science degree from a university, the explanations required by the curriculum would be the same as those currently being produced by scientists”. Science teachers have also to meet the expectations of the curriculum, the demands of public, and the interpretations of textbooks that represent a boundary between the requirements of the curriculum and the cognitive space created by teachers.
Science students are taught descriptive and interpretative explanations and the examples that are set in textbooks are usually chosen from these kinds of explanations because of the demands of the curriculum. Students learn to what extent people are able to control and change their environments. They can design intentional, causative and predictive explanations.
2.2.1 Why do we explain? In science, phenomena need to be explained to make them more easy to understand. Explanations provide some reasons about why the phenomenon behave as it does. The explanations depend on an causation. The observed behaviours of a phenomenon, its causes, and the effects are described either deterministically or probabilistically by explanations.
Hempel (1965) as cited in Gilbert, (et al., 1997, p.5), propose that deterministic explanations have three levels: a statement of a causal law; 7
a statement about phenomenon; a deduced causal explanation. Secondly, explanations are required in other cases. The phenomenon may perform differently in different conditions. The prediction is provided and the phenomenon is tested under other conditions and the explanation is then used and tested.
Teachers do not need to be asked questions in order to give an explanation. In general, they explain some phenomena which have never been thought of by students, for instance, the structure of atoms. Students acquire their ideas through their observations in everyday life or from their parents and they are often satisfied with them. This is the reason why explanations are not always directly accepted by students. Teachers’ ideas and explanations are generally accepted by students only for the purpose of passing examinations. If explanations are given in appropriate ways and by using appropriate models, they are usually accepted. However, if explanations are not appropriate, students will already have their own perceptions and ideas which are misleading. This reluctance in accepting new ideas does not affect only students but also others. Even science teachers tend to retain their own ideas which results in the simple transfer of ideas to their students. In other words transformation of ideas by the teachers does not occur in every case.
Scientists are generally more open to new ideas and explanations in their specific subject area. Therefore there is a need for teachers to explain phenomena in order to formulate people’s opinions and to transform new ideas. It is the responsibility of teachers to understand
children’s
misconceptions
concerning
phenomena
and
to
8
replace them with the appropriate ones. Unlike teachers, scientists work on models with the intention of testing their scope and limitations. Their work presents more appropriate and accurate explanations within the scientific sphere.
2.2.2 The teaching tools used for explaining of scientific phenomena In schools, teachers use some tools to explain some scientific phenomena that are abstract, unobservable or difficult to explain. By narrative stories knowledge can also be transferred. Ogborn, et al. (1996, p.15), suggest that stories can act as effective ‘knowledge carriers’ e.g. in the discovery of penicillin or through finding food having gone bad through personal experience. These narrative stories can make the phenomenon more easily understood, memorable, and recoverable.
On the other hand models, analogies, metaphors and demonstrations are used for transformation of knowledge in science classrooms.
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2.3 MODELS 2.3.1 Definitions Models occupy an important part in scientific methodology. Forming and testing of models is a major element of scientific methodology. Models are a major product of science, and in creating learning and teaching tools in science education.
It is a major learning tool because students form their own models in order to understand theories and the concepts. By determining their own models, students contribute to their own learning.
It is a major teaching tool because understanding abstractions of theories is very difficult. By using models, theories can become easier to grasp.
Gilbert (1993, p.5) defines a model as a representation of an object, event or idea, and modelling as the process of forming a representation. In other words, it is the outcome of representing a novel object, event, or idea, of scientific interest, in terms of a more familiar object, event, or idea.
MODEL similarity
TARGET
drawing analogy
SOURCE
Figure 2.1. The relationship between target, source and model. Source: Gilbert (1997b, p .6)
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In general terms, model “means that some aspects of the source of the model, from where it is derived, are transformed, in some way and to some extent, to that which is being described, to the target of the model. A model is the representation, the outcome, of that transfer” (Gilbert, op. cit.).
2.3.2 Model, Theory and Concept in science The model, theory and concept are different concepts in science. According to Gilbert (1997b, p.6) a theory may be taken as a set of abstractions which are mapped onto an imaginary. A model is seen as an intermediary between the abstractions of theory and the concrete actions of experiment, so they help to make prediction, guide enquiry, summarise data, justify outcomes and facilitate communication. In some cases one theory and one attendant model assist as a model to develop other theories and models (Nagel, 1987, p.110), as cited in Gilbert & Boulter, (1995a).
According to Carroll (1962), as cited in Gilbert & Boulter, (op. cit.) a concept is an abstract generalisation that emerges from experiences with more than one example of an event or object.
The concepts formed by children are called alternative conceptions, alternative frameworks, misconceptions, children’s science and so on. The concepts consist of the formation of propositions and models make use of images.
The above definitions have been clarified by the following example, given by Gilbert (op. cit.): An underground railway system has a complex series of tunnels and stations, therefore, an ‘underground map’ could be accepted as a formal diagram or model of this theory. 11
2.3.3 Analogy, Metaphor and Models In models, analogies and metaphors are used. According to Gilbert (1993) “in analogy, an object, event, or idea, is regarded as being like some other, more familiar, object, event or idea. Representation is made possible by some perceived similarity”. In metaphor the source of the metaphor and the target is different, but they are used for comparison at different levels of similarity.
2.3.4 Target- Model- Source connection The relationship between target, model and source is characterised by Gilbert & Boulter (1995a) such that a model of a target (that which is to be represented) is produced from a source (some other object, event or idea) by the use of metaphor in which the target is seen, if only initially for the sake of argument and for short time, as being very similar to the source.
2.3.5 The kinds of models in science education In science education, the expressed models of students are called alternative models, alternative conceptions, alternative frameworks, alternative misconceptions, children’s science. Gilbert & Watts (1983) claim that they are related to existing and the past conceptual models.
12
Depend on the duration and the public availability models were described by Gilbert (1993) as:
Mental Model
Alternative Conceptions
Scientist’s Conceptualisation
Conceptual Model
Duration
Transient
Long lasting
Public Availability
Private
Public
Figure 2.2. The kinds of models depend on the duration and public availability. Source: Gilbert (1993)
According to the literature of psychology, history and philosophy of science, mental models are accepted as the origin of all models Gilbert (op. cit.). All of these above models play an active role in science education.
Four major types of models received from different sources have been classified by Black (1962) as cited in Gilbert, (op. cit.): Scale Models: The target object is the source of the model. They can be in various proportions, smaller or bigger than the target object. The emphasise is on the important point of the target object. For example the architectural models; Analogue Models: The source of material is different from the real target object. In chemistry the structure of molecules is represented by ball and stick models and the material can be coloured polystyrene spheres or plastic straws but the real atoms do not possess colour. Mathematical Models: The symbols represent the target object and instead of using words symbols are used. 13
Theoretical Models: It is about the use of language. Metaphors and the analogies are used. Words represent the target object.
The categories of the models that have been discussed by Gilbert, et al. (1997, p.16) are: 1. MENTAL MODEL: The target is represented as personal and private by individual people. They use and develop their mental models to make sense of the world in which they live. They are transient in duration because as people receive more information and explanation, they construct, develop or alter their mental models from the existing one. 2. EXPRESSED MODEL: It is version of the mental model expressed by an individual via action, speech or writing. Expressed models are known by the public and everybody can benefit and construct their own mental models from them. 3. CONSENSUS MODEL: It is a model that has been tested and scientifically and socially agreed by scientists. 4. TEACHING MODEL: Teachers use and express teaching models (as aids for teaching) that have been accepted as consensus models.
2.3.6 The purposes and uses of the models Models have some purposes in explaining science. The advantages of the models have been summarised by Gilbert, et al. (1997a, p.15). Models provide five types of explanation:
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Intentional explanation; Descriptive explanation; Interpretative explanation; Causative explanation; Predictive explanation. Models maximise the appropriateness of explanation to compensate for the needs of the questioner. So, explanations need to be plausible, parsimonious, generalisable and fruitful.
Models enable the complex ideas, objects, events, processes or systems to be perceived easily and to be more visible and visualised. They also: make complex phenomenon easier to understand; promote the predictive capability of theories; promote development of theories provide imagination and insight for students; link theory to experiment and observation.
Models have been accepted and used as teaching aids. The important points of models have been summarised by Gilbert (1990, p.7,8): models are representations of the real things; they can be expressed in a number of forms, e.g. made of materials, pictorial or diagrammatic, written, mathematical or in combination of some or all of these forms;
15
they usually deal with only part of the total situation and more than one model may be needed to represent the total thing more accurately; they are useful aids to understanding; they give organisation to ideas; they aid remembering; they can help to produce new ideas. Models, then, are designed to create an idealised representation of reality.
Not only models are used in schools to explain scientific phenomenon but, Borges (1997) points out, people form simple mental models to account for their knowledge of the physical world. As they gain new knowledge, they assimilate their new knowledge into their existing knowledge and accommodate it into more advanced models.
2.3.7 The variance of the models According to Borges (1997), Haupt (1952) said that children’s ideas are parallel to the historical development of the scientific concepts. Children’s ideas have been accepted as similar ideas to ‘primitive ideas’ which are the main blocks of science.
In science lessons, for one phenomenon there should be more than one model but when the teachers are using these models they have to indicate that they do not provide fact that it is only one of the representations of the phenomenon. Barnea & Dori (1996) claim that many teachers who use models do not emphasise the notion that models are a simulation of theory. In one of the models that is used for explaining the structure of molecules, atoms
are
represented
by
coloured
balls
and
the
bonds
are
16
represented by the sticks. However, for scientists, it is well known that no bond looks like a stick and no atom is coloured blue. According to the research of Barnea & Dori (op. cit.) it is obvious that students perceive hydrogen atoms as shaped as balls and coloured blue. There is a need for teachers to emphasise that no molecule looks exactly the same as any of the models presented.
Various models should be used in order to compensate for the different needs of individual students. However, students also have the perception that the phenomenon looks like the model presented. For different purposes and for different age groups a different model can be used. For example; A woman can be called by different names and has different roles in a society, such as: a wife of her husband; a mother of her children; a friend of her friends; a daughter of her parents; a teacher of her students. Similarly, a phenomenon can be represented by different models for different purposes. It has also been suggested by Gilbert (1990, p.7) that “models usually deal with part of the real situation and more than one model may be required to adequately represent a total situation”.
2.3.8 Constructing new models in science classroom It is a fact that students cannot be expected immediately to perceive and to grasp models that are presented by their teachers. Every student has some models that they
17
feel comfortable in understanding. It has been stated before that models that children hold can be called “alternative models, alternative conceptions, alternative frameworks, alternative misconceptions, children’s science”. To benefit from teaching models, teachers need to aware of the students’ alternative mental models. This can be promoted by some activities. The students should share their ideas with other students and they need to explain why they believe the model that they have is appropriate for the special phenomenon. By discussion and the interchange of the ideas, students will be conscious of the other’s mental models. Just awareness is not sufficient. Before using a teaching model, teachers should provide an environment for students to apply their models on the specific phenomenon. They can then enable the students to see the limitations and the scope of their models and also of their own explanations. After these stages, students should be able to consider alternative models. This will help them to construct new models (Gilbert, 1993).
Mayer (1989) as cited in Gilbert, (1997b, p.12) states that the conceptual models should have some characteristics. These are: Complete: The model should contain all of the structural elements, Coherent: The model should be appropriate to the level of students, Concrete: The relationship between the model and all of the parts of the model should be obvious to the students, Conceptual: The model should have a clear explanation, Correct: The model has to represent the target precisely, Considerate: The appropriate vocabulary should be used. Correspondent: A sufficient number of analogies should be used in models. 18
Conceptual models need to be introduced systematically by the teachers and new analogies used to help to develop new conceptual models.
The model should be used without confusing the students. In some cases teachers use analogies with which students are unfamiliar and this causes some misconceptions. Teachers have to know the purpose for using a specific model in their explanations and it is important that they have also mentioned previously the similarities and the differences between the target and the model, the description of target, and source, and identified relevant features of source and target. Limitations and the scope of model should be indicated in order that students should not perceive the phenomenon, event, or object to look exactly like the model presented. They need to perceive that a model is one of the representations of the target object. Teachers also should use more than one model in explaining scientific phenomena to provide students with different perspectives.
2.4 Conclusion In this chapter the importance of explaining to students at different levels and the kinds of explanations that are usually given by different group of people such as, scientists, curriculum designers, science students has been mentioned. A crucial point that has arisen in this chapter is that is there is no single explanation for a specific phenomenon. Explanations need to be more explicit and to be appropriate to the student’s level. Teachers can give different explanations about a phenomenon based on the students’ ages in order to make the phenomenon more precise, visible and understandable. However, it has been shown in this chapter that teachers’ explanations are not always directly accepted by students. Students retain some private ideas (alternative conceptions), and,
19
so, as they feel comfortable about explaining the phenomenon with these ideas, and they do not easily accept the new idea.
On the other hand, in science classrooms, teachers use some models derived from different analogies, metaphors and sources in order to make explanations more explicit but it has to be accepted that some problems may occur if teachers do not use appropriate models for specific phenomena. In this case students may reject the new models and keep in their minds the old ones. The last part of this chapter has illuminated the way of constructing new models. There is often no single model to explain a specific phenomenon. It is possible for one model to be used for explaining different phenomena. In the case where the model is insufficient to explain all the properties of a phenomenon, more than one model should be used for the representation.
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CHAPTER 3
PRIOR RESEARCH FINDINGS ABOUT DISSOLVING
3.1 Introduction Dissolving has been seen as an important phenomenon in everyday life and in science education. There are many studies that analyse the findings from different perspectives such as: the history of dissolving; the models that were used to represent the dissolving by scientists and children; the development of the explanation of dissolving; the language that is used by children in explaining the concept of dissolving; the conservation of mass in solution; the role of solute and solvent in a solution; the process of dissolving, the interpretations of dissolving by drawing; the factors that affect solubility; differentiating dissolving from the other phenomena such as melting, evaporating; and some misconceptions in dissolving. These studies are discussed below.
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3.2 The technological use of dissolving in history It has been indicated by Oversby (1997) that dissolving was known as early as 5000 BC. The people at that time, without being aware of the importance of dissolving as a scientific concept produced beer by fermentation and then distillation of ethanol. Arabs used distilled ethanol in extracting some plant essences or perfumes and they used the products for medical purposes are also mentioned, (Brock, 1992). People dyed cloths with the extracted vegetable dyes. Non-aqueous liquids for example, linseed oil were used by artists for their paintings. Oversby (op. cit.) commented that “the use of coloured glass in medieval times for stained glass windows, indicates a technological understanding of the process of dissolving”. In Alchemists’ time solid solutions were known, such as bronze and brass. They separated contaminants from gold solution by dissolving gold in arsenic. Gold was extracted from the gold ores by dissolving gold in mercury. It is also important to note that for more than 5000 years some roots were used, such as cassava, as a staple. In these roots, toxic chemicals such as cyanides were removed by pounding and then the roots were washed in water (the cyanides are known to be water soluble).
In the nineteenth century a common inclination of chemists was to measure the properties of solutions. So, Raoult studied vapour pressure, Pfeffer explored osmotic pressure, Kohlraush investigated electrical conductivity and Arrhennius studied explaining the dissolving process.
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3.3 The conservation of mass in dissolving In science education, the earliest research about dissolving was conducted by Piaget and Inhelder (1941/74) as cited in Johnstone & Scott (1991, p.195), exploring four stages of children’s understandings of dissolving. These four stages were; Stage 1. absence of any conservation (4-7 years); Stage 2. conservation of substance (7-9 years); Stage 3. conservation of mass (9-12 years); Stage 4. conservation of volume (12 years +). In Piaget’s opinion by using and developing of the atomistic schema, the ability to understand conservation of mass and volume is developed.
There are also some studies that were carried out to find ideas of students about the conservation of mass. These studies are: Cosgrove and Osborne, (1981); Andersson, (1984); Wightman et al. (1986); Driver, (1985). In Driver’s opinion, however, some of children believed that there is a loss of mass in the dissolving process, but when they were challenged to think, they would respond that the sugar is still there. The studies of Tasker (1981) and Longden (1984) showed that some laboratory experiments, for example, evaporation, helped children to think of the conservation of mass in dissolving.
Johnstone & Scott (1991) claim that Driver & Russell (1982) believed that learner belief in conservation of the sugar increased with age. Driver (1985) reported that in her study to explore the conservation of mass in dissolving process, the two thirds of a
23
sample at the age of between 9 and 14 English students gave the opinion that there is a loss of mass in dissolving of sugar in water. (Prieto et al., 1989) Andersson (1984), as cited in Ahtee (1993), said that 60% of a sample of Swedish students believed that “the weight of sugar solution was the sum of the components”. Holding (1987), as cited by Johnstone & Scott (1991), who interviewed 90 students and gave a written survey to 588 students, concluded that the percentage of students who believed in the concept of conservation of mass were; 50% of 7-8 year-old students, 40% of 9-10 year old students, 70% of 16-17 year old students. He pointed out that children had difficulty in verbalising their mental models.
An intervention strategy was used for children between the ages of 12 and 13 years old by Johnstone & Scott (1991). Practising science teachers were asked to find out the effectiveness of teaching and learning strategies for assisting conceptual change in science, to determine the students’ conceptual difficulties, and to provide a bridge from concrete ideas to abstract ideas by demonstrations and group work. In this study there were three different stages. These are: To elicit the beliefs of the students in their sample before instruction using worksheet, through small group discussion, record predictions, and reasoning on the work sheet. A series of demonstrations; the demonstrations were carried by the teacher and the students observed and explained the event.
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Turning to the first question; the first question and the answer given was discussed in a group. There were three types of answers. These answers were; 1. The sugar would get lighter or weigh less because the students reasons were; “Sugar ‘bits’ become smaller. Each bit will go into microscopic pieces and make it lighter, If the size of the sugar in the water gets less, the weight of the sugar will get less because the crystals get smaller. When we mix the sugar to dissolve the particles, these get smaller so that sugar will weigh less than the other sugar. Liquids are lighter than solids”. 2. The mass of sugar must be conserved because “the sugar is still there, it will still have same weight, it is still there but in a different form”. 3. The third kind of answer was the gain of mass. Sugar has to be heavier because the students used the analogy of a sponge.
Johnstone & Scott (1991) stated that many non-conserver students only considered the change of sugar rather than the whole solution system.
On the other hand some misconceptions in using the concepts of density and mass were presented by Ahtee (1993). More than 40% of the secondary students thought that “at least part of the salt disappears” and half of the students believed that “salt does not contribute at all to the weight of the solution”.
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3.4 The effects of solubility in dissolving and the solute and solvent in dissolving Prieto et al. (1989) indicated that the children perceived the main factor that affects solubility is the solute. On the other hand, solvent has a passive role in dissolving such as, absorbing, taking in, changing colour or taste, etc.
Longden (1984) stated that children did not mention that solute and solvent gas could be solid, liquid or gas.
Longden et al. (1991) commented that the amount of the solute taken experimentally to dissolve in a solvent creates some difficulties in understanding dissolving. For small amount of the solute, students can understand intermingling at particle level but after saturation point even some everyday examples are not easy to understand.
In Ahtee’s (1993) study the students gave the temperature as an effective factor in the dissolving of sodium chloride. The examples were: “salt dissolves more/ faster/ better in hot than cold water. Only a limited amount of salt will dissolve in water”. Also some of students responded that “if you just stir long enough so all salt will dissolve”. He pointed out that none of the Finnish textbooks gave a definition of the concept of solubility.
3.5 Misconceptions in dissolving Prieto et al. (1989) found that the ideas of children are called in literature different names such as: misconceptions; preconceptions; alternative conceptions; children’s science; etc. According to Prieto et al. (op. cit.), Ausubel (1976) and Driver (1986) declared that in science education, exploring the children’s misconceptions helps to overcome this problem and to develop their understanding of scientific concepts. There are some misconceptions researched by Driver (1985), as cited in Prieto et al. (op. cit.), that 25% 26
of a sample of New Zealand students used the words melting and dissolving as synonyms. Longden (1984) conducted a study using interview- about- events technique and interviewed 20 students from the age of 11 to 12 year old. The importance of the choice of this age group, as stated by Longden, was that this age group is “a critical time in schooling [as] the transfer from primary to secondary education”. He showed some drawings on cards in which there were some everyday examples or related non-examples of dissolving. In the second study eighty-one children were required to write five sentences including the word “dissolving”.
In this study some children perceived dissolving only as disappearing of the colour of the solute. However, some children distinguished dissolving from melting, while some of them confused evaporating with dissolving. They did not relate their scientific knowledge to their everyday experiences. Longden pointed out the everyday meanings of dissolving by referencing Collins English Dictionary (1979). According to this dictionary, melting, vanishing, dismissing, ending, breaking down were meanings of dissolving. The sentences were: dissolve into thin air, dissolve parliament, the marriage was dissolved, dissolve into tears. In his opinion, children bring such associations to the classroom.
The definitions of solute, solvent and solution were given by Longden (1984, p.7) that “the solute is the substance being mixed, the solvent is what it is mixed into and the solution is the perfect mixture formed”. He indicated that even the scientists do not use the term of dissolving for gas dissolving in gas solutions. They prefer to use the term
27
mixing. On the other hand, teachers did not accept the word of dissolving for liquid dissolves in liquid solutions. However the events that have been used by Longden (1984) generally contained solid in liquid solutions and liquid in liquid solutions. There were no other examples of other kinds of solutions. However Longden (1984) has used the phrases of “mixing, perfect mixing, intermingling of different kinds of particle, dispersion, interactions between particles” instead of using the word of “dissolving”. He commented that (op. cit., p.51) the words of spreading and intermingling of atoms were similar to dissolving. He considered the dissolving into two levels: 1. Particle, microscopic, 2. Visible, macroscopic. In Happs’s (1980) opinion, as cited in Longden (1984, p.23), children prefer to use observable models not abstract models. The children described dissolving as particles disappearing, they decrease in size, they change from solid to liquid particles. Eighty-one children were required to write five sentences that contain the word “dissolving”. Dissolving was related to disappearing, making things softer, breaking down things and to spreading out. The examples were based on the solid dissolving in liquid solutions.
So, the ideas explored from his study were:
There was an opinion that the solute can be evaporated like solvents;
The children related the colour change and the disappearing of solid to dissolving but for some of them the colour change was confusing because they perceived that, in dissolving, the solvent retains its colour. It was the effect of the word of disappearing or vanishing;
28
For some of children some of examples such as paint, tea, nail varnish remover and stain remover were confusing because they only accepted the dissolving of crystals in a liquid;
the children recognised dissolving but there was a limited opinion that dissolving occurs only when the solute - solid disappears not a colour and liquid retains its original colour, the granular solid disappears;
the children perceived only water as a solvent. The non-aqueous solvents were considered as acids;
dissolving involved melting;
evaporation following to dissolving;
the solid retains its state in dissolving;
evaporation was referred as undissolving.
Nusirjan & Fensham (1987) interviewed thirty Indonesian students. They reported that there were some misconceptions in understanding dissolving. These misconceptions were: a. In the process of dissolving the solute becomes a molecule or atom or ion and mixes homogeneously; b. the solute becomes small particles (crystals) and mixes homogeneously; c. the solute becomes liquid and mixes homogeneously.
The children examined in this study used the homogeneous non-chemical dispersal model in their explanations. In these above explanations there is no mention of a chemical change in dissolving. Prieto et al. (1989) carried out a study in Malaga (Spain). The four written free answer questions asked of the 319 students the age from eleven to fourteen years old (6th, 7th, 8th grades) were: 29
to find out the different points of view and explanations of about the event of dissolving, and
to understand the origin of these ideas, whether they derive from social or school knowledge.
When the ideas of students about dissolving one substance in another were asked, there were two main aspects: that one of them was the mechanical (putting the two substances in contact with each other, shaking and stirring); and the second one was thermal (heating the substances together). The words that were used by the students were characterised as: requirements; actions; solute; solvent; interaction between solute and solvent; and characteristics. There was a diversity in the explanation of 8th grade students compared with 6th and 7th grades. 80% of students used the terms, mixing, adding, moving, joining, pouring, introducing actions in dissolving process. The children of young age interested in the solute its changes, and the distribution and the older children interested in the interactions that are physical and chemical between solute and solvent. The children had difficulty in expressing their ideas about the changes in the solution. Their ideas about the solute were ‘distributed’, and ‘it goes to the bottom of the solution’, ‘disappears’, ‘breaks up’, ‘breaks up into small particles’, or ‘its molecules are separate’, ‘melts’ and ‘decomposes’. According to Piaget & Inhelder (1941/ 1974), as cited in Longden et al. (1991), the use of ‘disappearing’ is the effect of metaphorical meaning. So, the misconceptions were: 1. dissolving is the same as melting; 2. solute and solvent interact to form a new substance having the properties both; 3. solute and solvent interact to form a new substance entirely different from both solute and solvent;
30
4. the term solution refers to solids dissolved in liquids; 5. a suspension is an example of a solution. Here the interaction between solute and solvent was accepted as a misconception. However in the dissolving process there is an interaction between solute and solvent due to the dissolving of the solute.
Bargellini et al. (1993) reported that there are most important concepts such as: change of state; solution; and chemical reaction. The multiple choice questions and drawing studies were conducted by Bargellini and his colleagues to investigate the Italian middle school students’ ideas concerning dissolving and the models used by them. Thirteen multiple choice questions were asked of three hundred and fifty-two students from the age of eleven to fourteen years old. The contents of these questions were:
melting of ice;
boiling of water;
solubility of sugar in water;
chemical reactions of effervescent aspirin in water; and,
thermal decomposition of sugar.
The findings were:
the majority of the students (94 %) had a qualitative dimension of the conservation of matter in the event of melting of ice;
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the majority of the students (94 %) believed that when the ice melts its nature does not change in the event of melting of ice;
the majority of the students (72 %) knew the concept of reversibility in the boiling of water;
some of the students (9 %) drew some particles for changing the form of water and they drew smaller or larger or triangular or square shapes and reduced the number of the particles;
21 % of the students believed that, in dissolving process, the lump of sugar changes into small grains of sugar;
4 % of the students stated that the water destroys the sugar;
5 % of the students did not have the idea of the conservation of matter. In their opinion the sugar has been changed by the water into another substance;
12 % of them said that after dissolving of the sugar it does not have any weight.
The drawings of the students were categorised into five groups: 1. No change; 2. Transformation on a macroscopic level; 3. Modification: The solute and solvent particles were modified and their forms were changed; 4. Change of state: There is a change from solid to liquid states; 5. Solution. It seems likely these drawings represented a physical change but there is no evidence to confirm this.
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Ahtee (1993) asked Finnish students from the ages between thirteen and fifteen years old about the dissolving of salt in water. There were five kinds of explanations: Concrete descriptions based on observations: water will become cloudy or salty, salt will disappear or dissolve; Macroscopic explanation: the salt will break up into small bits in water; Microscopic explanations: atoms, ions or molecules as structural parts of matter; Descriptions containing misconceptions, salt will melt or become liquid; Wrong ideas based on chemical knowledge, formation of new substance. ‘Taste and smell’ were perceived by some of the students as a result of interaction. This was called by Schollum (1982), as cited in Prieto et al. (1989), as a ‘conglomerate view’ and they are accepted by children as conceptual alternatives to the concept of chemical reaction.
Simpson and Arnold (1981) presented, as cited in Longden (1984, p.24), some misconceptions about dissolving; “dissolving will only occur where the molecules of solute are small enough to fit into spaces between solvent molecules. It is interactions (forces) between particles which determine solubility and not simply spatial factors”.
Longden (1984) pointed out that there is similarity between evaporation and dissolving because “dissolving involves disappearing and spreading out into liquid, evaporating, disappearing and spreading out into air”. So, he used the words “disappearing” and “spreading out” as a physical change for dissolving. However as we know that dissolving is not just a physical change but that there is often a chemical change in a solution. These
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two concepts of dissolving and evaporating were confused by children in Longden’s survey.
More than 25% of 7th and 8th year old children commented that in dissolving process there is an interaction that is chemical. Similar ideas were explored by Blanco’s (1988) study as cited in Prieto et al. (1989).
In Prieto’s (op. cit.) study, some of children at 8th grade believed that dissolving is a chemical change and in the dissolving process an entirely different substance is formed. These students viewed dissolving as a chemical transformation. However, some of the children used some expressions that are related to physical change such as, ‘solvent and solute are mixed’ and ‘they join’.
Briggs and Holding (1986), as cited in Ahtee (1993), said that one in ten students interpreted physical changes as evidence of chemical changes, i.e. melting and dissolving. So, the dissolving process was seen as a physical change in the Ahtee’s report.
Ahtee (op. cit.) used open ended questions in his study in searching the ideas of Finnish secondary level students the ages from 13 to 15 years. The students did not recognise that there is an interaction between solute and solvent. However it was stated that there is an interaction between solute and solvent in the dissolving process, but it was not accepted that this interaction is an indicator of chemical change.
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3.6 Explanation of dissolving in particulate level According to Prieto et al. (1989) children had only limited knowledge in explaining the dissolving at the particulate level. So, the degree of generality was classified into three groups, that are G-general, A-average, and C-concrete. The degree of generalisation increased by the age. There were two contexts that affect the ideas of the students. For young students (6th and 7th grades) the effects of their everyday experiences were important and for older students the school teaching takes a more important place. The only idea of solutions was that solid dissolves in liquid solutions. Only a few older students knew that there are also liquid - liquid solutions. Homogeneity was used in characterising a solution. The examples that were given by children are generally from daily life. In Longden’s et al. (1991) study children used some of words such as ‘particle’, ‘grain’, or ‘bit’ without referring the particulate level of matter. Some of the children used some phrases in interpreting of dissolving such as, ‘dissolving into tears’ and ‘dissolving into thin air’.
3.7 Language Prieto et al. (1989) stressed that the language used by children was not related to the scientific context. The ideas of children in science are affected by their everyday experiences and their school based experiences. In Ahtee’s (1993) study Finnish students had difficulty in finding exact scientific meanings. He reported that the everyday language affects the interpretation of children such as in everyday talking it is said that the salt melts in the mouth.
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According to Longden et al. (1991) the science teaching needs to be related to the everyday life but there is also a need to use some everyday language in science lessons.
3.8 The change of children’s ideas Longden et al. (1991) claims that children’s ideas can be changed by learning about particles. In cognitive psychologists' views, in time there should be considerable change in children’s intellectual development but Longden’s study showed that there was only a little change in the intellectual development of eleven to fourteen years old children. It was unexpected that there was an inconsistency in students’ interpretations of dissolving between school context and everyday context. For the students who had not learnt any idea in school about dissolving, their interpretations were based on everyday contexts and these ideas were consistent with their particulate interpretations. This was commented on by Longden in terms of these students’ having difficulty in learning new views. The children found it more comfortable to apply different explanations for in-school and outof-school situations. Posner et al. (1982), as cited in Longden et al. (1991), believed that one description must eliminate the other. In contrast to Posner’s idea, Solomon commented that for different fields two different kinds of explanations can be found. The gradual evolutionary process was suggested by Nussbaum (1983), as cited in Longden et al. (1991), that one type of explanation can be derived from the other. Longden pointed out there was inconsistency in the above views about context effect.
They also commented that children hold some views that are helpful in understanding some everyday events. It was suggested by Longden that insufficient attention to
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models creates a lack of understanding. In order to provide consistency between the children’s ideas about dissolving for in school and out of school, it was suggested by Longden that teachers should point out in their lessons that there are different ways of looking at dissolving. So, children could link the different views consistently.
Johnstone & Scott (1991) reported that the children’s ideas were affected by demonstrations, students’ everyday experiences, and peer pressure within the small groups.
Schollum & Osborne (1985), as cited in Ahtee (1993), pointed out the importance of being aware of children’s existing ideas and they suggested that there is a need to relate teaching to children’s existing ideas. Hesse and Anderson (1992), as cited in Ahtee (1993), commented that the traditional teaching techniques are not efficient in providing conceptual change.
3.9 Reversible change Ahtee (1993) found that filtration was the most common mentioned wrong method suggested for retrieving of the solute. On the other hand nearly every Finnish student mentioned distillation, 3% of 7th grades gave pouring the water away, and 6% did not know.
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3.10 Melting and dissolving Longden (1984) determined that there was difficulty in differentiating between melting and dissolving but children were enable to make some distinction between these two events. They used some sentences: For dissolving; “two different substance, a solid is mixing with a liquid, the solid disappears, it is getting smaller”; For melting; “only one substance, there is nothing to mix into, it is still there, not gone away, there is a heat/ temperature difference”.
3.11 Models Holding (1987) referred to some historical models of solution. These were; 1. An Interstitial atomistic model: Democritus (ca. 400 BC) and Plato (427-347 BC) believed that “matter” consists of “atoms” and “the void”. According to this model the empty interstices between atoms of one substance are filled by the atoms of another substance. 2. A Continuous model: Aristotle did not accept there are the voids between atoms. In his opinion “all cosmic space is filled with body”. This model does not accept the filling of the empty spaces by the other atoms. 3. The (pre-shaped) Pore model: Pierre Gassendi (1592-1655), said that “salt crystals were composed of very small particles, called corpuscles, and that they, like the (visible) crystals were cubic shaped”. The water was assumed has empty cube-shaped pores. So, cubed
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shaped salt corpuscles enter into the cube shaped pores of water. He also gave the explanation about saturation that “when all the cube- shaped pores had been filled, no more salt could dissolve”. 4. The Gravitational- forces - between -particles model: Newton (1643-1727) assumed that “a salt can dissolve in water if the salt particles have a greater (gravitational) attraction for water molecules than they have for each other”. The interactions between solute and solvent were accepted firstly by Newton. Holding pointed out that Newton used the expression of ‘minute bodies’ that “after observing the rapid dispersal of dissolved material...repulsive forces between ‘minute bodies’ could be responsible for that effect” According to Newton a combination of attractive and repulsive forces causes dissolving. 5. The ‘Like Dissolve Like’ model: Georges - Louis Buffon (1707 - 1788) believed that “substances having similar characteristics would be made up of ‘bodies’ of similar form and so fulfil the stated requirement”. This idea is being used in many chemistry textbooks today. 6. The solute- solvent chemical combination model: Claude- Lois- Berthollet (1749-1822) thought that in all proportions the substances can react and ‘real chemical changes’ occurred in the dissolving of some substances in water. 7. The hydrate model: Dimitry Ivanavich Mendeleev (1861-1880) pointed out that the solute is hydrated by the water and they are dispersed throughout the liquid.
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8. The mutual interaction model: According to this model dissolving is based on mutual interaction between solute and solvent particles. 9. The kinetic energetic model: By Robert Hook (17th cc.) the motion of solute particles was seen as analogous to the motion of gas particles. Robert Brown considered ‘Brownian motion’ as analogous to molecular motion. 10. Mathematical modelling of kinetic - molecular ideas about solutions: Albert Einstein (1879-1955) devised the mathematical model of Brownian motion.
The process of dissolving has been summarised in Longden (1984, p.86) as that two different substances, solute and solvent, form a clear liquid mixture (solution) by the process of perfect mixing or intermingling, and this process is divided into two groups, of disappearing of colour or solid bits and spreading out of colour or substance. The general model for dissolving was a non-chemical dispersal model. Holding (1987) claimed that all the above theories show that “there is no single rational route from the making of an observation to formulating a theory” and modern scientists often avoid the use of a ‘physical’ model. They use the mathematical terms that do not allow for a physical explanation.
3.12 Drawings The common finding in Holding’s (1987) research in drawings was of individual molecules of solute dispersed throughout the water molecules. The drawings contained
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some figures that cannot be seen with the naked eye but they have been imagined by the students. Similarly it is possible to find this kind of imaginative pictures in textbooks. Holding (1987) asked students that if they had superman’s ability to see inside the objects, how they could draw a picture of the one drop of solution. He also required them to explain their opinions. The students drew shading, dots, squares, circles, etc., to show the sugar particles and the water was shown as plain, this was seen as continuous. Holding categorised these drawings into three groups: 1. kinds of depiction of dissolved sugar: dots, irregular shapes, square shapes, circles, liquid; 2. kinds of distribution of sugar parts: homogeneous, heterogeneous (near to bottom, near the top, sides, middle, corners); 3. names used to denote sugar parts: bits, pieces, grains, crystals, liquid, sugar. The depictions have been categorised by Holding into three groups: a. depictions of a continuous liquid. There were no atomistic ideas about the solution. The conservation of sugar has been examined; b. depictions of parts (gross particles) of (continuous) solid or of solution. The sugar’s and solution’s representations have been analysed; c. depictions of molecular particles of sugar in the solution. The presentations of the sugar and water molecules have been analysed. Also the drawings were: a. the ‘continuous’ conception of a solution. There were drawings in which water was presented as ‘plain’, ‘misty’, ‘murky’, ‘blurry’ and ‘black’;
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b. the dispersed ‘piece’ and/ or ‘drop’ picture. In these pictures the sugar was presented as dots, squares, circles or irregular shapes and labelled sugar. Some of students drew cells to show sugar particles; c. the dispersed ‘molecular particle’ picture. In the plain background the sugar and water molecules intermingled; d. pictures that depicted species other than sugar and water. Some of children drew pictures of solution including air or bacteria, germs, bugs, little creatures, dust, glass, impurities, chlorine and fluoride rather than the particles of sugar or water. The results of Holding’s study were: 20% percent of the students had a ‘continuous’ view of solution; 60% per cent of the students had a view of solution that contains ‘continuous bits’ of sugar. The non-chemical (dispersal) model was common in these drawings. Some of children commented that the particles ‘stuck together, joined, bound, fit together, packed tight, attracted’. These comments show that these children were aware of some changes in a solution rather than just dispersing. This study also includes the explanations and drawings of undissolved sugar. There were eight types of drawings in Prieto’s et al. (1989) study. The homogeneity of solution and discontinuity of the solute, some small dots that can be either assumed to be visible or invisible but it is because some of children put the concept of solution and suspension in the same category. There were drawings that show the homogeneous coloured solutions. The plain solution referred to the disappearance of the solute in a solvent. There were also some non-homogeneous discontinuous or continuous solutions’ drawings in which the solute was assumed to sink to the bottom of the beaker. These 42
drawings were explained that they are related to weight ‘the water dissolves it and it then falls to the bottom because of its weight’ or to density ‘since the salt is denser than the water it falls to the bottom and is deposited there until the solution is stirred again’. These kinds of drawings are also related to children’s everyday experiences. From these drawings two criteria came out: the homogeneity of a solution and the discontinuity of the solute in a solution. Three quarters of students believed in the homogeneity of the solutions. On the other hand the continuous state of the solute was accepted by more than half of the students. In Longden’s et al. (1991) study, two kinds of drawings were given to from eleven to fourteen years old children and they were asked to draw a picture about dissolving. One drawing was to explore the general explanation about dissolving and the second one was to explore the particulate explanation about dissolving. However the particulate interpretations of the students are more accurate than everyday interpretations for both eleven to twelve years old group and thirteen to fourteen years old group children.
3.13 Conclusion In spite of the use of dissolving for many centuries the explanation of this concept is still difficult. Dissolving is valuable to study in science education because it presents so many different issues in science. The issues that play important roles in dissolving are: the conservation of mass; the effects of the solute and the solvent on solubility; misconceptions; explanations of dissolving at particulate level;
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language; reversible change; the concepts of melting and dissolving; models which have been used to explain this phenomenon by scientists during history; drawings.
According to research findings there is an increase in the understanding of conservation of mass in dissolving for children from the age of 7 to 17. However some of findings show that the children consider only the change of solute rather than whole solution system in solution.
The solute is also considered as the significant factor in dissolving whereas the solvent is perceived as having passive role. To alter ideas of children it is important to explore these ideas and their origin. Research also informs us that the misconceptions of children do not always arise from the scientific knowledge and children do not always relate school knowledge to their everyday experiences. It also indicated that most of misconceptions were derived from their everyday experiences. The children were reluctant to use scientific words in their explanations.
Children gave ideas at two levels, one of them is particulate, microscopic, and the other visible, macroscopic. In general, children preferred to think of dissolving at the visible, macroscopic level. Dissolving was often perceived as same as melting.
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There were some findings in which children thought about the interaction between particles but these ideas were sometimes considered as misconceptions by researchers. Many researchers accepted the process of dissolving as a physical change as did the students.
Filtration and distillation were the common methods suggested for reversing dissolving.
In the history of science there is a similarity between the models suggested by scientists and the children’s ideas. The intermingling or simply mixing (dispersal) model was used by children. Solutions were seen as homogeneous (macroscopic) and discontinuous (microscopic) by some of children. However, the research findings emphasised the process of dissolving as a physical change. There are still some issues that need to be explained. In the solution system, changes should be correctly analysed and models that explain some exceptions in dissolving should be devised.
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CHAPTER 4
WORKING TOWARDS A FULL EXPLANATION OF DISSOLVING
4.1 Introduction Explaining dissolving is very important in understanding many of the occurrence in chemistry. However it is much more complex to explain dissolving than it is expected. This complexity led me to start searching the degree level chemistry textbooks. In the following sections, the definitions of solution, solute, solvent and dissolving will be discussed. The models that have been used in these books will be described, the process of dissolving and the solubility depending on changes in enthalpy and entropy will be explored.
4.2 The definitions of the solution, solute, solvent and dissolving A solution can be defined as a homogeneous mixture which consists of only one phase. In many solutions there are two substances. Conventionally, one of them is said to be the solute and the other one is the solvent. Often, it is very difficult to define which is the solute and which is the solvent. However, some chemistry books provide definitions for ‘solute’ and ‘solvent’. For example: “...to refer to the component present in larger quantity as the solvent”. (Hughes & Maloney, 1964, p.102) “The component which is present in the largest amount is commonly called the solvent and the other components solutes.” (Hägg, 1969, p.35) “If a solution is prepared from two substances of the same phase, the solvent is conventionally the substance present in the larger amount”. (Baum & Scaife, 1975, p.138) 46
“The substances used to specify the composition of a solution are known as components. One of the components, usually the one present in greatest quantity, is called the solvent; any other component is called a solute.” (Mahan, 1980, p.148) “The solute dissolves in the solvent. Usually, there is much more solvent than solute. In some cases, however, the substances are miscible, that is, soluble in each other in any proportion, so it is not very meaningful to call one the solute and the other the solvent. The simplest solution consists, in general, of a smaller amount of one substance, the solute, dissolved in a larger amount of another substance, the solvent.” (Silberberg, 1996, p.464 and p.114)
Table 4.1 The comparison of the definitions of the solute and solvent (based on their proportions)
SOLVENT
SOLUTE
a larger amount of another substance the largest amount greatest quantity
a smaller amount of one substance the other components any other component
These books define the solvent as a substance which is present in greater amount than the other substance(s) (solute) but this definition becomes inadequate for some solutions when their proportions (mass, volume or mole) are equal. In order to decide which substance is solute and which one is solvent, which quantity will be chosen, mass, volume or mole? For example: Mass: 18.0 g water and 18.0 g ethanol. mwater = methanol They have equal mass but Mole: 18.0 g/18.0 gmole-1 H2O = 1.0 mole H2O 18.0 g/46.0 gmole-1 C2H5OH = 0.39 mole C2H5OH
nwater > nethanol
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The amount of water is more than the amount of ethanol, so water is the solvent. Volume: d = m/V dwater = 1.0 gcm-3
mwater = 18.0 g
V= m/d
Vwater = 18.0 g/ 1.0 gcm-3 = 18.0 cm3 water. dethanol = 0.789 gcm-3
methanol = 18.0 g
Vethanol = 22.817 23 cm3 ethanol 18 cm3 water and 23 cm3 ethanol. The volume of ethanol is more than the volume of water, so ethanol is the solvent. It is therefore, very problematic to determine which of substances is solute and which one is solvent. None of the chemistry books discussed this point.
On the other hand, in some chemistry books the definitions of solute and solvent are tautological. For example: “A mixture formed by dissolving one substance in another is called a solution. The dissolved material is the solute and the substance in which it is dissolved is the solvent.” (O’Connor, 1974, p.138) “The components of a solution are referred to as solute and solvent. The solute is the substance that is dissolved; the solvent the substance that does the dissolving”. (Baum & Scaife, 1975, p. 138) “Solutions are homogeneous mixtures of two or more substances (the dissolved substance is called the solute, the dissolving substance is called the solvent.” (Garvie et al., 1979, p.15) “The substance dissolved is called the solute, and the liquid is referred to as the solvent.” (Gray et al., 1995, p.85)
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In some chemistry books water is the only solvent mentioned in these explanations. For example: “The substance in excess, water, is called the solvent, and the dissolved substances are the solutes.” (Atkins, 1989, p.14)
Some of books accept only liquids as solvents: “With solutions of solids or gases we refer to the liquid as the solvent and the other as the solute”. (Hughes & Maloney, 1964, p.102) “Whenever a solid dissolves in a liquid, the solid is called the solute and the liquid is referred to as the solvent”. (Greenstone & Harris, 1975, p.182) “Often a solid or a gas may be dissolved in a liquid. In this case the original liquid is termed the solvent, and the dissolved substance is the solute.” (Hairison et al., 1991, p.41)
Additionally, some books define the solvent as a determinant of the phase of the solution. “When a solution is prepared from two substances of different phases, the solvent is the substance that is of the same phase as the resulting solution, and the solute is the substance dissolved in the solvent”. (Baum & Scaife, 1975, p. 138) “The component that determines whether a solution is solid, liquid or gaseous is called the solvent; any other component is a solute.” (Radel et al., 1990, p.117)
Sharpe (1964) explained that the solvent is the component that dissolves the solute, and retains its own physical state after addition of the solute.
The definitions for the solute and solvent can be summarised in the Table 4.2:
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Table 4.2 The comparison of the definitions for the solute and solvent.
SOLVENT
The substance which does the dissolving is called the solvent. The dissolving substance is called the solvent. The liquid is referred to as the solvent. The substance in which it is dissolved is the solvent. The substance in excess, water, is called the solvent. A liquid. The component that determines whether a solution is solid, liquid or gaseous called the solvent. Solvent is the component that dissolves the solute, and retains its own physical state after addition of the solute. It is the substance that is of the same phase as the resulting solution.
SOLUTE
The substance which dissolves is called the solute. The dissolved substance is called the solute. The substance dissolved is called the solute. The dissolved material is the solute. A solid or a gas. Any other component is a solute.
In chemistry books generally, liquid-liquid and liquid-solid solutions are given as the range of solutions and water is accepted as the most common solvent. However in chemistry and everyday life there are many materials such as; olive oil that cannot dissolve in water.
Solutions are classified on the basis of their physical state (gaseous, liquid or solid). There are nine possible kinds of solutions. Some examples described by Sharpe (1964, p.75) and Holum (1975, p.106) are; Gas in gas: Air, all gases that do not react with one another, are soluble in each other. Liquid in gas: Water (vapour) in Nitrogen (N2), humid air. 50
Solid in gas: Iodine (vapour I2) in Nitrogen (N2), smoke. Gas in liquid: Oxygen (O2) in water, carbon dioxide(CO2) in water (carbonated beverages). Liquid in liquid: Ethanol (C2H5OH) in water (H2O), antifreeze (ethylene glycol) in water, acetic acid (vinegar-CH3COOH) in water. Solid in liquid: Salt (sodium chloride-NaCl) in water, sugar (glucose-C6H12O6) in water. Gas in solid: Hydrogen (H2) in palladium (Pd), oxygen (O2) in copper (Cu). Liquid in solid: Mercury (Hg) in Silver (Ag), benzene (C6H6) in rubber. Solid in solid: Alloys; copper (Cu) and zinc (Zn), carbon (C) and iron (Fe) (steel), copper (Cu) and nickel (Ni), sterling silver is the solution of copper in silver.
4.3 Models of Dissolving Some chemistry books use the words of disappearing, distributing, diffusing, mixing and melting to associate with dissolving; “When crystals of sugar are stirred with a sufficient quantity of water, the sugar disappears and a clear mixture of sugar in water is formed...In this solution the molecules of sugar are uniformly distributed among the molecules of water; i.e., the solution is a homogeneous mixture of sugar and water molecules. The molecules of sugar diffuse continuously through the water”(Nebergall et al. 1968, p.171).
4.3.1 Dispersal Model (Physical Change) Some chemistry books do not explain dissolving at all, simply describing the process and some of them explain dissolving by the “dispersal model (physical change)”. “In (a) we have portions of two species of molecules, A and B . In (b) and (c) we have two possible outcomes of putting them together; in one they remain apart whilst in
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the other they intermingle freely”(Hughes & Maloney, 1964, p.102)
a) Two substances,
b)may be immiscible,
c) or miscible “Gases mix readily in all proportions to form solutions. The tiny, discrete gas molecules have large distances between them and thus vast amounts of empty space available for additional molecules...The formation of solid solutions is relatively difficult. The tiny, discrete atoms, molecules, or ions in a solid are in direct contact. Free space for additional particles is minimal. The particles forming a solid have strong attractive forces between them. This inhibits mixing and results in distinct packing arrangements in the crystal. An incoming solute particle must be neither too large nor too small so that it does not disrupt the orderly packing arrangement of the solvent particles.”( Baum & Scaife, 1975, p.139). The ease of formation of liquid solutions varies considerably. A particular liquid solvent may dissolve one solute readily, another solute only with difficulty, and a third solute not at all. The tiny, discrete particles composing liquids are in close contact, but have more space for additional particles than do solids” (Baum & Scaife, 1975, p.140). “Millions years ago the water in the ocean contained less salt than it does today. Gradually, as the rain washed the salt and other minerals of the earth’s crust into the sea, the water became saltier. Since the particles of all substances are in constant motion, the minerals intermingled and became 52
thoroughly mixed the sea water...suppose we take a sample of sea water and add more salt to it. After gentle stirring, the salt “disappears” and the sample of water becomes homogeneous once more”. (Greenstone & Harris, 1975, p.180)
According to the dispersal model, chemical interaction between particles is not mentioned. Solute and solvent particles simply intermingle in this model (Oversby, 1997). In the dispersal model (figure 4.1.), the spaces between the particles are used for explaining solubility. The solvent particles have some spaces between each other. If solute is added into a solvent, the particles of solute will fit into these spaces. In this model insolubility is explained on the basis of the spaces being too small. Saturation occurs when the spaces are full. However sugar (glucose) has a big molecular structure and it can dissolve in water so the explanation becomes inadequate. Also this explanation cannot adequately explain the saturation. So, dispersal model is insufficient to explain dissolving.
Solvent particle Solute particle
Figure 4.1. Dispersal Model (Physical change)
On the other hand, dissolving of iodine in hexane and ethanol may give an opportunity to determine which is the most suitable model for the dissolving process. The vapour of iodine has a purple colour. If iodine is dissolved in hexane, the colour of solution 53
will be purple. According to the colour of the resulting solution it can be supposed that iodine and hexane molecules are simply intermingling. However, the dissolving of iodine in ethanol produces a bulk brown coloured solution. The change of colour and energy of solution gives an idea of distortion of electron arrangement of iodine molecules. Because ethanol has polar molecules, the electric field of ethanol molecules distorts the electron arrangement in the iodine molecules. The interaction between iodine and ethanol molecules cause this distortion that is called a chemical bond (Oversby, 1997).
4.3.2 Chemical Model The “Chemical Model” is an alternative model to explain dissolving. In this process some books use the rule of “like dissolves like”. “generally, polar solvents dissolve ionic and polar solutes, and nonpolar solvents dissolve non-polar solutes (like dissolves like). A polar solvent such as water is a good solvent for a polar gas like hydrogen chloride, a polar liquid like methanol, or an ionic solid like sodium chloride. The polar or ionic nature of these substances creates particularly strong dipole-dipole or ion-dipole solutesolvent interactions, and thus favours solubility”. (Baum & Scaife, 1975, p.143) “A useful guide to the possibility of dissolution is the axiom that ‘like dissolves like’. Thus ionic or very polar substances tend to be soluble in polar liquids such as water, whilst covalent organic molecules are usually soluble in organic solvents of low polarity”. (Buttle et al., 1981, p.98) “When a covalent compound dissolves in a solvent without ionisation, solubility is governed by the polarity of the solute and solvent, the rule being that like dissolves like. Highly polar compounds such as glucose and ethanamide dissolve in highly polar solvents such as water and methanol, but have only a low solubility in non-polar solvents such as hydrocarbons and tetrachloromethane. Conversely, non-polar solutes, such as naphthalene, are insoluble in water but readily soluble in hydrocarbon solvents...However, the molecules of a polar compound will attract other molecules of the same sort rather 54
than those of a non-polar substance; consequently there is little mixing and the solubility is low”. (Brockington et al., 1981, p.137) We use the word of “like” for compounds that have similar polarity. According to this rule, polar compounds can only be dissolved by polar compounds and similarly non-polar compounds can only dissolve in non-polar compounds. This comparison of the polarity sometimes provides a good prediction for dissolving. For instance tetrachloromethane (CCl4) has a non-polar molecule and tetrachloromethane does not dissolve in water. Iodine (I2) is non-polar molecule that can dissolve in tetrachloromethane (CCl4). Methanol (CH3OH) is a polar molecule that dissolves in water by hydrogen bonds.
Water has dipolar molecules and there are dipole-dipole attractions, specifically called hydrogen bonds between molecules. Because of the strong ion-dipole attractions formed, water dissolves many salts. Sodium chloride does not have polar molecules because it is an ionic compound. Between the ions (Na+ and Cl-) there are ionic bonds (Figure 4.2.), creating a three dimensional lattice.
Na+
Cl-
Figure 4.2. Three dimensional lattice of sodium chloride
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On the other hand hexane (C6H14) and heptane (C7H16) do not have polar molecules. There are weak dispersion (London) forces between the molecules. Therefore, salts are insoluble in hexane and heptane because the weak ion-induced dipole forces their ions could form with these non-polar solvents cannot substitute for attractions between ions in the solid.
Similarly, olive oil (C17H36O2) (figure 4.3.) has an ester group that is polar. Therefore, it is expected that olive oil should dissolve in water but it does not. Most of the water molecules are adjacent to non-polar parts of the olive oil molecule and only form weak dipole-induced dipole bonds. Only some water molecules interact strongly with the adjacent ester group. On the whole, few water molecules bond strongly to the olive oil molecule.
CH2O CHO
CH2O
O ║ C R O ║ C R O ║ C R
O║ C+ O.. H O ..
H
Figure 4.3. The molecular structure of oil molecule.
Olive oil dissolves in hexane, because the dispersion forces in one substitute well for the dispersion forces in the other (Silberberg, 1996, p.466).
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It is not straightforward to measure polarity. We can measure the polarity through measurement of the refractive index or the dielectric constant. According to Chiswell and James (1969, p.139) heteronuclear diatomic molecules consist of different atoms which have different electronegativity are polar. The charge is distributed in the molecule which has electrically positive and negative ends. It is possible to find the polarisation of charge in a diatomic molecule if there are atoms that have different electronegativity and in a non-symmetrical molecule that has no centre of symmetry. So, it can be said that the polarity of bonds and the shape of the molecule determines the polarity of molecule. The dipole moment () is a measure of polarity. The vector product of the partial charges (in coulombs, C) and the distance between them (in meters, m) is measured in debye (D) units (1 D3.34x10-30 Cm). (Silberberg, 1996, p.381)
In this case water has polar molecules. Each molecule has an angular structure and the oxygen end of molecule is electrically negative, the hydrogen ends of molecule are electrically positive. With the angular structure, water possesses polar character. If the structure was linear the water molecule would have non-polar character. ( D)
..
.. (
OH +
D)
H+
Figure 4.4. The structure of the water molecule
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In a contrast tetrachloromethane molecules have polar bonds but it has non-polar molecules overall because of its symmetrical molecular structure. ( D)
Cl-
C+ -
(0 D) Cl-
Cl Cl-
Figure 4.5. The structure of the tetrachloromethane molecule
The carbon dioxide molecule (CO2) has atoms that have a large electronegativity difference and the bonds are quite polar but the structure of the CO2 molecule is linear, so, the net dipole moment is zero.
+ - O
=C= O -
( D)
Figure 4.6. The structure of the carbon dioxide molecule
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In liquids a measure of the polarity of the molecules given by the dielectric constant. Table 4.3 Dielectric Constants of some common solvents at temperatures (K) given. Source: (Chiswell & James, 1969, p.139-140)
Liquid ammonia Liquid hydrogen fluoride Water Methanol Benzene Carbon tetrachloride Chloroform Glacial acetic acid
22 83.6 81.1 31.2 0 0 5.0 9.7
240 273 291 293 293 293 293 291
It can be seen from the Table 4.3 that water has a relatively high dielectric constant. This means that water is a good solvent because it has polar molecules. However, water is not a suitable solvent for every solute and water is not a very good solvent for non-polar solutes.
Some of chemistry books also use the word “like” for similar forces. We can say compounds whose particles bond with similar forces can dissolve each other. “...substances that exhibit similar intermolecular attractive forces tend to be soluble in one another. This observation is often stated very simply as ‘like dissolves like’. Non-polar substances are soluble in non-polar solvents, while polar or ionic compounds dissolve in polar solvents”. (Brady & Humiston, 1986, p.401) “Substances will dissolve in each other readily when the intermolecular forces in the solution are similar to those in the pure components” (Radel & Navidi, 1990, p.533)
In sodium chloride (NaCl) there are very strong forces between the ions. According to Brady & Humiston (1986, p.407) +766 kJ/mole energy is needed to break the bonds of sodium chloride between the ions.
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NaCl(s) Na+(g) + Cl-(g) U = +766 kJ/mole H Hydration = -770 kJ/mole The calculated H solution = -4 kJ/mole and The measured H solution = +3.9 kJ/mole
Na(s)
+
1/2 Cl(g)
HatNa Na (g)
Hf
NaCl(s)
1/2 HatCl Cl(g)
U Lattice Energy
1st I.P - eNa+(g )
Cl-(g)
U = Lattice Energy Hat = Heat of atomisation I.P
= ionisation potential
E.A = electron affinity Hf = HatNa + 1/2 HatCl + 1st. I.P.Na - E.ACl - U Figure 4.7. Energy diagram for the formation of sodium chloride. Source: (Millen, 1969, p.74)
The boiling point of sodium chloride is 1740 K and the melting point is 1074 K (Brockington et al., 1981, p.132). In liquid water the forces are relatively strong.
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H2O(l) H2O (g)
H= +40.7 kJ/mole
40.7 kJ/mole energy is needed to break the bonds in water (= +40.7 kJ/mole) (Silberberg, 1996, p.230). The forces between the particles in water and in sodium chloride are not of the same magnitude. The comparison of the forces of bonds before and after dissolving gives a limited explanation of solubility.
The models for dissolving that have been explained can be summed into two groups. Dispersal model (Physical change) Interaction - chemical model(Chemical change) In dispersal model it is supposed that there is no interaction between solute and solvent, they are just spreading out, intermingling or mixing. This idea is related to the change of entropy.
In interaction (chemical) model there is an interaction between solute and solvent particles. The bonds of solute and solvent are broken and new bonds are formed. The process of bond breaking and formation is related to the energy change of the system. Small energy changes magnify the effect. This can be derived from; G = -RT lnK which means that small amount of energy supplied from the increase of the entropy of surroundings, magnify the solubility effect. Small change of system’s enthalpy gives massive changes in solubility. So, we can say that the driving force of dissolving is the change of system’s entropy while the modifying force is the enthalpy change.
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4.4 Chemical Change In chemical change while some bonds are broken, new bonds are formed. There are two kinds of chemical changes: 1- Reversible Chemical Change 2- Irreversible Chemical Change
In the dissolving process chemical changes often occur. Dissolving can be a “Reversible Chemical Change” in which some bonds are broken and new bonds are formed. There are some processes including Evaporation, Crystallisation, Extraction or Chromatography which can reverse the chemical changes in solutions.
Distillation is based on volatility, the tendency of a substance to vaporise or become gas. For example: The dissolving of sodium chloride in water is a reversible chemical change. By evaporation of water, we can get salt back again. By evaporating existing bonds between water and ions are broken and then the ionic bonds between Na+ and Cl- ions form their ionic three dimensional lattice.
Crystallisation is based on the differences in solubility of the components of a solution. Many of the substances are more soluble in the solvent at high temperatures than at the low temperatures. For example white sugar is refined from the raw sugar by a crystallisation process utilising this property.
Extraction is based on the differences in solubility of the components of a solution. If the solution cannot be separated by distillation and crystallisation this process may be 62
used. The solute that is wanted to extract, is dissolved in another solvent but the solvents must be immiscible. After that, the two immiscible solvents form two different phases because of their differing densities. Thus can be separated the solute and solvent. By crystallisation, distillation or chromatography it is possible to separate one solute from a second solute.
Chromatography is also based on the difference in solubility. A solution, which cannot be separated by other methods, may be separated by chromatography. A solution or mixture is dissolved in a gas or a liquid and the components are separated over a solid (or viscous liquid) in a column chromatography at different speeds. According to their solubilities, components move over the solid (or viscous liquid) at different rates. In chromatography process some suitable solvents must be used because the polarities of all components must be considered. If the solvent used is very polar, the solvent dissolves all of the substances that are in the column and we can get all of the solution back but the aim is to separate all of the different substances in a pure state. So, different proportion of solvent mixtures are controlled in the thin layer chromatography and then by using the best proportion of a solvent mixture, we can get all the pure solute back. After this process distillation may be used to separate solute from the solvent.
4.5 Solubility Solubility is a measure of the amount of solute that dissolves in a given amount of solvent. The idea of solubility is explained in different books in some different ways. According to Brown & Rogers (1980, p.222-223) a substance is when more than 1.0g
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of that substance dissolves in 1.0 litre of solvent and insoluble substance is when less than 1.0 g substance dissolves in 1.0 litre of solvent. They indicated that ‘miscibility’ is the same meaning as ‘solubility’ but it is usually used for both the solute and the solvent are liquids, and ‘immiscibility’ is the same meaning with ‘insolubility’.
Brown and Rogers (op. cit.) also pointed out that the solubility of the substances is different at different temperatures. For example; 35.7 g sodium chloride dissolves in 100 cm3 of water at 20 oC, in this case the solubility of sodium chloride is 35.7 g/100cm3 water and it was called as a moderately soluble salt. Sodium nitrate’s solubility is 92.1 g/100cm3 water and it was called as a very soluble salt. On the other hand in 100cm3 water only 2.3 x 10-4 g barium sulphate dissolves, so, it was called as an insoluble salt. It seems likely that a definition of absolute solubility is too problematic. The solubility of the substances is compared with each other and the solubility and insolubility of the substances is determined.
Similarly, Baum and Scaife (1975, p.140) explain solubility in two groups; qualitatively and quantitatively. The solubility of different solutes in water has been expressed qualitatively: calcium chloride (CaCl2) is very soluble, mercury (II) chloride (HgCl2) is moderately soluble, lead (II) chloride (PbCl2) is only slightly soluble, silver chloride (AgCl) is relatively insoluble. At 20oC the solubilities of same solutes have been expressed quantitatively; 74.5 grams calcium chloride (CaCl2) dissolves in per 100 ml. water, 64
6.9 grams mercury (II) chloride (HgCl2) dissolves in per 100 ml. water, 0.99 gram lead (II) chloride (PbCl2) dissolves in per 100 ml. water, 2.9 x 10-4 gram silver chloride (AgCl) dissolves in per 100 ml. water. In Baum & Scaife’s opinion the solubility depends on the bonding interactions that are broken and formed during the dissolving process. In this process there are two kinds of interactions that are bonding interactions in the solute that is called solute-solute interactions and bonding interactions in the solvent that is called solvent-solvent interactions. The bonding interactions between solute and solvent that is called solventsolute interactions. Baum & Scaife (op. cit.) pointed out “a substance dissolves in a given solvent if the solute-solvent interactions are great enough to overcome the solute-solute and solvent-solvent interactions. The substance does not dissolve if the solute-solvent interactions are small compared to the solute-solute and solvent-solvent interactions” (Baum & Scaife, op. cit., p.141-142). Methanol, ethanol, hydrogen chloride and sodium chloride dissolve in water, iodine dissolves in carbon tetrachloride, because the interactions of solute-solvent compensate for the interactions of solute-solute and solventsolvent interactions. However, silver chloride does not dissolve in water and water does not dissolve in carbon tetrachloride. The interactions of solute-solvent are not strong enough to overcome the solute-solute and solvent-solvent interactions. Similarly, Brady & Humiston (1986, p.400) stressed that “In order for the solute particles to enter into a solution, the solute-solvent forces of attraction must be sufficient to overcome the attractive forces that hold the solid together”. In molecular crystals there are relatively weak bonds-called dipole-dipole or London forces that can dissolve in nonpolar
solvents
but
cannot
dissolve
in
polar
solvents
because
in
polar
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solvents there are very strong bonds. While non-polar iodine dissolves in carbon tetrachloride, it hardly dissolves in water. However, sodium chloride being an ionic substance, dissolves in water that is polar but cannot dissolve in some other polar solvents such as: methanol and ethanol. In the case in which the solvent is water, the water molecules surround opposite charged ions with opposite ends. This process is called “hydration”. In Figure 4.8. the ionic substance is attracted by the dipole molecules in order to break the bonds between ions. The anions are surrounded by the positive ends of the dipole molecule and the cations are surrounded by the negative ends of the dipole molecules. There are electrostatic forces between the ions and the dipole molecules. Therefore, hydration neutralises the ions’ charges and prevents opposite charged ions from the attractions of each other. In Brady & Humiston’s opinions the solvent acts as an insulator of ions from each other.
Figure 4.8. Hydration of ions in solution. Source: (Brady & Humiston op. cit., p.401)
According to Brady & Humiston (op. cit., p.401) ionic solutes cannot dissolve in nonpolar solvents because non-polar solvents “can neither tear an ionic lattice apart nor do they offer any shielding for the ions. In a non-polar solvent, ions quickly congregate and separate from the solution as the solid”.
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4.6 The Dissolving process for Sodium Chloride and Glucose in water Sodium chloride has a three dimensional lattice of positive (Na+) and negative (Cl-) ions held together by strong electrostatic forces called ionic bonds. On the other hand sodium chloride being a solid that has a regular lattice structure, has lower entropy than liquids and gases. Na+ Cl-
Figure 4.9. The structure of sodium chloride
Water (H2O) has polar molecules and the molecules have polar covalent bonds. The oxygen end of each O-H bond acts as a slightly negative pole (represented as -) and the hydrogen end acts as a slightly positive pole (represented as +).
..
.. O
H
H
Figure 4.10. The structure of water molecule
In water the more positive end of one water molecule attracts the more negative end of another water molecule strongly. This attraction is repeated throughout the solid and liquid. These dipole-dipole forces are called hydrogen bonds (figure 4.11.).
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..
H O
..
H+
..
H O-
Hydrogen bond ..
H
Figure 4.11. Hydrogen bond in water
In contrast to sodium chloride, water has higher entropy, because the molecules in liquids and gases are arranged more randomly than in solids. In spontaneous processes there is a tendency for arrangements to become more disordered.
In the dissolving process, by the interaction of solvent molecules, the bonds of the solute molecules are broken and solute molecules dissolve as an ionic or as a molecular material. For example: sodium chloride has ionic bonding. By the interaction of water molecules (sodium ions and chloride ions attract the water molecules) the ionic bonds in sodium chloride between sodium (Na+) and chloride (Cl-) ions are broken, Sodium and chloride ions are surrounded by water molecules and bond to them. So sodium chloride dissolves as ions, hydrated by water molecules, and becomes more disordered compared with the solid state of sodium chloride.
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Cl-- ion
Na+ ion
Water molecule
Figure 4.12. Dissolving of sodium chloride in water
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On the other hand glucose (C6H12O6 - sugar) (figure 4.13.) is a molecular compound and there are covalent bonds between C-C and C-O atoms. Also there are hydrogen bonds between glucose molecules in the solid. In the dissolving process the intermolecular hydrogen bonds are broken but intramolecular covalent bonds are not. So glucose dissolves in water as molecules, bonded by hydrogen bonds to water molecules.
H C
CH2OH
HO
C C
HO
H
H
O C
H
OH
HO
C H
Figure 4.13. The structure of glucose molecule
H C
CH2OH
HO
C C
HO
H
H
O C
H
O
HO
H +
C
Hydrogen bond
H
..
.. O
H
H
Figure 4.14. Dissolving of glucose molecule in water
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After breaking the bonds of solute molecule some new bonds are formed between solute and solvent molecules. A homogeneous solution is formed and the system becomes more disordered, so, the entropy of the system increases.
4.7 Energy and Entropy In the dissolving process there are two types of energy change. One of them is in terms of the change in enthalpy and the other one is the change in entropy of the system. Entropy and Enthalpy of the system are both important factors in determining solubility. The changes in enthalpy and entropy together give a good explanation for solubility or insolubility. In the following paragraphs the effects of the entropy (S) and the enthalpy (H) in solubility will be explained.
In the system all the substances have a tendency to reach the lowest energy and the greatest entropy. Entropy is a measure of a system’s disorder. When the system becomes more randomised, the entropy increases. Naturally, all systems have a tendency to become disordered (increase their entropy). So, the change of the entropy of the system is almost always positive. The solids have the lowest entropy because of their regular lattice structure on the other hand liquids have higher entropy than the solids. Gases are have greater entropy than solids and the liquids because they are distributed randomly.
On the other hand, a solution's entropy is always greater than the pure solute and solvent. The entropy change of the process is shown by S and the enthalpy change of
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the process is shown by H. The Gibbs free energy is denoted by G. The change of the free energy is given by: G H - TS (for constant temperature) The dispersion of the particles in each other and the relative forces between the solutesolute, solvent -solvent and the solute -solvent particles are both contribute to the dissolving process. Naturally, the free energy of a system has a tendency to decrease. When a solute is dissolving in a solvent, the entropy usually increases. Sometimes, especially when gases are dissolving in a liquid, their entropy decreases. So, these solutions are more ordered or less disordered than their pure solute and solvent. H is expected to be negative because systems have a tendency to have lower energy. On the other hand TS is positive, it means the system is more disordered. If a solute dissolves in a solvent spontaneously, G must be negative.
The changes in the system are also affected by the surroundings. The changes surroundings determine whether a process can happen spontaneously or whether it needs an input of energy. In the dissolving process the surroundings have an important role. According to Burton et al. (1994, p.60) “...the entropy of the surroundings increases by a quantity equal to the energy they gain divided by the temperature”. The gained energy of surroundings are equal energy that is lost by the chemical system, so it is shown by -H. The entropy change of surroundings is; S surroundings = -H / T The total entropy change is;
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S Total = S System + S surroundings The sign of the total entropy change gives an information about the process. S
Total
> 0 is known as the second law of thermodynamics and in the spontaneous
changes the entropy increases.
It is possible to understand why the sodium chloride dissolves in water but calcium carbonate does not. It is also possible to find under the which conditions, such as: temperature, some substances can dissolve. In the case of the system needing a small amount of energy to break the intermolecular bonds between the particles, the surroundings compensate this need of energy with an increase of entropy, therefore the substance can dissolve. If the need for energy is very large the substance cannot dissolve in spite of the increase of the entropy.
Also there is a magnification effect that can be denoted by: G = -RT lnK Many times it is very problematic to explain the solubility of some substances however small energy changes arises large differences. According to Oversby (1997); “Dissolving is seen to consist of two aspects: an overall enthalpy change explains change as a balance between the forces holding the separate solute and solvent together, and those holding the solute to the solvent; an overall entropy change accommodating the generally ordered state of most solid solutes and the generally disordered state of the dispersed solution. Taken together, these are combined to give the free energy change for the solution as an indicator of the degree of dissolution in any particular case”.
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This explanation can be summarised in this formula: G/T = -S + H/T G/T = Overall entropy change S = The entropy change after mix the solute and solvent in each other H/T = The entropy change of surroundings arising from enthalpy of mixing It is possible to analyse the solubility in the Table 4.4: Table 4.4 The change of solubility depends on the H/T
H/T is POSITIVE insoluble
dissolves a bit
H/T is ZERO (0) or NEGATIVE 0
dissolve a lot
decreasing solubility
perfectly soluble
increasing solubility
Generally in dissolving process the interparticle forces are small or there is a balance of in both solute and solvent, so, the entropy change of surroundings is nearly zero. However, the silicon dioxide does not dissolve in water because the bonds are very strong in the silicon dioxide than the bonds of water. This is the reason that the H/T is highly positive which means a decrease of the entropy of surroundings.
In the light of the above explanations, the solubility depends on the comparison of the entropy and the enthalpy changes of the system. For dissolving one substance in other substance the system’s enthalpy and entropy must produce an exoergic (negative G) process.
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As has mentioned above, in the dissolving process the intermolecular bonds of solute molecules and the intramolecular bonds of solvent molecules are broken and bonds between solute and solvent particles form and the energy of system changes. Because of the breaking of different bonds in solute and solvent, the solute changes in solution such as, the dissolving of sodium chloride to separate into ions, on the other hand, there is no difference in solvents due to the breaking of the intramolecular bonds in water. This can only be applied to interaction (chemical) model.
4.7.1 Hydration Energy For breaking a bond, there is always a need for energy input. On the other hand, for forming of a bond, energy is always released. If we take water as a solvent and an ionic substance as a solute, in a solution the ions of solute are surrounded by water molecules, and the ions are hydrated by water.
H H H
H
O
O
H
H
M+
H O H
O
H
O O H
H
O
H Y-
H
H
H
O H O
H
H
O H
H
Figure 4.15. Diagrammatic representation of hydration of ions. Source: (Kneen et al., 1985, p. 200)
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Generally all ions are hydrated in a dilute solution. Some ions are hydrated by more than one water molecule.
For example sulphate(VI) (SO42-) ion is hydrated by four water molecules:
H O
H H
O
O
H
O
O
H
O
2-
S O
H
O
H
H
Figure 4.16. The hydrated sulphate molecule. Source: (Mortimer, 1988, p.355).
The beryllium ion (Be2+) is hydrated by four water molecules:
H
H
H
2+
O
O
H
O
O
H
H
H
Be H
Figure 4.17. The hydrated beryllium ion. Source: (Mortimer, 1988, p.355).
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The aluminium ion (Al+3) is hydrated by six water molecules: OH2 H2O
OH2 Al+3
H2O
OH2 OH2
Figure 4.18. The hydrated aluminium ion. Source: (Brady & Humiston, 1986, p.401).
or, Hydrogen chloride (HCl) gas can dissolve in water;
H2 O
H+
Cl-
Figure 4.19. The dissolving of hydrogen chloride in water. Source: (Baum & Scaife, 1975, p.142).
It should be stressed that the numbers of hydrated water molecules are only average numbers. In the solution the other water molecules have no direct bonds with the ions, but form hydrogen bonds with water molecules that bond with ions.
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Why are there a strong attractions between ions and water molecules? The ions that have the greater charge, attract water molecules more strongly. The small ions that have greater charge density attract water molecules strongly.
BeCl(s) + 4 H2O Be(H2O)42+(aq) + 2 Cl-(aq) If we suppose that the hydrated ions are formed from gaseous ions at this step, the energy is released and this energy is called as Hydration Energy (H). For example: K+(g) + Cl-(g) K+(aq) + Cl- (aq)
H Hydration = -686 kJ/mole
4.7.2 Dissolving Enthalpy (Enthalpy of solution) The enthalpy change of dissolving is the change in the enthalpy during the dissolving process. It is the enthalpy change of the infinitely dilute solution. For example: KCl (s) K+ (g) + Cl-(g) K+ (g) + Cl-(g) K+ (aq) + Cl-(aq)
H Lattice = +690 kJ/mole H Hydration = - 686 kJ/mole
the total reaction is an endothermic reaction: KCl (s) K+ (aq) + Cl-(aq)
Calculated H Solution = + 4 kJ/mole Measured H Solution = + 17 kJ/mole
Brady & Humiston (1986, p.407) [Books differ slightly in the values they calculate but they all produce small endothermic values].
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K+(g) + Cl-(g)
H hydration = - 686 kJ/mole
Enthalpy, H
H Lattice = +690 kJ/mole K+(aq) + Cl-(aq)
(measured) H solution = +17 kJ/mole KCl(s) Figure 4.20. The change of enthalpy in the dissolving process of potassium chloride.
When we divide the dissolving process into hypothetical steps we could determine/ inspect the changes in the system’s enthalpy. At the first step the intermolecular bonds of solute molecules are broken by enthalpy. SOLUTE(Aggregated) + HEAT SOLUTE (the intermolecular bonds broken)
H Solute > 0
Secondly, the intermolecular bonds between the solvent molecules are broken by enthalpy. SOLVENT (Aggregated) + HEAT SOLVENT (the intermolecular bonds broken) H
Solvent
>0
Finally, new bonds between solute and solvent occur. This process must be exothermic. SOLUTE + SOLVENT SOLUTION + HEAT Particles Particles H
Solution
H Mixing < 0
is the total enthalpy change in solution. It can be found by combining these
three individual enthalpy changes.
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H Solution = H Solute + H Solvent + H Mixing If H Solvent + H Solute is smaller than H Mixing, H Solution will be exothermic. Therefore, the solution will become warmer. If H
Solvent
+ H
Solute
is bigger than H
Mixing,
H
Solution
will be endothermic and the
solution will become cooler. However, most dissolving processes discussed in schools have H Mixing close to zero. In the case of H
Solvent
+ H
Solute
is greater than H
Mixing,
however, the solute may
dissolve in that solvent, because entropy effect can overcome the small differences of enthalpy.
If we think of dissolving of sodium chloride in water, we could understand why the dissolving process of sodium chloride is slightly endothermic. NaCl(s) Na+ (g) + Cl-(g)
H Lattice = +766 kJ/mole H Hydration = -770 kJ/mole
Na+ (g) + Cl-(g) Na+ (aq) + Cl-(aq)
H Solution = + 3 kJ/mole
(Brady & Humiston, 1986, p.407)
The hydration enthalpy depends on the concentration of the solution. It can be accepted as an indicator of the level of the attractions between ions and solvent molecules in the solution. If the hydration enthalpy is large, it means that more energy is released and there is a strong hydration between the ions and the water molecules.
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Na+(g) + Cl-(g)
H Lattice = +766 kJ/mole
Enthalpy, H
H Hydration= -770 kJ/mole
Na+(aq)+ Cl-(aq) (measured) H Solution = + 3.9 kJ/mole NaCl(S)
Figure 4.21. The change of enthalpy in the dissolving process of sodium chloride.
If, instead of water, other solvents are used, the term ‘solvation enthalpy’ is used in place of hydration enthalpy. On the other hand the attractions between molecular materials are not as strong as in the ionic crystals, therefore the solvation enthalpies of molecular crystals are smaller.
4.8 Conclusion It has been explained that in chemistry textbooks there are many different kinds of definitions of the solute, the solvent and the solution. The definition of the solute and the solvent based on their proportions is not used in every case in chemistry. The tautological definitions cannot provide an understanding of these two concepts. In the books, water has been accepted as solvent but cases in which water is insoluble are not considered.
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The other point was that the authors only used solid dissolved in liquid solutions in their explanations. They should think about other kinds of solution.
It has also been determined that the authors in those books which have been analysed used various words in explaining dissolving without attention to the meaning of the words that they have been used. However some of the words may lead students to have some misconceptions.
All of the books used the rule of ‘like dissolves like’ but in the same books even in the same chapter there were conflicting explanations in which some of the explanations depended on similar polarity yet on the other hand, some of them depended on similar forces. The solubility of olive oil was the crucial in this research because, according to books, the substances that have similar polarity should dissolve in each other. However, olive oil having an ester group that is polar, has similar polarity as ethanol. Yet olive oil cannot dissolve in water but ethanol can dissolve. Explanations based on similar polarity and similar forces were insufficient for explaining solubility.
The model that is the most commonly used in explaining dissolving was the dispersal physical model and generally many of them did not mention chemical change in a solution. However the dispersal model only provides a good explanation in some cases. There is a need to explain dissolving using both the dispersal and the chemical model.
There was the important thing that has been rarely reflected in the books by surveyed is that the energy and entropy are crucial factors on dissolving. In the dissolving process, 82
the solubility of the substances depend on the factors of entropy and the energy. The contributions of the overall enthalpy and overall entropy change are important in dissolving process. The change of the solubility that depends on the /T should be explained in textbooks.
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CHAPTER 5
EXPERTS’ EXPRESSED EXPLANATIONS OF DISSOLVING
5.1 Introduction In this study, three graduate chemists and one graduate physicist were interviewed and eight questions were asked. The interviews lasted from twenty minutes to half an hour. The interviews were audio-typed and as soon as possible the interviews were transcribed. The four physical scientists are lecturers in science education in their respective universities.
A comparative study of the transcripts was characterised into fourteen different items. These items can be classified into five groups these are: 1. Ideas about dissolving in general Dissolving Ionic compounds Dissolving Covalent compounds Generalised Dissolving 2. Ideas about the constituents of a solution Solute Solvent 3. Ideas about dissolving and melting Dissolving-Melting Words associated with dissolving Words associated with melting 84
4. Ideas about the process of dissolving Chemical Change Physical Change Reversible changes Conservation of matter 5. Examples and Drawings
5.2 Ideas about dissolving in general Three of the four of experts explained dissolving by the chemical model, and they called it is chemical because the bonds of substances are broken and new bonds between the solute and the solvent are formed. However, only one of them explained dissolving by the dispersal model that “The substances just intermingle”. The molecular idea about dissolving can occur with both ionic and covalent compounds. In Lecturer A’s words: “If you give ionic substances and if you put in a water and mix you can say you have chemical process because we have bonds are broken and new pieces you break the structure of the ionic substances. If you think about substances they are not ionic so cannot the process will be a little different because there are not bonds been broken so we have a mixture, things and molecules being separated and we can think in these two points of view.”
So, there is an important view that the chemical change in dissolving occurs only between ionic substances and the physical change in dissolving occurs only between covalent compounds. Lecturer C believes that the process of dissolving is same in both ionic and covalent compounds:
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“If it is molecular solid are broken and at the same time bonds between molecules of solvent are broken and the solute molecules attach themselves in some way to the molecules of solvent electrostatically... in a solution one or usually one single ion or single molecule of solute surrounded by a sphere of solvent molecules normally held together but always held together electrostatically...” In the lecturer D’s opinion there was no idea about ionic compounds. There was a molecular idea: “The molecules that make up the solid are taken up into the liquid...the constituents of the solid molecules separate, must become loosely bond the constituents of the liquid molecules, so that the combination of that particular temperature remains the liquid...”
5.3 Ideas about the constituents of a solution Three of the four of experts defined solute and solvent similar way. Lecturer A says that: “The solute is a substance that has a small quantity, the solvent is a substance that has a big mole quantity...”
According to lecturer B the solute can be solid, liquid or gas and he said that it is difficult to identify solute and solvent in some cases. “If you are taking the solutions of solid, liquid or gas, the solvent is the liquid. If you are taking solution solid-solid that is more problematic. Because which is which?... Solute would be of a lower concentration when we have a mixture which has a variable composition. We have less of the solute than the solvent”.
Lecturer C brought an idea about the strength of the bonds in solute and solvent molecules: “...The solute is the minority substance the solvent is the majority substance in mole terms...A solvent have always quite weak internal bonding, the molecules of solvent very weakly held together we can
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easily break up. Whereas the solute often has very strong internal bonding but...the primary difference is the relative quantities”.
Lecturer D did not explained solute and solvent on the basis of their relative quantities. He explained that the solute is a solid and the solvent is a liquid that captures the solute.
It is obvious that even in experts’ opinion there is an uncertainty in defining the solute and solvent. However Lecturer C’s idea about solute and solvent concerning the bonds in them may be a good way to identify the solute and solvent in a solution.
5.4 Ideas about dissolving and melting All of the experts stressed that dissolving and melting are different concepts. Lecturer A related the word fire to melting. Lecturer B believes that for dissolving there is a need for two substances but for melting only one substance is enough. He explained the difference between dissolving and melting in terms of the energy changes: “Dissolving can be endothermic or exothermic but melting is always an endothermic change”.
Lecturer C reported that melting is only a phase change. On the other hand lecturer D identified dissolving and melting on the basis of physical and chemical change. In his words, dissolving and melting are explained in this way: “Melting is obviously related to phase, change of phase, latent energy, boiling, freezing,...those sort of words which indicates phase change. I think melting is a physical change...with dissolving...limited concept of in my mind but the words ion, sort of ionic...the word of a mobility, I think dissolving is a chemical change or chemical process”.
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He pointed that he does not want to call dissolving as a chemical change. In his opinion in a solution, substances and their identity do not change. So, he found it useful to call dissolving a chemical process rather than a chemical change.
5.5 Ideas about the process of dissolving In lecturer A’s opinion in dissolving process there is a physical and a chemical change. For ionic substances the process is chemical, for molecular substances the process is the physical. Lecturer B, C, and D said that dissolving process is chemical. Lecturer C reminded that “historically the process of dissolving was seen a physical change”. Lecturer D again pointed out that “It is a chemical change but not a chemical reaction”. All of the experts were in consensus about the idea of dissolving as a reversible change. Distillation-evaporation is recommended for getting the substances back. Also, lecturer C and D suggested the crystallisation. Lecturer D said that we may get back the substances back by electrolysis but “we need a further process to bring the substances together”. Three of the four experts referred to the conservation of matter when getting the substances back. Lecturer A said that: “I am not sure that some ions could be involved [with] water molecules and ions evaporated”. In lecturer B’s opinion “In universal (isolated system) nothing is lost. Energy may go. In an open system there could be something lost”. Lecturer C said that “by crystallisation you always lose a certain amount...some of these always retain in the solvent. You have to keep going a number of times in order to get back all the solute you put in”.
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5.6 Examples The examples of lecturer A from daily life were: “salt in cooking, making refreshments different substances dissolve, the soap dissolves in water, the substances in alcohol (in refreshments), air”. The examples of lecturer B were “sugar into the cup of tea, salt in water, nail varnish and nail varnish remover, using some petrol, paraffin, grease”. The example of lecturer C was only salt dissolves in water. No other examples were given. According to Lecturer D “salt into water, salt onto icy road and sugar in coffee” are the examples of dissolving.
5.7 Drawings The drawings of four experts were analysed and three of the four experts used the chemical model. Only one of them (lecturer D) in the contrast to his explanation about dissolving as a chemical process, drew a non-chemical (dispersal) model.
5.7.1 The drawings of the lecturer A
Figure 5.1. The dissolving of sodium chloride (salt) in water. Chemical model- particles -detailed.
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Figure 5.2. The dissolving of glucose (sugar) in water. Chemical model- particles- general.
5.7.2 The drawings of the lecturer B
Figure 5.3.a. The dissolving of sodium chloride in water (above picture). Chemical model-particles- detailed. Figure 5.3.b. The dissolving of glucose in water (below picture). Chemical model- particles- detailed.
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5.7.3 The drawing of Lecturer C
Figure 5.4. The dissolving of sodium chloride in water. Chemical model- particles -detailed.
5.7.4 The drawing of the lecturer D
Figure 5.5. The dissolving of sodium chloride in water. Non-chemical model- particles- intermingled.
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5.8 Conclusion The interviews of four physical scientists showed that the general explanations for dissolving used the chemical model. They mainly (3/4) mentioned bond breaking. As in the chemistry textbooks there were various definitions for the solute and solvent. The common definition given depended on the proportions of the solute and the solvent.
The experts differentiated dissolving and melting as different concepts having different processes. Dissolving was accepted as a reversible change by distillation, crystallisation. Except one of the experts, they used the conservation of matter in the dissolving process. The examples given were mainly solid dissolved in liquid solutions but there were also some examples that included gas-gas and liquid-liquid solutions.
In the drawings, they generally (3/4) used the chemical model. In this model the particles, atoms, molecules and bonds were detailed. The experts’ views also showed that they gave no more different explanations than the chemistry textbooks.
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CHAPTER 6
STUDENT PRIMARY TEACHERS’ VIEWS ON DISSOLVING
6.1 Introduction In this study student teachers of different speciality were asked eight open ended questions and were required to draw a picture of dissolving. In the following three sections the ideas of four fourth year science specialist students, nine first year primary science specialist student teachers, thirty-nine non-specialist student primary teachers will be considered. The drawings of the students and the models that have been used in drawings will be analysed.
6.2 SCIENCE SPECIALIST STUDENTS’ VIEWS In this study four students who are fourth year primary science specialists were asked eight open ended questions.
6.2.1 Ideas about dissolving in general Their views about dissolving in general can be described as a non-chemical model. According to one of them (S1) this concept of dissolving is given: “a substance is dispersed within and throughout another substance”. In the second student’s (S2) view of dissolving “can be either a chemical reaction or a natural effect of dispersion (entropy)” S3 used the particles-intermingled model and in his opinion “particles of solute suspencially in the liquid”. In S4’s opinion there is a conflicting idea that is “dissolving is a
physical
process
whereby
a
solvent
surrounds
a
solid
to
make
a 93
solution. If the process is ‘hydration’ water molecules physically surround the atoms of the solid. If the ‘hydrolysis’ occurs, the solvent actually reacts with the solid.” So in contrast to her belief of dissolving is a physical process, she comments that there is an interaction between solid and solvent. Table 6.1 The responses of science specialist students on dissolving Students
The responses of students
S1
A substance is dispersed within and throughout another substance.
S2
A solute is dispersed in a solvent this can be either a chemical reaction or a natural effect of dispersion (entropy).
S3
It is where you put a solid sort of particles of solid into a solution into a liquid. They suspencially in the liquid.
S4
It is a physical process whereby a solvent surrounds a solid to make a solution. If the process is ‘hydration’ water molecules physically surround the atoms of the solid. If ‘hydrolysis’ occurs, the solvent actually reacts with the solid.
6.2.2 Ideas about the constituents of a solution A solvent is seen as the ‘acceptor’ of the other substance and a solute is the ‘accepted substance’. The main idea is that only liquid is supposed to be a solvent and only solid is supposed to be a solute. Only one of them pointed out that solvent can be a gas but she did not mention other kinds of solute and solvents. There is a confusion between the concept of solute and solution in S4’s explanation that “solute is the product from solid and solvent and the solvent is the substance which actually does the dissolving”. S1 has no idea about solute.
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Table 6.2 The ideas of science specialist students about the solvent and the solute.
S1 S2
S3 S4
SOLVENT
SOLUTE
A solvent will allow a solute to dissolve unit. Water is a good solvent. Solvent is a substance into which a solute disperses. The solvent is the substance which ‘accepts’ the most of the other substance. Solvents are often liquids or gases. The solvent is the thing you put in the or you dissolving something into. i.e. solvent is tea. The solvent is the substance which actually does the dissolving.
no response The solute is the substance which is ‘accepted’. Solutes are often solids.
The solute what you put in. i.e. solute is sugar. The solute is the product from solid and solvent.
6.2.3 Ideas about dissolving and melting One of the four students responded that “dissolving is similar to melting” and energy is the only word that was associated with both dissolving and melting, however there was not any more explanation about how does energy associate with these two concepts.
Two of the four students stated that dissolving and melting are completely different concepts. In one of their opinion, while dissolving does not need heat, melting needs heat and dissolving can be different concentration, melting is always in same concentration. S4 differentiated the concept of dissolving and melting as “dissolving has two species Solvent + Solid Solution and melting is a physical change of state due to energy breaking bonds within a solid lattice, only one substance is present”. The numbers of species are enough for this student to state the difference between dissolving and melting.
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Table 6.3 The ideas of science specialist students about dissolving and melting.
S1 S2 S3 S4
DISSOLVING AND MELTING
WORDS ASSOCIATED WITH DISSOLVING
WORDS ASSOCIATED WITH MELTING
dissolving is similar to melting dissolving is not melting dissolving is completely different from melting dissolving is not the same as melting
Energy
energy
no response
no response
different concentration does not need heat
same concentration need heat
Two species (Solvent + SolidSolution)
Only one substance. Physical change of state due to energy breaking bonds within a solid lattice
6.2.4 Ideas about the process of dissolving One of the four students responded that dissolving is a chemically reversible and a physical change. The other three students believe that dissolving is a physically reversible change. In all of their answers we can get the substances back by crystallisation, evaporation, chromatography and boiling.
Table 6.4 The ideas of science specialist students about the process of dissolving and the conservation of matter in this process.
Chemical change
Physical Change
Reversible Change
Conservation of matter
S1
no response
Yes
Yes
S2
Yes
Yes
Yes
S3
no response
Yes
Yes
S4
No
Yes
Yes
Yes, by crystallisation Yes, by evaporation Yes, by boiling, but not always Yes, by distillation, evaporation and chromatography 96
6.2.5 Examples and drawings If we compare their examples and drawings about dissolving, one of them (S3) could not draw any picture. His examples were all solid-liquid solutions. The first student drew pictures using the non-chemical (dispersal) model in which the solute is particulate, the solvent is continuous and they just intermingle and the chemical model in which particles are detailed. Likewise with (S3) in all her examples the solute is a solid, and the solvent is a liquid. (S2) used both the chemical and the non-chemical model, (dispersal) model. In the chemical model particles of solute and solvent were drawn in a general way. In the nonchemical model the solvent and solute are particulate and they simply intermingle. The examples are only solid-liquid solutions. S4’s examples are also solid in liquid solutions. She drew a picture of the dissolving of sodium chloride in water by using chemical model in which the ions, bonds between atoms are drawn detailed. O (-) end of water molecules bond with sodium and chloride ions. Table 6.5 The examples given by science specialist students and the models that has been used in their drawings.
EXAMPLES
DRAWINGS
S1
Sugar in water, salt in water, ink in water, gold in platinum (gold ring), coffee in water, tea in water and jelly in water.
S2
Salt in water, detergent in water, and sugar in tea.
S3 S4
Sugar in tea, dirt in a puddle in water Sugar cube in hot cup of tea, bath crystals in bath of water
Chemical and non-chemical (dispersal) model Chemical model - particles - detailed Non-chemical model-solute is particle, solvent is continuous-intermingledNon-chemical (dispersal model) solute and solvent are particles intermingled and Chemical model, particles - general No drawing Chemical model Solute and solvent particles detailed
97
1. The drawing of the S1 1.a. Non-chemical model (Solute is particle, solvent is continuous -intermingled-) In this model the solute is represented by small dots and the solvent is continuous. The particles of the solute simply dispersed (intermingled).
Figure 6.1. Non-chemical model, Solute is particle, solvent is continuous - Intermingled
1.b. Chemical model, particles - detailed In this model the particles of solute (salt - sodium chloride) and solvent (water) are drawn detailed and as can be seen in the drawing below the different ends of water molecules surround the sodium and chloride ions.
Figure 6.2. Chemical model, particles - detailed
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2. The drawing of the S2 2.a. Non-chemical model, Solute and solvent are particles - intermingled However S2 pointed that there is an interaction, she drew a non-chemical model. There are particles of solute and solvent that is presented as black and white coloured balls. In the dissolving process they simply dispersed (intermingled) throughout the solution.
Figure 6.3. Non-chemical model, Solute and solvent are particles - Intermingled
2.b. Chemical Model, particles -generalIn this model the particles of solute and solvent are again represented by black and white dots. It is important that S2 drew the particles of solute and solvent firstly split up and then they recombine to form new substances.
Figure 6.4. Chemical model, Solute and solvent are particle, general
3. The drawing of the S3 He could not draw a picture of dissolving.
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4. The drawing of the S4 4.a. Chemical model, particles - detailed In S4’s drawing the ionic lattice and the bonds between atoms are drawn detailed. The water molecules bond with sodium and chloride ions but with the O (-) ends.
Figure 6.5. Chemical model, particles - detailed
6.2.6 Conclusion This study showed that four science specialist students used the non-chemical model in their explanations. The definitions of the solvent and the solute were different from the experts’ definitions. One of them defined the solvent as the acceptor of the solute. Also there was a misconception that one of the students commented that the solute is the product from the solid and solvent. Three of the four students knew that the dissolving and melting are different concepts. They were able to give the words that are associated with melting and dissolving.
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The process of dissolving was seen by three students as a physical reversible change by evaporation and crystallisation.
The examples were mainly solid- liquid solutions, only one of the students gave an example of the solid- solid solutions. Except for one of them, they drew pictures and used both chemical and non-chemical models in their drawings. This shows that they were not very clear in their views about dissolving.
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6.3 FIRST YEAR PRIMARY SCIENCE SPECIALIST STUDENT TEACHERS’ VIEWS In this study, nine first year ‘Primary Science Education’ students were asked eight questions. In this chapter the understandings of students about dissolving will be explored and compared with each other.
6.3.1 Ideas about dissolving in general In this study five of the nine students gave explanations at the particulate level and they mentioned the breaking of the bonds in a solution. We can classify their opinions into two groups. Firstly, the idea of physical change and secondly, the idea of chemical change.
Table 6.6 The ideas of first year primary science education students about dissolving
Physical Change
Chemical Change
S1- A soluble [solute] mixed in a solution [solvent],
the
solution
[solvent]
molecules push between the molecules of soluble [solute].
S2- An ionic substance e.g. NaCl(s) added to a solvent and changes to Na+(aq) and Cl-(aq). S5- Particles that make up the substance
S3- The solid is suspended in the water.
separate from each other into ions.
The solid molecules move in between
These are negatively and positively
the water molecules.
charged and are attracted to different
S4-
The
water
molecules
and
solid
molecules mix together to form a
ends of the water molecule. S6- The molecules are broken down, the
solvent [solution]. The solid molecules
bonds
get in between the water molecules.
reformed with the solvent.
S9- It is the complete absorption of a compound in a solvent.
are
broken.
They
can
be
S7- THIS ANSWER WAS GIVEN TWICE. S8- Atoms or molecules form bonds with solute molecules or atoms and the bonds in the solid break.
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If we analyse the responses of students who believe that dissolving is a physical change, the ideas are: mixing of the two substances in a solution, pushing, get in between the solvent molecules, or it is a complete absorption of a compound in a solvent. On the other hand using the idea of chemical change, two of the five students believe that it is a process in which the ionic substance separates or changes into ions. One of them gave a good explanation about the attraction of charged ions to different ends of water molecules. However they do not express any idea about non-ionic substances. The other three students responded that the atoms, molecules, or bonds are broken down and that they form the bonds with the solvent.
6.3.2 Ideas about the constituents of a solution SOLUTE AND SOLVENT: Table 6.7 The ideas of first year primary science education students about the solute and solvent
SOLVENT S1 S2 S3 S4 S5 S6 S7 S8 S9
Solvent is medium (usually fluid). Solvent is liquid in which the solidsolute is added. i.e. water-salt. Solvent is the liquid that contains the solid. Solvent is the liquid that contains the solid. Solvent is the substance in which the solute dissolves. Solvent what dissolves the substance (Solvent = solute- substance). Solvent what dissolves the substance. Solvent is the liquid in which the solute is dissolved. Solvent is water. Solvent is the medium in which the solute is to be dissolved (water).
SOLUTE S1
Solute is a substance.
S2
no response
S3
Solute is the substance that dissolves. Solute is what dissolves
S4 S5 S6 S7 S8 S9
Solute is the substance which dissolves. Solute what is formed when a substance dissolves. When a substance dissolves in solution. Solute is the substance in solution. Solute is ionic substance. Solute is the matter to be dissolved (sugar).
103
The ideas about solute and solvent are very restricted. The general idea is that the solute is a substance that dissolves in a solution. There is no more information except this idea. The students state that a solvent is a substance that dissolves the solute, is a medium, liquid, or fluid. Solute is seen as a solid and solvent is seen as a liquid. One of the nine students says that the solute is formed when a substance dissolves, the solvent dissolves the substance (Solvent = solute-substance). These statements show that students do not have any idea about other states of solute and solvent.
6.3.3 Ideas about dissolving and melting According to the students’ answers six of the nine students believe that dissolving and melting are not the same but there is no further explanation about their ideas. The words associated with dissolving are: ionic substance, liquid, bond breaking. Similarly, the words associated with melting are; liquid and bond breaking.
6.3.4 Ideas about the process of dissolving In the answers to the seventh question, six of the nine students state that dissolving is a chemical reversible change. On the other hand one of them does not express any idea. One of them says that it is a chemical reversible change but not always. Similarly one of the students responded that it is a chemical reversible change but only sometimes. These responses show that most of them believe that dissolving is a chemical reversible change but there is no more information. All of the students answer that it is possible to get the substances back, and the processes of getting the substances back are; evaporation, boiling, condensation, filtration, and crystallisation.
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6.3.5 Examples The general example for dissolving is salt or sugar in water, tea, or coffee. Also, there are other examples; bath salts in water; coffee granules in water; and washing powder in water.
6.3.6 Drawings Except for one, all of the first year primary science education students drew pictures about dissolving. As the drawings can be analysed in Table 6.8, six of the drawings were non-chemical model and four of the drawings were chemical model. It is important to note that some of the students used both the non-chemical and the chemical models in their drawings. Table 6.8 The models that has been used in drawings by first year primary science education students
MODELS Chemical model (Solute and solvent- detailed) Non-chemical (dispersal) model continuous - disappear Non-chemical (dispersal) model - intermingled Non-chemical model (dispersal) model solute is particle, solvent is continuous, - breaking bits
NUMBER OF PICTURES 4 pictures 3 pictures 2 pictures 1 picture
(Some students drew more than one pictures such as S3 and S8.)
1. S1’s drawing (S1) drew a non-chemical model and in this drawing black dots represent the salt particles and green dots represent the water molecules. In the dissolving process they just intermingle. 105
Figure 6.6. Non-chemical model, solute and solvent are particles - intermingled
2. S2’s drawing However S2 drew a detailed picture in which we can see the particles of water and salt. The resulting solution is the mixture of all of the substances.
Figure 6.7. Non-chemical model, solute and solvent are particles - intermingled
3. S3’s drawings In S3’s drawing in a beaker there is a solvent and solute but the solvent is continuous. After dissolving, the solute is breaking bits. In the other picture by using the chemical model s/he appears to wish to draw the dissolving of sugar molecules in water. In this chemical model the particles are detailed.
106
Figure 6.8. Non-chemical model, solute is particle, solvent is continuous - breaking bits (above) and Chemical model, particles - detailed (below).
4. S4’s drawing No drawing 5. S5’s drawing S5 drew the chemical model in which the particles of salt and water are detailed and different ends of water molecules are attached sodium and chloride ions.
Figure 6.9. Chemical model, particles - detailed 107
6. S6’s drawing The drawing of S6 is a non-chemical model, that the solvent is continuous and the solute is particulate. After dissolving, the solute particles disappear. However S6 mentions the breaking of the bonds but there is no detailed drawing that shows the bonds.
Figure 6.10. Non-chemical model, Solvent is continuous, solute is particle - disappear
7. S7’s drawing Likewise S6, S7 mentions about the breaking of the bonds but the drawing is a nonchemical model, the solute is particulate, the solvent is continuous and in the dissolving process the solute particles disappear.
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Figure 6.11. Non-chemical model. Solute is particle, solvent is continuous - disappear
8. S8’s drawing In this picture the lattice of sodium chloride and the attractions by water molecules are drawn detailed. S8’s drawing is a chemical model, the particles are detailed.
Figure 6.12. Chemical model, Particles - detailed
9. S9’s drawing Figure 6.13. was drawn using a non-chemical model, the solute is particulate and the solvent is continuous, and in dissolving process the solute disappears. On the other
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hand Figure 6.14. is a chemical model in which solute and solvent particles are detailed.
Figure 6.13. Non-chemical model, solute is particle, solvent is continuous - disappear
Figure 6.14. Chemical model, solute and solvent particles - detailed
6.3.7 Conclusion The study of nine first year primary science education students showed that there were two different views about dissolving. Four of nine students believed that dissolving is a physical change and five of nine students believed that it is a chemical change. The solvent generally was seen as a liquid and the solute was seen as a solid. They were not aware of the other states of the solute and the solvent. 66% of students knew the difference between the concepts of melting and dissolving but they could not make any explanation about it. 66% of students considered that dissolving is a chemical reversible 110
change by evaporation, boiling, condensation, filtration and crystallisation. However it is known that the method of filtration is not appropriate for separating the solute and the solvent from each other. The examples were solid dissolved in liquid solutions. 44% of the students drew pictures using chemical model and 56% of the students used nonchemical model in their drawings.
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6.4 NON-SPECIALIST STUDENT TEACHERS’ VIEWS In this study the same questions were asked to thirty-nine non-specialist students student teachers.
6.4.1 Ideas about dissolving in general The ideas about dissolving can be analysed into two groups; physical and chemical changes. Table 6.9 The ideas of non-specialist student teachers about dissolving based on the physical and chemical changes
PHYSICAL CHANGE
CHEMICAL CHANGE
S1- Disappearing in a liquid. S4- Particles from the solute split up and Solid form which turns to liquid. particles from the solvent split up and S2- Making a powder turn to liquid. bond together. S3- Solid breaking down and S22- Break down of a solute in a solvent being incorporated into liquid. to form a solution- can be chemical S5- A solid form disappears into another change or an energy spread. form. S6- Break down of substance in liquid. S7- Dissolving involves a solid combining with a liquid to make a different liquid. S8- A substance ‘disappears’-but is still in whatever it was dissolved in- you just cannot see it. S9- A substance like sugar disappears in hot water, its molecules are changed from solid to liquid, but remain in the liquid. S10- An item changing its matter, becoming invisible. S11- Solid into liquid form. S12- A substance joining with a liquid. S13- A solid /liquid or gas changes into another form. S14- There are two ideas: A substance is put into a liquid. The substance either combines with the liquid or the substance spreads out in the liquid. S15- A substance is absorbed within another. S16- Adding water. S17- Soluble things disappear in water. S18- A soluble substance is disappeared
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into a solution. S19- Disappearing in water. S20- When solids dissolve into liquid. S21- A solid becomes part of a liquid. S23- Solid forms into a liquid. S24- A solid’s molecules disperse in a liquid. S25- A solid changes into a liquid. S26- A solid substance breaking up to form part of a liquid. S27- A solid is placed in a liquid and it dissolves and becomes a liquid. S28- Solid breaking down and being incorporated in a liquid. S29- A solute with disperse into a solvent until saturation point. S30- A particle breaking up in a liquid. S31- When something mixes in completely with other substances. So, it appears to have disappeared. S32- Dissolving is where something solid is made into something liquid. S33- Solid disintegrates into the liquid. S34- It melts into the solution. S35- Solid becomes liquid. S36- Dissolving is when the substance disappears or melts in the liquid. S37- A solid changes in to a liquid. S38- A solid becomes part of a liquid. S39- When one solid item breaks down into a liquid until no longer visible.
As can be seen in the table, students have various explanations for dissolving. The majority of explanations (37/39) for dissolving depend on physical change. The words that mainly has been used for explaining dissolving are; Table 6.10 The ideas of the non-specialist student teachers about dissolving
The ideas about dissolving changes into a liquid disappearing joining with a liquid melts disperse into a solvent dissolve into a liquid absorbed within another mixing in completely
students (%) 52% 25% 7% 5% 5% 2% 2% 2% 113
5% 7%
5%
2%2% 2%
changes into a liquid disappearing joining with a liquid melts
25%
disperse into a solvent 52%
dissolve into a liquid absorbed within another mixing in completely
Figure 6.15. The ideas of the non-specialist student teachers about dissolving
Only two of the thirty-nine students have the idea of chemical change for dissolving and only one student has the better explanation that “particles from the solute and particles from the solvent split up and bond together". However it is understood that in dissolving there is no particle breaking but it is possible she means bond breaking. On the other hand the second student says that dissolving “can be a chemical change or energy spread” but no more information is provided.
The majority of students perceive dissolving as mixing, disappearing, disintegrating, melting, becoming invisible, changing into a liquid, or becoming a part of the liquid.
No explanation uses the particulate model. It seems likely that they are content with a simple physical description of dissolving.
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6.4.2 Ideas about the constituents of a solution In this section similar responses are summarised in Table 6.11. Twelve of the nonspecialist primary students gave no response about the solvent. Nine of students believe that liquid is a solvent and seven of the students defined solvent as a substance in which the solute dissolves rather than to indicate whether it is a solid, a liquid or a gas. Only two of the students had an opinion that the solid is a solvent but they gave no examples to support this idea. It is possible that these two students confused the concepts of the solute and the solvent. Four of the students gave different comments that were the solvent is a smell, not soluble, most quantity and what it does. However one of the students indicated that the solvent is present in most quantity but s/he did not explain whether it is mole, volume or mass. Table 6.11 The ideas of non-specialist student teachers about the solvent
The ideas of the SOLVENT liquid a substance in which the solute dissolves solution solid others no response
The number of ideas 9 7 5 2 4 12
Thirteen of the students gave no response about the solute. Eleven students commended that the solute is a substance which dissolves. On the other hand solid and liquid has been accepted as a solute but gas was not considered. Table 6.12 The ideas of non-specialist student teachers about the solute
The idea of the SOLUTE
The number of ideas
substance solid liquid soluble solution no response
11 5 4 3 3 13
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The answers for solute and solvent showed that many students have difficulty in differentiating solute, solvent and, even, solution. These answers demonstrate that many of these students know the solute as a solid or a substance dissolved and the solvent as a liquid. One of them confuses the concept of solute and solution. Two of them commented that a solute is a soluble substance. One of them responded that the solute is a hard substance. This study shows that other kinds of solutions such as, solid- solid, solid- gas, liquid- solid, liquid-gas, gas-gas, gas-liquid, and gas-solid are not recognised by nonspecialist primary students. There is a range of ideas about solid dissolves in liquid solutions and according to them only water is seen as the solvent.
6.4.3 Ideas about dissolving and melting This study shows that there are three groups of students; one of the group of students still perceives that dissolving and melting are similar concepts or the same concept. The answers are; Table 6.13 The ideas of non-specialist student teachers about dissolving and melting
The ideas about dissolving and
The number of students
melting dissolving is the same as melting
1
dissolving is like melting
1
dissolving is similar to melting
4
dissolving means melting
3
dissolving is different to melting
9
No response
21
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Table 6.14 The ideas of non-specialist student teachers about the words associated with dissolving and melting The number of WORDS ASSOCIATED WORDS ASSOCIATED students WITH DISSOLVING WITH MELTING liquid liquid 3 solvent solvent 1 solid solid 1 suspension suspension 1 disappearing no response 1 suspended no response 1 integrating-suspended integrating-suspended 1 heat heat 1 becomes a solution becomes a liquid throughout heat 1 stirring no response 1
NO RESPONSE
27
The second group of students believes that dissolving and melting are different concepts.
So, nine of the thirty-nine students believe that dissolving and melting are similar or the same concepts. Nine of the thirty-nine students believe that dissolving and melting are different concepts. Finally, twenty-one of the thirty-nine students do not propose any idea of the similarity or the differences between these concepts.
The words that are associated with dissolving and melting were not given by many of the students. Only twelve students gave an answer for this question. Heat, liquid, solvent, suspension, integrating-suspended and solid are seen as associated words with both dissolving and melting. Stirring was given as a word that was thought to indicate that to dissolve a substance needs stirring. However we know that stirring is not necessary for dissolving. One of the students made a comment that melting occurs through heat and for dissolving
no
word
was
given.
On
the
other
hand
another
student
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who could not make any comment about dissolving and melting answers that “one becomes a solution through addition of water, one becomes a liquid through heating”. One of students relates disappearing to dissolving and for melting there is no idea.
6.4.4 Ideas about the process of dissolving Only two of the thirty-nine students believe that dissolving is a chemical process and only five students believe that dissolving is not a chemical change, that it is a physical change but they did not express any idea about chemical change in their explanations. Ten students answered that dissolving is a physical change. This study showed that these students express these ideas about the process of dissolving: Table 6.15 The ideas of non-specialist student teachers about the process of dissolving Chemical change (YES) Chemical change (NO) No response
2 5 32
Physical change (YES) Physical change (NO) No response
10 no response 29
6.4.5 Reversible Change: Nineteen of the thirty-nine students answered that dissolving is a reversible change, eleven students answered that it is not reversible, one student responded that it is possible but only sometimes and eight students do not express any idea of what is a reversible change. Table 6.16 The ideas of non-specialist student teachers about the reversibility of dissolving THE IDEAS OF STUDENTS dissolving is reversible dissolving is not reversible sometimes NO IDEA
THE NUMBER OF STUDENTS 19 11 1 8
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Students responded that we can get back the solute and solvent back by various methods such as; distillation, freezing, boiling off, heating, evaporation, crystallisation, and drying out. The methods of distillation, boiling off, heating, drying out were evaluated as an evaporation. This can be analysed in Table 6.17:
Table 6.17 The ideas of non-specialist student teachers about the methods for retrieving of dissolved solutes number of students Answers for methods evaporation 27 crystallisation 2 freezing 2 we can not get back 1 no response 10
It is obvious from the table that three quarters of the students had some idea for retrieving the solutes but one quarter had no idea. Evaporation was the most well known method for retrieving the solutes among these students. It is possible to say that these students who commented various methods such as: evaporation, boiling off, heating, distillation, and drying out, understood the importance of heating for getting the solute back. On the other hand two students commented that by freezing it is possible to retrieve the solute. I believe, these two students perceived dissolving and melting as the same meaning.
6.4.6 Examples The examples which were given by students were almost all examples of solid- liquid solutions such as; sugar in tea, coffee, and water, salt, bath salts, coffee, disprin, Andrews, coffee granules, gravy granules, jelly, oxo cubes, lemon sip, aspirin, gelatine, soap and washing powder in water. One of the examples was a liquid-liquid solution, that
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is, orange squash diluted in water. There were no more examples for other states of dissolving.
6.4.7 Drawings There were only two examples that display a chemical model, four students could not draw any picture and twenty-three students drew a picture in which they used nonchemical models. In one of the chemical model, the solute and solvent are particles and they were drawn detailed. Another student drew a picture in which solute and solvent are generalised particles. Table 6.18 The models that have been used by the non-specialist student teachers
THE NUMBER OF MODELS WHICH WERE USED BY STUDENTS
the number of students
Non-chemical model solute is particle, solvent is continuos - disappearing Non-chemical model solute and solvent are particles - intermingled Non-chemical model solute is particle, solvent is continuos - breaking bits Melting
21
Chemical model solute and solvent are particles - general Non-chemical model solute and solvent are particles - disappear Chemical model solute and solvent are particles - detailed NO DRAWING
2
4 3 2
1 1 7
In this study there is a remarkable point that these non-specialist student teachers perceive the process of dissolving as a non-chemical process. Generally, there are no micro level drawings. The particles of substances such as molecules, atoms, ions are not drawn by these students.
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It can be seen in above table two students drew pictures as melting:
1. MELTING
Figure 6.16. Melting
Figure 6.17. Melting
These drawings show that for these two students dissolving is only a phase change from solid to liquid. On the other hand twenty eight students drew pictures using non-chemical models, such as; 2. NON-CHEMICAL MODEL 2.1. Solute is particle, solvent is particle 2.1.a. Non-chemical model (solute is particle, solvent is particle - breaking bits) There is no drawing in this type. 2.1.b. Non-chemical model (solute is particle, solvent is particle - intermingled) There are four drawings in which the solute and the solvent particles were represented by small ball models with different colours or with letters inside these balls. There are
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not any change of the solute and solvent particles. It can be seen in below drawings that the particles of solute and solvent simply intermingle in each other.
Figure 6.18. Non-chemical model, solute is particle, solvent is particle - intermingled
Figure 6.19. Non-chemical model, solute is particle, solvent is particle - intermingled
Figure 6.20. Non-chemical model, solute is particle, solvent is particle - intermingled
2.1.c. Non-chemical model (solute is particle, solvent is particle - disappear) In this model the particles of solute and solvent are presented by small ball models. In dissolving process the particles of solute disappear.
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Figure 6.21. Non-chemical model, solute is particle, solvent is particle - Disappear
2.2. Solute is particle, solvent is continuous 2.2.a. Non-chemical model (Solute is particle, solvent is continuous - breaking bits) In this model the solute is particle and the solvent is continuous. It is seen in the drawings below after the solute are put into solvent, that the particles of the solute are breaking bits. In the second drawing, firstly, the particles of the solute are breaking bits and then they disappear.
Figure 6.22. Non-chemical model, Solute is particle, solvent is continuous - breaking bits
Figure 6.23. Non-chemical model, Solute is particle, solvent is continuous, breaking bits and disappearing 123
2.2.b. Non-chemical model (Solute is particle, solvent is continuous - intermingled) There is no drawing in this type. 2.2.c. Non-chemical model (Solute is particle, solvent is continuous - disappear) There are twenty-one drawings of this type. The solute is particulate and the solvent is continuous. The visibility of the solute is no longer present. According to these twentyone students the particles of solute disappear in the solvent.
Figure 6.24. Non-chemical model, Solute is particle, solvent is continuous- disappear
Figure 6.25. Non-chemical model, Solute is particle, solvent is continuous- disappear
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Figure 6.26. Non-chemical model, Solute is particle, solvent is continuous- disappear
Figure 6.27. Non-chemical model, Solute is particle, solvent is continuous- disappear
Figure 6.28. Non-chemical model, Solute is particle, solvent is continuous- disappear
Figure 6.29. Non-chemical model, Solute is particle, solvent is continuous- disappear
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2.3. Non-chemical model, Continuous - disappear There are no drawings in this type. 3. CHEMICAL MODEL (Particles, solute and solvent) 3.1. Particles - general In this model the solute and solvent are represented by small different coloured balls. As it can be seen in the above drawings in dissolving process, the solute and solvent particles join together and the appearance of the substances is quite different from the beginning.
Figure 6.30. Chemical model (Particles, solute and solvent), general
Figure 6.31. Chemical model (Particles, solute and solvent), general
3.2. Particles - detailed There is only one student who drew a chemical model that is detailed. In this picture the sodium and chloride ions bond with different ends of water molecules. Sodium ion bond with the O (-) end of water molecule and chloride ion bond with the H (+) end of the water molecule.
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Figure 6.32. Chemical model (Particles, solute and solvent), detailed
6.4.8 Conclusion The ideas of the non-specialist students were analysed in this section and the ideas about the concept of dissolving were mainly (52%) were the changes into a liquid and secondly with 25% was disappearing. This result shows that these students considered only the appearance of the solute and the solvent in the solution rather than considering the particulate changes. The majority of the explanations (37/39) for dissolving was based on the physical change.
On the other hand it was determined that there was a misconception of the concepts of the solution, the solute and the solvent. Some of the students confused these three concepts with each other.
The majority of the students who gave response believe that the solvent is a liquid on the other hand the majority of the students believe that the solute is the substance which was called as an ingredient, a chemical, a powder, a substance etc. dissolves in the solute. It was founded that there were still some students who believe that dissolving and melting are similar or same concepts, and there were twenty-one students (a majority) did not have any idea about the difference or similarity of these two concepts.
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Most (29/39) of the non-specialist students have no idea about the process of dissolving and only two of the students believe it is a chemical process. 19/39 of the students commented that it is a reversible change and there were many students (11/39) who do not believe its reversibility.
In contrast to previous findings the majority of the students recommended some methods for retrieving the solute and the solvent back. These methods were mainly evaporation, freezing, crystallisation. Evaporation was the most recommended method by 27 students.
Solid dissolved liquid solutions were the common examples of dissolving and there was only one example of the liquid dissolved in liquid solutions.
Twenty-nine of the students drew pictures using the non-chemical model and three of the students used the chemical model. It is important that there were two students who drew pictures of the melting
So, this study showed that the non-chemical model was the popular model for explaining dissolving and drawing the pictures in non-specialist primary students’ answers.
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CHAPTER 7
SCIENCE EDUCATION TECHNICIANS’ VIEWS ABOUT DISSOLVING
7.1 Introduction This study consists of interviews of three technicians who are the responsible for preparing curriculum materials and laboratory materials in science education department. Like the experts’ interviews, eight questions were asked between twenty minutes and half an hour. In this chapter, the ideas of technicians will be described and their ideas will be compared. The technicians are identified as T1, T2, T3.
7.2 Ideas about dissolving in general The ideas of academic experts were evaluated in the chapter 6. In contrast to the experts’ ideas of a chemical model for dissolving, the interviews of technicians showed that they used the dispersal model to explain dissolving. None of them mentioned ionic and molecular compounds and their importance in the dissolving process.
According to T1 “dissolving is to add solute to a liquid and usually with stirring, it would dissolve into the liquid...they are taken up by the liquid”. So, it is obvious that she does not provide a micro level explanation for dissolving.
T2 believes that the “particles disappear because they spread out, getting smaller”. This idea led to me to ask what did T2 mean by particles and the answer was “not molecules but the particles that you are dissolving get smaller and disappear”. This 129
explanation does not give any information about the bonds between molecules and ions and their role in dissolving process as described by T1.
In T3’ s opinion dissolving is the diffusion of substances through the liquid. Like diffusion of gases throughout each other in this idea there is a similarity. According to above ideas the macro level of explanation is dominant. The explanation does not use atoms, molecules, ions, or bonding.
7.3 Ideas about the constituents of a solution In T1’s opinion the solute is the solid and the solvent is the liquid. T2 has a tautological view about solute and solvent, that is: “the solute is a substance that we want to dissolve, the solvent is a substance in which we are dissolving”. T3 believes that “the solvent is a liquid and solute which is dissolving in the solvent”. These opinions showed that different kinds of solution such as gas-gas, gas-liquid, gassolid, solid-solid are not considered by technicians.
7.4 Ideas about dissolving and melting To differentiate dissolving and melting from each other in T1’s words “dissolving has a connection with a liquid and a solid” and she commented that for melting heat and a solid are needed.
T2 pointed out that dissolving and melting are not same. The words of heat and temperature are associated with dissolving and melting in T2’s opinion. She said that “in dissolving, the solid’s particles are still there, you cannot see them. They just 130
spread out. If you try to melt something like ice, you just have one substance. It stays the same and becomes liquid. It is changing from solid to liquid often”.
T3 believes that “dissolving is similar to melting, both are heat dependent, temperature dependent”.
7.5 Ideas about the process of dissolving According to one of the three technicians, the process of dissolving is a chemical change but the others believed that it is a physical change. All of the technicians said that dissolving is reversible and by evaporating the liquid we can get back the solute and solvent. They all stated that there is no loss of solute or solvent and T1 pointed out that “if you collect the evaporating vapour, you should be able to get both of them exactly but you have to devise a system this is good enough to collect them”.
7.6 Examples and drawings The examples of the technicians are: T1’s examples “sugar in a tea and salt in water”, T2’s examples “sugar in tea, bubble gas in lemonade drinks, using alcohol to get grease, and to get stain”. T3’s examples “sugar in tea, and salt in vegetables when we are cooking in the water”. It has been mentioned above that the technicians did not express any particulate idea of dissolving. At the end of the interviews the structures of sugar water and salt were shown and they were asked to draw pictures to explain dissolving. Unfortunately, none of them was be able to draw a representative picture of the process of dissolving. 131
7.7 Conclusion The results of the interviews with the technicians indicated that these three technicians described the non-chemical (dispersal) model in their explanations of dissolving. As with the non-specialist students they concentrated on the appearance of the constituents of the solution There was no mentioning of atoms, molecules, ions or bonding in their explanations. The solute was described as a solid and the solvent was described as a liquid and the tautological view was used in one of the explanations. Only solid dissolved in liquid solutions were considered. Heat was suggested to differentiate dissolving from melting. Dissolving process was stated generally (2/3) as a physical reversible change by evaporation and they believed in the conservation of matter in dissolving process. The examples were generally from solid dissolved in liquid solutions and only one of the examples was from the gas dissolved in liquid solutions. The technicians did not draw any pictures.
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CHAPTER 8
OTHER FINDINGS
8.1 Introduction This study consists of some drawings of people about dissolving. Thirteen people who drew these pictures inclined a primary school teacher, secondary school chemistry and physicist teachers, PGCE students, and a laboratory technician. The data was collected from the attendance at The Association for Science Education conference on the 29th of June 1997. In this conference one of the speakers presented the dissolving of iodine (I2) in heptane (C7H16) and ethanol (C2H5OH) and sodium chloride (NaCl) in water (H2O) by doing experiments.
8.2 The dissolving of iodine in heptane Firstly, iodine was dissolved in heptane and the purple colour of iodine did not change. The colour of the resulting solution was purple. In this case, it was the expectation that people would draw a picture by using a non-chemical (dispersal) model in which the particles of iodine and heptane just intermingle.
1. Non-chemical (dispersal) model: 1.a. Solute is particle and solvent is particle - breaking bits In this drawing the solute and solvent are particles and they are breaking bits. In her/his words “iodine molecules moving apart”.
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Figure 8.1. Non-chemical (dispersal) model. Solute and solvent are particles - breaking bits
1.b. Solute is particle, solvent is particle - intermingled In the drawings below the particles of iodine (solute) and heptane (solvent) are represented by different coloured balls and different shapes. They simply intermingle. In those people’s words the iodine is mixed, dispersed through heptane.
Figure 8.2. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
Figure 8.3. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
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Figure 8.4. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
Figure 8.5. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
Figure 8.6. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
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Figure 8.7. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
Figure 8.8. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
Figure 8.9. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
Figure 8.10. Non-chemical (dispersal) model. Solute and solvent are particles - intermingled
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2. Chemical model: 2.a. The solute and solvent particles - general It can be seen in this drawing there is a difference in the shapes of particles between the solute (iodine) and solvent (heptane) particles and the particles of the resulting solution. There is also the energy change that was represented by the arrow. This person may perceive the process of dissolving as chemical change.
Figure 8.11. Chemical model. Solute and the solvent are particles- general
8.3 The dissolving of iodine in ethanol Secondly, iodine was dissolved in ethanol. The expectation was that the colour of the resulting solution would be as same as the iodine-heptane solution, that is purple. However the colour of iodine-ethanol solution was brown. In this case there should be some changes in drawings. The change of colour confused some of people because the model of intermingling (dispersal) was appropriate for these people.
1. Non-chemical model 1.a. Solute is particle, solvent is particle - intermingled There are only two pictures that were drawn using non-chemical model and in the first picture s/he states that there are some interactions between iodine and ethanol.
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Figure 8.12. Non-chemical model. Solute and solvent are particles - intermingled
Figure 8.13. Non-chemical model. Solute and solvent are particles - intermingled
2. Chemical model 2.a. Solute is particle, solvent is particle - general The particles of iodine and ethanol are not as same as in the beginning and it is possible to say that there is a chemical change. In people’s words they associated, joined together, joined together to make a new particle, there are some interaction between iodine and ethanol.
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Figure 8.14. Chemical model. Solute and solvent are particles - general
Figure 8.15. Chemical model. Solute and solvent are particles - general
Figure 8.16. Chemical model. Solute and solvent are particles - general
Figure 8.17. Chemical model. Solute and solvent are particles - general
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8.4 The dissolving of sodium chloride in water There were some drawings about the dissolving of sodium chloride in water. 1. Non-chemical model 1.a. Solute is particle , solvent is particle - intermingled
Figure 8.18. Non-chemical model. Solute and solvent are particles - intermingled
Figure 8.19. Non-chemical model. Solute and solvent are particles - intermingled
Figure 8.20. Non-chemical model. Solute and solvent are particles - intermingled
1.b. Solute is particle, Solvent is continuous - intermingled The water is seen in this picture water as continuous and the sodium chloride (solute) is particulate. The particles of solute intermingle with the solvent.
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Figure 8.21. Non-chemical model. The solute is particle, the solvent is continuous - intermingled
2. Chemical model 2.a. Solute is particle, solvent is particle - general
Figure 8.22. Chemical model, the solute and the solvent are particles- general
Figure 8.23. Chemical model, the solute and the solvent are particles- general
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2.b. Solute is particle, solvent is particle - detailed In this picture, the sodium and chloride ions and the molecules of water can be seen detailed. The sodium and chloride ions surrounded (bonded) with the opposite ends of the water molecules. The sodium ions bonded with the O (-) end of water molecules and The chloride ions bonded with the H (+) end of water molecules.
Figure 8.24. Chemical model, the solute and the solvent are particles- detailed
In this picture the ions and the bonds inside the water molecules are detailed but s/he commented that salt and water mixed.
Figure 8.25. Chemical model, the solute and the solvent are particles- detailed
The drawings can be summarised in Table 8.1: Table 8.1 The models that have been used for explaining different phenomena
Non-chemical model Chemical model No drawing
The dissolving of Iodine in heptane 12
The dissolving of iodine in ethanol 4
The dissolving of salt in water 7
1
4
4
-
5
2
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The people who attended the ASE conference were content with the non-chemical (dispersal) model in their explanation of dissolving. However the dissolving of iodine in heptane and the iodine in ethanol did not give the same result. There was a colour change from purple to brown. People thought that the change of the colour is an indicator of chemical change but they used different words such as: there is an interaction between the particles; the particles joined together; joined up to make new substances; or associated.
It can be seen in table 8.1. the models of dissolving that were used by people in their drawings are various and there is a significant result that is these people used the nonchemical (dispersal) model when they were drawing the dissolving of iodine in heptane but the case of dissolving of iodine in ethanol confused many of them. However, there is an increase in the number of drawings the chemical model from 1 to 4 and decrease of the drawings of the non-chemical model from 12 to 4. However for the drawings of the dissolving of salt in water, the majority of people still used the non-chemical model.
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8.5 Conclusion The drawings of people these from different parts of the education system showed that the non-chemical (dispersal) model was the common model that has been used but the models used were changeable for different events. In the case of dissolving of iodine in heptane the non-chemical model was common but the colour change in dissolving of iodine in ethanol due people to rethink the process of dissolving and there was incredible decrease in using non-chemical model. However for dissolving of salt in water people did not consider the change of salt in water because there was no visible change. So, they used again the non-chemical model. It is obvious that people had some evidence of chemical change for dissolving of iodine in ethanol and it changed their views. Although the terms of interaction, joining of particles, association were used by these people, they avoided to use the term of chemical change in their views.
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CHAPTER 9
CONCLUSION
9.1 EVALUATION
9.1.1 Introduction In this study fifty-nine people who were four physical science specialists, four science specialist student teachers, nine first year science specialist student teachers, thirty-nine non-specialist student teachers, three science education technicians were interviewed or requested to respond eight open-ended questions. All the student teachers were aiming at the primary level. Their ideas and comments were described in the previous chapters. Their responses were classified into five main groups: 1. Ideas about dissolving in general; 2. Ideas about the constituents of a solution; 3. Ideas about dissolving and melting; 4. Ideas about the process of dissolving (chemical change, physical change, reversible change, conservation of matter); 5. Examples and drawings. These ideas are compared and discussed below.
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9.1.2 The analysis of the responses a. The explanations The explanations of people about dissolving can be summarised in Table 9.1; Table 9.1 The explanations of different group of people about dissolving Group of people Experts Science specialist student teachers First year science specialist student teachers Non-specialist student teachers Technicians TOTAL
Physical change 25% 75%
Chemical change
45%
55%
95%
5%
100% 82%
0 18%
75% 25%
100 90 80 70
Physical change
60 50 40 30
Chemical change
20 10 0 Experts
SSS
FYS
NSS
Technicians
Total
Fi gure 9.1. The explanations of different group of people about dissolving.
NOTATIONS: SSS: Science specialist student teachers; FYS: First year science specialist student teachers; NSS: Non-specialist student teachers.
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As can be seen in the Figure 9.1 that 82% of people explained of dissolving with physical change. They only considered the appearance of the constituents of a solution. There were some people who believed that the dissolving is a chemical change who also used particulate explanations in their explanations.
b. The definitions of solute and solvent The definitions of the solute and the solvent were very complex in the chemistry textbooks studied. This research has indicated that people have various definitions for the constituents of a solution. For solvent the responses given can be outlined in Figure 9.2.:
liquid
20%
7% 3% 3%
a substance which dissolves greater quantity solution medium
7% 34% 7%
solid others no response
19%
Figure 9.2. The ideas of people about solvent.
34% of people considered that only liquids could be solvents and there were some people who confused the concepts of the solution and the solvent. 19% of people believed that the solvent is a substance which dissolves but they did not consider that there are some solvents in which some substances dissolve and the others in which they do not. 7% of people defined the solvent as the material in greater quantity but they did not comment whether it is measured as mole, volume or mass.
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On the other hand 36% of people defined the solute as a soluble substance but they did not think that there are also some substances which can dissolve in one solvent but not in another solvent. For example, salt can dissolve in water and in this case it has to be considered as a soluble substance but it cannot dissolve well in alcohol. 15% commented that the solute is a solid and 7% commented that the solute is liquid but there are gases that can be solute. Likewise for solvent, 9% confused the concepts of the solution and the solute, and 3% confused the concepts of the solvent and the solute with each other. 3% described the solute as the small quantity in respect to its mole in the solution and a quarter of the people could not define the solute.
For solute the responses given can be summarised in Figure 9.3;
25%
soluble substance solid
3% 5%
solution
7%
liquid 36%
solvent
9% small quantity (mole) 15%
no response
Figure 9.3. The ideas of people about solute.
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c. The concepts of dissolving and melting In this study the other aim was to determine whether people can differentiate the concepts of dissolving and melting from each other. The results were;
37%
different meaning
same meaning 39% 24% no response
Figure 9.4. The ideas of people about the concepts of dissolving and melting.
39% of people believed that these two concepts are different from each other and they have different processes. However there were still 24% who perceive these two concepts as a same concept. It shows that they might only be considering the appearance of the solute in a solution. 37% did not give any idea about these two concepts. It can be suggested that people should focus on differentiating these two concepts. The words associated with dissolving were;
Table 9.2 The words associated with dissolving WORDS liquid- solid more than one substance heat bond breaking suspension others (chemical change, energy, concentration, ionic substance, disappearing, stirring) no response
Percentages (%) 10 5 5 5 5 17 53
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According to 10% of people the words of liquid- solid are associated with dissolving. This idea is for solid- liquid solutions. The solute was considered as a solid and the solvent was considered as a liquid. In 5% of people’s views in dissolving more than one substance are needed. The same percentage of people commented that heat is necessary in dissolving. 5% presented that there is a bond breaking in dissolving. This is a different factor from melting. There was an idea that suspension was accepted as associated word with dissolving. 17% indicated that the words of chemical change, energy, the concentration, ionic substance, solvent, disappearing and stirring are the words that related to dissolving. However these ideas were not common. They were in small proportions, so, these ideas were evaluated as others. More than half gave no idea about the words associated with dissolving. Table 9.3 The words associated with melting WORDS ASSOCIATED WITH MELTING becoming liquid heat dependent bond breaking one substance energy physical change solid suspension others (concentration, solvent, phase change) no response
Percentage (%) 13 10 5 5 3 3 3 3 5 50
There was no majority of people who gave a word that is peculiar to melting. The words are in small proportions. However 13% of people suggested that in melting the solid becomes liquid, that this is the factor that differentiates the two concepts of dissolving and melting from each other. 10% said that melting is always heat dependent. However 5% could not differentiate melting from dissolving and they commented that in melting we have bond breaking. 150
According to the same proportion of people one substance is enough for melting. 3% of people associated the energy with melting and claimed that the process of melting is physical change. In 3%’s views, solid is the word that is related to melting but it should be indicated that some people referred to the solid also for dissolving. It seems likely that they considered this word as an associated word with both melting and dissolving. Likewise for the words associated with dissolving, 3% gave the word of suspension as a related word to melting. In small proportions pointed out that concentration, solvent, phase change are words that associated with melting. Half of the respondents gave no idea about words associated with melting.
d. The process of dissolving When respondents were asked them whether the process of dissolving is physical or chemical change. There were four kinds of answers. They were summarised in Figure 9.5.:
Physical change 50% Chemical change 21% Both
13%
16%
No idea
Figure 9.5. The kinds of answers that has been given for the process of dissolving.
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21% of people believed that the process of dissolving is a physical change, but 16% think it is a chemical change. 13% commented that it is both the chemical and the physical change but half gave no idea about the process of dissolving.
So, most people considered the process of dissolving as a physical change. If the views of people about dissolving are analysed it is obvious that they were much more enthusiastic to explain dissolving as a physical change.
17% 3% Physical change
Chemical change
Both 80%
Figure 9.6. The explanations of people in dissolving.
Explanations based on physical change were the major proportion, 80% explain dissolving without mentioning any chemical change or any particulate details. They preferred to explain dissolving at a macro level that is based on the appearance of the constituents of the solution. 17% explained dissolving using the terms of atoms, molecules, bond breaking, change of energy and so on. Only 3% used both chemical and the physical change in their explanations. This may be because of the lack of clarity of the models that they chose or difficulty in explaining dissolving by chemical change.
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e. Reversibility The respondents were asked to think of the reversibility of dissolving. The results were:
5%
13%
Reversible
19% Irreversible
Not always
63%
No response
Figure 9.7. The ideas of people about reversibility in dissolving process.
The majority of people pointed out that dissolving is a reversible change. It is clear that the reversibility of dissolving led people to consider dissolving as a physical change but it has been determined that even when people used in their explanations the words of association, joining together, interaction between solute and solvent, they believed that the process of dissolving is a physical change.
19% commented that it is impossible to retrieve the solute and the solvent from the solution. This idea might be come from the idea of chemical change in dissolving. There were some (5%) who stated that it is normally reversible but they did not give any further explanation to support their ideas. 13% of people gave no idea about reversibility.
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In the dissolving process there are some methods for retrieving the constituents of solution. Methods for undissolving were requested of respondents. The answers were:
2%2% 3%
15%
3%
Evaporation Crystallisation
8% Freezing Filtration Chromatography Cannot 67% No response
Figure 9.8 The methods for undissolving that have been suggested.
Evaporation was the common method (67%) although people used different words such as; heating, distillation, boiling off, drying out. It shows that people were aware of the effect of heating in dissolving. 8% recommended the method of crystallisation however there were also two methods that were freezing (3%) and filtration (3%) have been suggested for retrieving the substances from the solution. Freezing may be suggested because of the confusion of the concepts of meting and dissolving and only 2% pointed out by chromatography we can get back the solute and the solvent. 1% commented that it is impossible to get the substances from the solution. 15% of people gave no suggestion for reversing of dissolving.
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f. The examples People gave various examples for dissolving but there was a common example that just includes the solid dissolves in liquid solutions. The minority of people gave some examples for other kinds of solutions. The examples were; Table 9.4 The kinds of solutions that has been considered in examples
THE KIND OF SOLUTION SOLID-LIQUID LIQUID-LIQUID LIQUID-SOLID GAS-GAS GAS- LIQUID SOLID-SOLID
EXAMPLES sugar, salt, coffee granules, tea, bath salts, disprin, Andrews, gravy granules, jelly, oxo cubes, washing powder, soap, lemon sip, aspirin, gelatine in WATER orange squash diluted, alcohol in WATER using alcohol to get grease, to get stain, nail varnish and remover, paraffin, petrol air bubble gas in lemonade drinks gold in platinum (gold ring)
g. Drawings People drew various pictures in which they used different models. These models can be analysed in Table 9.5: Table 9.5 The models that have been used in drawings
The models
Percentages (%)
Non-chemical model, solute is particle, solvent is continuous, disappearing Chemical model, particles, detailed Non-chemical model, solute and solvent are particles, intermingling Non-chemical model, solute is particle, solvent is continuous, breaking bits Chemical model, general Melting Non-chemical model, solute and solvent are particles, breaking bits Non-chemical model, solute and solvent are particles, disappearing Non-chemical model, solute is particle,
36% 17% 12% 6% 6% 3% 2% 2% 2%
solvent is continuous, intermingling No drawings
14%
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17% Non-chemical model 23%
Chemical model
60%
No drawing
Figure 9.9. The models that have been used in drawings.
The drawings showed that 60% of people drew pictures using the non-chemical model in which the solute and the solvent are particles. There were three kinds of change. Firstly, the particles of the solute and the solvent were intermingling, secondly, breaking bits, and lastly, they were disappearing. Especially, the solute particles were disappearing among the solvent particles.
In the second kind of non-chemical model the solute is particulate, the solvent is continuous. It has been drawn plain. The solute particles are intermingling, breaking bits or disappearing in the continuous solvent.
Twenty-three per cent of people drew pictures in which the chemical model has been used. The chemical models were two kinds; in the first one the solute and the solvent particles were drawn in general and in the second kind of chemical model the solute and the solvent particles were drawn in detailed. The atoms, molecules, ions and the bonds were clear. Seventeen per cent drew no picture to explain dissolving. 156
On the other hand for the group of people from ASE conference the drawings of dissolving salt in water were:
15% Non-chemical model 31% Chemical model
54% No drawing
Figu re 9.10. The models that have been used by people in ASE conference.
So, the drawings also showed that people perceive dissolving as a physical change and the idea of chemical change are increasing by the level of science education and by the consideration of the changes in a solution.
The models that have been used by both group of people can be analysed in the Figure 9.11:
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MODELS Breaking bits Solute and solvent are particles
1. NON-CHEMICAL
Disappear
Particles Intermingled
Breaking bits Solute is particle Solvent is continuous
Intermingled
Disappear
General
2. CHEMICAL
Particles
Solute and Solvent Detailed
Figure 9.11. The kinds of models of dissolving that have been used by people in this research 158
9.1.3 Conclusion The responses show that there is an increase in the use of idea of physical change in this order: Experts < FYS < SSS < NSS < Technicians The use of the concept of chemical change is in the reverse order. The definitions of the solute and solvent were problematic because there were various definitions for the solute and the solvent. The similar responses were classified in Figure 9.2 and Figure 9.3 that in the majority of responses the solvent is a liquid or with nineteen percentages a solvent is a substance that dissolves. The other types of answers were that a solvent is: a greater quantity; a solution; a medium; a solid. On the other hand the majority of people (39%) believe that the solute is simply a soluble substance and the second majority of people perceive that the solute is a solid. The other types of responses for solute were: a solution; a liquid; a solvent; a small quantity (mole). The above definitions for the solute and the solvent show that some have difficulty in differentiating
the
solute,
the
solvent
and
the
solution
from
each
other.
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From the ideas of the solute being a liquid and the solvent being a solid it can be conjectured that these people knew about other types of solutions. However there was no extra explanation to confirm this idea.
Differentiating the concepts of ‘dissolving’ and ‘melting’ from each other is a problem for sixty-one people who perceive that these two concepts are the same or who give no idea. Only thirty-nine people can understand and explain the difference between these concepts.
This study also indicates that for half of the respondents the process of dissolving is not conceptualised. The second majority accept the process of dissolving is physical. There are some people (16%) who believe and explain that the process of dissolving is chemical or thirteen per cent who believe it both physical and chemical changes are important in dissolving process.
In contrast to these results, eighty per cent of people explained dissolving as a physical change, seventeen per cent explained by chemical change and only three per cent explained by both physical and chemical changes.
Most identify dissolving is a reversible change and the great majority suggested a variety of methods of evaporation, heating, distillation, boiling off and drying out for reversing dissolving.
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The solid dissolved in liquid solutions were the majority type of solutions. There were some examples for liquid- liquid, liquid- solid, gas- gas, gas- liquid, solid- solid solutions that have been given by the smallest quantity of people.
There are various sorts of drawings that were given by these people. The drawings can be analysed into three groups: Melting; Non-chemical model; Chemical model. In the first group the attention is on the particles of the solution. Two people drew this. This idea is derived from the difficulty in differentiating of the concepts of ‘dissolving’ and ‘melting’ from each other. This model was evaluated as non-chemical model in this chapter. There was no drawing that includes the melting model in ASE conference attendance’s’ drawings.
The non-chemical model was used by the greatest percentage of people (60%) and in the ASE group of people’s drawings similarly fifty-four per cent of people used nonchemical model.
The chemical model was used by only twenty-three per cent of people. In ASE attendance’s drawings more drawings showed the chemical model. Thirty-one per cent of this group used the chemical model.
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9.2 IMPLICATIONS
9.2.1 Introduction Dissolving can be used as an example in order to emphasise the importance of explaining and modelling in chemical education. It may be assumed that dissolving is a simple scientific concept. However, this research shows that this is not the case.
Explaining dissolving is in fact one of the most complex scientific areas of study, even though it is and has been for centuries a part of everyday human life. One of the main issues in science education today is to discover children’s ideas. I feel these could be used to develop improved models in order to promote conceptional development. In the second chapter the necessity for explanation and the production of appropriate models concerning various age groups of students were examined, and in conjunction with their models, their purposes and uses were described. In the third chapter, the results of previous research undertaken in dissolving within the age group 7 to 17 is discussed. In the previous chapters, the ideas taken from the sample group have been analysed.
The main purpose of this section is to indicate the difference in models and explanations in dissolving and their implications for teachers. It is also to point out the limitations of models which are used in this process. This section will conclude by offering suggestions to teachers of how to make a conceptual change in children’s perceptions of dissolving.
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9.2.2 Research findings The analysis of chemistry degree level textbooks was the first stage in this research, with the intention of finding a deeper explanation for the process of dissolving. However, there was already a variety of existing explanations in these textbooks. The definitions of the solute and the solvent were tautological. In some definitions only water was accepted as a solvent, in other definitions emphasis was placed upon the solid dissolving in liquid solutions. Other kinds of solutions e.g. solid- solid, gas- solid, gas- liquid were rarely considered in some of those books. Some of the definitions were based on the quantities of the constituents of the solution, although there was no evidence to define these quantities, whether they are mass, volume or mole.
The solubility of these substances was another issue where the books offered a variety of explanations. In some of these, the salt/peas model or dispersal model was commonly used. The solute and the solvent were considered as particles which simply mix. However, this model is inadequate in explaining insolubility.
Generally books tended to use the rule of “like dissolves like”. Some books regarded this rule as similar polarity or some as similar forces, however, this explanation is invalid in some cases. For example, olive oil and water have similar polarity but they cannot dissolve in each other. If the base of this rule is similar forces it is difficult to explain the dissolving of salt in water even though they have forces of different magnitudes.
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There were also other explanations in which the dissolving of salt in water was pictured with the water molecules surrounding the separated ions. In certain books it was possible to find a variety of explanations and models for dissolving, this made establishing the concept of dissolving difficult for the student.
The language and vocabulary used in books to explain the process of dissolving was varied: mixing, spreading out, interacting, association, joining up, disappearing, distributing, diffusing, intermingling. These words are not chosen carefully by authors and this may result in misconceptions of students ideas regarding the concept of dissolving.
In these books the process of dissolving was considered as only a physical change, however, they used the words interacting, association, joining up. The important point is that many of these books rarely paid attention to the change of energy and entropy in the dissolving process. Also the change of solubility depends on H/T but this was not taken into account.
My research concerned people who are the most effective factors in promoting children’s conceptual development. These people were: experts, science specialist student teachers, first year science specialist student teachers, non-specialist student teachers and science education technicians. They were all individually questioned: experts and technicians by interview and others by an open ended questionnaire. The models which these people held
in
their
minds
were
also
important
in
this
study.
In
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order to understand their ideas regarding this scientific process, they were asked to draw pictures. As a result of this study various explanations and models were revealed. This research examined the following: the process of dissolving; the definitions of the solute and the solvent; the ability to differentiate the concepts of ‘dissolving’ and ‘melting’, and the language used by respondents to distinguish between these two concepts; the reversibility of dissolving; the examples given models that have been used in drawings.
This research demonstrated that people’s ideas concerning dissolving were influenced through their experiences of everyday life. Their responses tended to concentrate only upon the visible properties of the solution and not on an atomistic level. The explanations of respondents generally depended on their everyday life.
The process of dissolving was considered as physical change some of them used the words of bond breaking, joining up, joining together, association.
The solvent was perceived as liquid and solute as soluble substance. Some of them confused the concepts of solute, solvent and solution. There was also difficulty in differentiating the concepts of dissolving and melting as in prior research.
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The reversibility of dissolving was recognised by majority of those questioned. The method of evaporation was the most popular response in reversing dissolving.
The examples were chosen from daily life were generally solid dissolving in liquid solutions.
The models used in drawings to express dissolving were various. The three major groups of models were: melting; non-chemical model; chemical model. The drawings were drawn using six types of non-chemical models and also two types of chemical models (see Figure 9.10).
The previous and the present research shows that the concept of dissolving including many scientific area have different types of explanations. As has been already mentioned teachers need more appropriate models to explain scientific theories, to alter children’s ‘alternative conceptions, alternative frameworks, preconceptions’. However this is not as easy as is expected. According to research findings there are many models which are strongly held by children and are also presented in textbooks. The limitations of dispersal model has been indicated in fourth chapter. In spite of this model being used in textbooks and by teachers, there is also mention of an interaction, an association and joining together but both scientists and textbooks are reluctant to use the term of ‘chemical
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change’ in their explanations. If consideration is based on the change of enthalpy and entropy in the system, the limitation of the dispersal model can be seen. The books also used the rule ‘like dissolve like’. However this is not appropriate in every case. In books the main concentration is on solubility rather than insolubility.
9.2.3 Some suggested possible models for different age group of students In a qualitative approach the dispersal model in either a continuous or a particulate solvent is considered as dispersing or simply mixing with the solute. This is driven by the increase of entropy, or in other words the system’s disorder. The increase of the entropy in a system can be represented by an entropy model. This idea is generally held by lower secondary level of students.
At the upper level of secondary, students start to think about the forces between the particles of the solute and the solvent (Interaction model) but they do not compare this idea with the entropy explanation.
In the quantitative approach, the model used is the Hess Cycle model of dissolving. The separation of the solute and the solvent particles and the solvation process is considered. This idea is rarely used by chemistry (post-16) students.
At chemistry degree level, the attention is on the free energy model in which the changes of enthalpy and mostly the surrounding entropy are taken account and from qualitative results the explanations of dissolving are derived.
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The models which are used by different level of students can be summarised in Table 9.6: Table 9.6 The models suggested for different age group of students
MODELS emphasise on consideration types of approach
lower secondary level DISPERSAL MODEL entropy
upper secondary level INTERACTION MODEL enthalpy
Chemistry Post-16 HESS CYCLE MODEL Enthalpy
chemistry degree FREE ENERGY MODEL entropy
qualitative approach
quantitative approach
Qualitative Approach
qualitative approach
Each of these above models are only parts of the explanation of dissolving and for different levels of students only part of the full explanation is usually used and accepted.
9.2.4 Implications for teachers The models of dissolving and their appropriateness for different levels have been indicated. In order to understand the concept of dissolving students need access to explanation used by different models. These models are developed for different age group: Dispersal model (lower secondary); Interaction model (upper secondary); Hess cycle model (chemistry post-16); Free energy model (chemistry degree). Teachers can see that some students are reluctant to accept a new model. All people use models in order to find satisfying explanations. Teachers need to explore the ideas of students for challenging their cognitive development. This could be done by:
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providing an environment in which students can discuss their ideas with their friends;
provide an environment in which students can make explicit their models;
providing an environment in which students can test the limitations and the scopes of their models;
using suitable models for different age groups in order to avoid the student’s reluctance to accept new models.
However, teachers need to describe their models’ advantages, scope and limitations.
The teachers should emphasise that the model used in classroom is ‘not the target’. They should state that the model is ‘only one of the possible representations of that specific phenomenon’.
They should know that there is no single explanation for each specific phenomenon and the models only represent that phenomenon from different perspectives.
Teachers need to have high level of understanding of their own subject knowledge in order to give satisfactorily explanations.
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APPENDIX 1
QUESTIONS
1. Can you explain the concept of dissolving? 2. Please give some examples taken from daily life? 3. Are these two concepts related in science? ‘dissolving’ and ‘melting’. a. Is there any word that relates these two concepts? 4. After dissolving where do you think the substances go? 5. Can you explain the difference between ‘solute’ and ‘solvent’? 6. Can we retrieve the solute and the solvent from a solution? a. If the answer is yes, How? b. Is there anything lost or added when retrieving the solute and solvent? 7. Do you think dissolving is ‘a reversible chemical’ or ‘a reversible physical change’? 8. Can you explain ‘dissolving’ by drawing pictures?
THANK YOU FOR YOUR CO-OPERATION YASEMIN GÖDEK
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APPENDIX 2
THE TRANSCRIPT OF THE INTERVIEWS
2.1 INTERVIEW WITH THE LECTURER A
1. Can you explain the concept of dissolving? Please give some examples from daily life? -
We can look at the dissolving in from two points of view from a chemical one and from a physical one. If you give ionic substances and if you put in a water and mix you can say you have chemical process because we have bonds are broken and new pieces you break the structure of the ionic substances. If you think about substances they are not ionic so cannot the process will be a little different because there are not bonds been broken so we have mixture, things and molecules being separated and we can think in these two points of view.
-
If we cook salt dissolves, when we make some refreshments, different substances dissolve, when you have a bath the soap dissolves in water, when you make some drinks we are dissolving substances in alcohol. Because this is other thing that is important generally we think only water is solvent it is important to consider other solvents. We have a big solution and we have a lot of different gases dissolve in each other (air).
-
I do not think that people use the word of dissolving in their general life. They have some idea about what is the process, I do not think they use much in their life.
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2. What is the relation between these concepts? Dissolving, melting, evaporating, disappearing, reacting, spreading out, separating. -
They are completely different concepts if we think chemically we can say that well... Melting is we can get some material if we put in fire and then it becomes melted because ... Evaporating is ... and it becomes gas. Disappearing is something that there is no chemical sense... no chemical movement, generally people think when you dissolve something it disappears because they can see the solute so we can say all disappears but it is something else it is a popular conception no chemically lose. Reacting, in general way is we say just substances are formed just substances react when they are put together and they produce ... there is a process that the results in other substances different from we have before so the substances change its characteristics.
3. After dissolving the substances where do they go? -
If you are asking this question to a child they say it disappears, I do not know. If you think chemically the substances that dissolve solvent are in the system just in another form. When you dissolve sugar in water we have big pieces of sugar then you can see nothing just because They are so small. You cannot see them but they are in the system being circulated by water molecules. So the substances are all in the system.
4. What is the difference between solute and solvent? -
This is a kind of convention I think, there is a convention that solvent and solute have a mole quantity and the solute is substance has a small quantity but I think this is
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useful because it makes it easier to think about systems everyone understands when you say easily solvent and solute. 5 . Can we retrieve the solute and solvent from the solution? How? -
It depends on the substances. If we distil the mixture then it becomes water and salt because it is possible using physical method of separation so ... and even we think complex solution idea and it should be some process that you can get for instance O2 from air. It is possible to separate.
6. Is there anything added or lost? -
It is not lost just the water and substances will be in other form that is not lost. If you think liquid the substances will be in the air. a. What about salt?
-
It should be possible. I am not sure that some ions could be involved by water molecules and ions evaporated but I am not sure. It is possible or not possible. So if this process we have the salt in the water.
7. Do you think dissolving is chemical reversible change? -
If we think ionic substances in water for instance we have bonds being broken and then only this the process is chemical one and chemically it is reversal... Not exactly in the same form of course you have little crystals being formed but have the ionic bonds being formed so in this way it is reversible.
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2.2 INTERVIEW WITH THE LECTURER B
1. Can you explain the concept of dissolving? Please give some examples from dailylife? -
I guess that dissolving is into the piece of ... I am not sure. Solute dissolving in liquid into the piece of particles intermingle. The examples are sugar into the cup of tea, salt in water, nail varnish and nail varnish remover, using some petrol or paraffin, grease.
2. After dissolving the substances where do they go? -
It is ... If you are taking covalent substance dissolving then if you take sugar in water it is less problematic because in the final mixture we still have water and that sense original substances are still identifiable. Ionic salts dissolving are problematic. Because we may have two different kinds of ionic solids dissolving and then notions of substances are rather than problematic. I am not sure, in that case it is more difficult to say substances are identifiable.
3. Can you explain the difference solute and solvent? -
If you are taking the solutions of solid, liquid or gas, the solvent is the liquid. If you are taking solution solid-solid that is more problematic. Because which is which? It depends on some points in the middle. Solute would be of a lower concentration when we have a mixture which has a variable composition ... 100 %. Then that easy cases, solute in generally is ... We have less of the solute than the solvent.
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4. Can we retrieve the solute and solvent from the solution? How? -
If we just have single salt. If we evaporate it get back the solute. If we have got mixture of salts then getting back the salt what we are originally started it is more problematic. Because you do not know what the original started with.
a. Is there anything added or lost? -
The universal system, isolated system nothing lost. Energy may go. Open system there could be something lost.
5. Do you think dissolving is chemical reversible change? -
Strictly speaking, Yes, but in everyday language dissolving is in a broader sense so alca-seltza dissolves in water or Zn dissolves in HCl.
6. Can you fill in the gap with a suitable word: “Dissolving...............................................................Melting.” a. Is there any word that related these two concepts? -
Dissolving is some ways of like melting. Dissolving in other ways is not like melting. The most fundamental thing they have in common is that they both involve particles becoming more disordered. Similarly they both involve disordered particles. Dissolving particles becomes more disordered. One thing which would characterised of dissolving would be that more than one substance is necessarily involved. In melting, it may involve one substance. Chocolate melts. The principle could be one substance. They have different process. The other thing is energy change. Melting will always be an endothermic change. Dissolving may be both.
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2.3 INTERVIEW WITH THE LECTURER C
1. Can you explain the concept of dissolving? -
Most common way we are talking about dissolving is dissolving of a solid in a liquid a lot of, of course, one could dissolve anything of solid, liquid or gas any of the others the issue is relationship of quantity between the two. The biggest volume, the biggest numbers of moles present as usually refer to as the solvent a minority of the substance as the solute but it is usual to talk about these things about thinking about a solute dissolving in a liquid. That is I do recognise all the others are possible. As a solid dissolves in liquid, the bonds between the ions of the solid or between the molecules. If it is molecular solid are broken and at the same time bonds between molecules of solvent are broken and the solute molecules attach themselves in some way to the molecules of solute electrostatically and the energy of the process is driven by what basically the released of energy as the solid molecules and solvent molecules come together. So what you left in a solution is one or usually one single ion or single molecule of solute surrounded by a sphere of solute solvent molecules normally held together but always held together electrostatically and with a great important being given towards ... often it bonds, that is how I see.
2. Please give some examples from daily life? -
Dissolving of salt in water as you go ... cooking will be one example. An interesting different example would be way two liquids together that is not dissolving it is mixing is not it? That is interesting. It is mixing. Two different liquids together are mixing they are in the same physical phase. That is different from dissolving. There is 176
a difference in my mind. I was going to say. You know, mixing an orange drink in water. That is not really dissolving. I do not think because so much solvent in the orange drink. So everyday example: Salt in water. Everyday life ... dissolving ... in the kitchen ... I found hard to come up with everyday examples.
3. Can you fill in the gap with a suitable word: “Dissolving...............................................................Melting.” a. Is there any word that related these two concepts? -
Dissolving is different from melting. I guess the word is solute in some ways it does in dissolving. I said you take the minority of substance put into majority of substance. The minority of substance is solute. In melting you do not have a solvent you just retain, you know, a substance which is called the solute you mean it changes its phase you change of a solid to liquid.
4. After dissolving the substances where do they go? -
Oh! They are distributed. The solute is distributed throughout the solvent with stirring distribution is even. You have the ions or molecules of the solute well separated from each other with a sphere of solute solvent molecules around it and indeed some non-attached molecules in between that after you stir it. You stir it after make it mix.
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5. Can you explain the difference solute and solvent? -
It is nature of quantity. I think in the first instance, the solute is the minority substance the solvent is the majority substance, in mole terms. I guess, as usually for a solvent to have always quite weak internal bonding, the molecules of solvent very weakly held together we can easily break up. Whereas the solute, often has very strong internal bonding but I think the primary difference for me the relative quantities. So could talk about for instance about dissolving concentrated HCl in water or you could talk about dissolving water in concentrated HCl. It depends which is the majority substance.
6. Can we retrieve the solute and solvent from the solution? How? -
Yes. You can get it back because the definition of solution is no permanent change takes place and the normal ways there are number of normal ways one of which is to distil of evaporate the solvent because it has weak intermolecular bonds moving the evaporate off leaving the solute behind. That is one way or you can increase the concentration of the solute in the solvent until the its saturation point and crystallise out the solute. Of course you do not get whole solute back if you do crystallisation because some of these always retain the solvent you have to keep going number of times in order to get back all the solute you put it.
a. Is there anything added or lost? -
If you do it by crystallisation you always lose certain amount. If you do it close substance you would do get all the solute back. You will just physically separate the solvent. Of course the definition of the solution is no permanent change takes places no in a narrow sense in chemical reaction you do get back the solute and solvent. The 178
point is in crystallisation you lose some of the solute and liquid. It is hard to get back pure solvent.
7. Do you think dissolving is chemical reversible change? -
That is interesting point isn’t it? I would think what is chemical change. I mean this is difficult question. I mean claimed to think it is chemical change because the ... present separating, regroup from each other and of course it is reversible that is the definition of the solution. So I think I probably would agree reversible chemical change. Yes. I would but historically it was not seen that way it has just seen that, a kind of mixing. Now we know more about the bonding that goes on inside. We are more able to say it is reversible chemical change.
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2.4 INTERVIEW WITH THE LECTURER D
1. Can you explain the concept of dissolving? -
It is a process where a solid and liquid are brought together and you end up with something that is liquid which in some sense has both of in it, say, the molecules that make up the solid are taken up into the liquid. It is often accompanied, well it is sometimes certainly accompanied by things like temperature change, so it must be in some sense a chemical process whether it is reaction but I never sure some reasons I don’t want to call it chemical reaction but certainly the constituency of the solid molecules separate, must become loosely bond the constituency of the liquid molecules, so that the combination of that particular temperature remains the liquid So, dissolving is the process where by solid is taken into a liquid and a combination of the liquid and solid remains liquid it is a chemical process but the resulting liquid has the properties of the original liquid and solid rather than something totally new.
2. Please give some examples from daily life? -
If you throw salt into water salt dissolves in water and if you boil eggs in that water some reasons already you get better boiled eggs. That is certainly common. The scattering of salt onto icy roads produce some sort of salt of solution which has lower melting temperature and so remains liquid. That is daily life here in Britain. I suppose we put sugar in coffee, things like that.
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3. Why some substances cannot dissolve in each other? -
We have the molecules of the solvent break up which probably suggested has some into the kind of bonding that they have to start with. So, I guess some solids are hard to dissolve because the molecular structure is hard to break up in the right way it is obviously get something to do with the molecules of the liquids as well and I guess there are some combinations where the equilibrium, energy levels of the bonds are such that it is lower energy to stay solid to stay separately Liquid it would be to combine.
4. Can you fill in the gap with a suitable word: “Dissolving..............................melting.” -
My first reaction is totally separate process. Dissolving is not melting is that I would put in the middle. I can see why the two are often confused in the sense that you start of with something it is solid and you end up something it is not solid. So, another sort of thing that can go on the middle could be dissolving could be confused with melting but I can’t think a word which connects the two, other than one isn’t the other but one could be confusable than the other.
a. Is there any word that related these two concepts? -
Melting is obviously related to phase, change of phase, latent energy, boiling, freezing, you know, those sort of words which indicate phase change, are and there is a sort of invariant concept of it is the same material and so molecular structure and so on, with dissolving we are talking about rather ... limited concept of in my mind but the words ion, sort of ionic come in to my mind the word of a mobility, I want to say chemical change but I’m using the word change there very strange way I don’t mean 181
it in the sense that one chemical becomes another chemical but I suppose, there is, I think of melting is a physical change I think dissolving is a chemical change or chemical process.
5. After dissolving the substances where do they go? -
Obviously, the liquid solvent where it is and the solute, a solid distributed through the liquid and again it depends what you mean by substances I suspect that if it is that, whatever makes up the molecule of the solid is not only distributed, it isn’t necessarily distributed as itself. That is make divide two or more parts each of which are distributed and therefore remain association with each other but not when it was a solid, it, how some structure were by let say it has got two parts, both parts would remain together all the time. When it say, in solution I would ... two parts are separated and ... around so that part A will associate with different parts B’s as it ... around. So, it becomes mobile, moves around, there was .... motion with in the liquid and therefore for passed the time, its parts must be associated with parts of the liquid. if you imagine yourself as a sort of little part of this thing in solution you don’t no longer got as a life partner you will not be with you, sort of obtain but rather you are wondering round you keep seeing other bees and you behave other associated with bees but the different ones and you are also saying ... the liquids and so you become associated with them as well.
6. Can you explain the difference between solute and solvent? - Well, solute is the solid bit, solvent is the liquid bit. It is my simple explanation and we are talk of the solute being dissolved in the solvent which is an interesting statement it is first, I never thought about that, that is way we use the word so it does 182
look as it is a symmetry that is actually may be much more symmetrical change in other words, you know two things become associated in much the same way but because the solute starts of the solid and solvent starts of the liquid and the solution is liquid you can see why people make the assumption that is some how rather it is the solvent that is capturing the solute and so, one is dissolving in the other but the first time in my life I have actually thought it may well be the case that some more symmetric than that if we forgot the physical phases, the original might actually on more in terms of there are two things which together make a solution.
7. Can we get back the solute and solvent from the solution? How? - Experimentally clearly yes, it must be able to. Okay well I understand the ....by evaporation so heat it up the usually the solvent which is its boiling point boils of and you condense it somewhere else that is common process, industrially elsewhere and you can clearly separate parts of it electrically so, and electrolysis will take things out of the solution you might have to then do something else to bring them together again and so it may involve further chemical reactions down the line before you get back the solute but crystallisation I mean stick in, crystals of the solute and until a solution get saturated and then start returning and you can crystallise about two. I suppose there is other processes where things go straight from liquid to gases which is slightly different from or could be. No, I suppose it is just another form of evaporation but I was thinking if you just sort of spreading out the surface in the latter on ... but basically it is another form of evaporation.
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a. Is there anything lost or added? - Does all set on energy changes? There is nothing in the process which destroys the nuclei, so in principle all the solute and all the solvent remain in one form or another. So, you should be able to get it all back, providing efficient methods of capturing, and of course I haven’t mentioned it but there is a notion of gas being dissolved in a liquid that might slightly be hard to make sure. You have actually contain dissolve so providing efficient Yes you can get all back.
8. Do you think dissolving is chemical reversible change? - Yes, I think it is chemical change and I think it is reversible. I keep worrying why I don’t want to say to chemical reaction probably my own limited view what chemical reaction is, I suppose you don’t usually write down equations I don’t usually write down equations for ... chemist do.
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APPENDIX 3.
TABLES OF EXPERTS’ INTERVIEWS
Dissolving Ionic compounds
Dissolving Covalent compounds
Generalised Dissolving
LECTURER
LECTURER
LECTURER
LECTURER
A
B
C
D
If substances are ionic the process will be chemical. Because the bonds between ions are broken.
If the substances are ionic, to identify the ions in the final mixture are very problematic.
The bonds of the ionic substance are broken and one single ion of solute is surrounded by a sphere of solvent molecules always held together electrostatically.
If substances are covalent, the process will be physical because no bonds are broken. We only have a mixture.
If the substances are covalent, in the final mixture the substances will be identifiable.
The bonds of molecular solute and molecular solvent are broken and the solute molecules attach themselves to the solvent molecules electrostatically.
The molecules that make up the solid are taken up into the liquid. The constituents of the solid molecules separate, must become loosely bound to. The constituents of the liquid molecules, so the combination at that particular temperature remains liquid.
Chemical Model
Non-chemical (dispersal) model Intermingling
Chemical model
Chemical model
-
185
LECTURER
LECTURER
LECTURER
LECTURER
A
B
C
D
Can be solid, liquid or gas. A substance that has a lower concentration.
The minority substance, in mole terms. A solute often has very strong internal bonding.
It is a solid.
Solute
The substance that has the smaller mole quantity.
Solvent
The substance that has a big mole quantity.
Liquid A substance that has a bigger concentration.
The majority substance. A solvent has always quite weak internal bonding. They can easily broken up.
That is capturing the solute. It is a liquid.
They are completely different concepts.
Dissolving is in someways like melting. In other ways it is not like melting. Both of their particles become more disordered. They are different processes. More than one substance is necessarily involved. Dissolving can be endothermic or exothermic. Only one substance is needed. Always an endothermic change.
Dissolving is different from melting.
Dissolving is not melting.
The minority substance is put into the majority substance.
It is a Chemical Change but not a Chemical Reaction. Ion, and mobility.
Only a phase change. We do not need a solvent.
It is a Physical Change. Same material, phase, change of phase, latent energy, boiling and freezing.
DissolvingMelting
Words associated with dissolving
Words associated with melting
-
Fire
186
LECTURER
LECTURER
LECTURER
LECTURER
A
B
C
D
Chemical change
For ionic substances it is chemical.
Yes.
Yes, I agree.
It is a chemical change but not a chemical reaction.
Physical change
For molecular substances it is physical.
Reversible change
Conservation of matter
Yes. By Distillation.
But historically it was seen as a physical change.
-
Yes. If we have just a single salt solution, by evaporation. If we have got mixture of salts, getting back the salts are very problematic.
They will be in another form, that is not lost, but maybe some ions of salt could be involved with water molecules and ions are evaporated.
Yes. By distillation, and crystallisation.
In universal (isolated system) nothing is lost. Energy may go. In an open system there could be something lost.
-
Yes. By evaporation, crystallisation and electrolysis but in electrolysis we need further processes to bring the substances together again.
We can get back all the substances back but in crystallisation we can lose some of the solute and liquid. It is hard to get back pure solvent.
Yes, we can get all back.
187
Examples
Drawings
LECTURER
LECTURER
LECTURER
LECTURER
A
B
C
D
Salt in cooking, Making refreshments different substances dissolve, the soap dissolves in water, the substances in alcohol(in refreshments) Water is not the only solvent. There are also some different solvents for example: In air a lot of gases dissolve in each other.
Sugar into the cup of tea, salt in water, nail varnish and nail varnish remover, using some petrol, paraffin, grease.
Salt in water.
Salt into water, salt onto icy road, and sugar in coffee.
Chemical model Particles detailed and general
Chemical Model Particles detailed
Chemical Model Particles detailed
Non-chemical model particles intermingled
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APPENDIX 4. EVALUATION OF EXPERTS’ INTERVIEWS
4.1 Evaluation of first interview Lecturer A has an opinion about dissolving that is a chemical and a physical process. According to her opinion in the chemical process, the substances are ionic and during the dissolving the bonds of the ionic substance are broken. On the other hand the molecular substances undergo physical process in which the bonds of molecules are not broken but molecules are separated from each other and they just mix with the solvent molecules. The examples of dissolving from daily life were salt in cooking, some refreshments, a soap in water, and some substances dissolve in alcohol in some drinks. She emphasised that as a solvent only water is seen but there is another big solution that is air in which a lot of different gases dissolve in each other. She believes that the concept of dissolving and the concept of melting are completely different. For melting the substance needs heat. After dissolving the substances are in the system but in another form. For example, at the beginning sugar has big pieces of particles, after dissolving the sugar the particles become so small and the system is circulated by water molecules. The difference between the solute and solvent was explained according to their mole quantity so the solute was accepted that has a small mole quantity. She believes that this is a useful convention that makes it easy for everyone to understand. The separation of the solute and solvent is possible by using a physical method but it depends on the substances (water and salt). She thought that for some complex
189
solution there is a need for some other processes, for example, the separation of oxygen from air. When we are separating the solute from solvent there is no loss. However she said that some of ions which are involved with water molecules may be lost, and that, some ions may be evaporated.
In her opinion, dissolving is a chemical process for ionic substances because the bonds of ionic substances are broken. On the other hand, it is reversible but after the ‘undissolving’ process the crystals are not as same as the before because their sizes are smaller than the crystals of the beginning. I think, she wants to stress that after undissolving process the structure of the ionic substance is not as same as the beginning because in the regular pattern of the ionic crystal some ions may not locate and in the structure we may have some spaces that have not accommodated by the ions. So it is so difficult to retrieve the same size of the crystals.
190
4.2 Evaluation of second interview He explained dissolving by the dispersal model. He believes that in dissolving, the solute particles intermingle with the liquid particles. As for ‘solvent’ he states that solvents are only liquids. The examples of dissolving are sugar into a cup tea, salt in water, nail varnish,
nail
varnish
remover,
using
of
petrol,
paraffin
and
grease.
It is interesting that, he mentioned that dissolving and melting are similar because for in both of them the particles become more disordered. He emphasised that there are two things that characterise dissolving. The first one is the number of substances. In dissolving more than one type of particle is needed. On the other hand, in melting only one substance may be involved. Secondly, the energy change is different for dissolving and melting. Melting always will be an endothermic change but dissolving may be either. According to his opinion, in the dissolving process, after the dissolving of covalent substances (sugar in water) we can identify the original substances but for ionic substances he accepts that it is problematic and it is very difficult to identify the substances especially if there are two different ionic salts. The difference between solute and solvent was explained that if we have the solutions of solid, liquid or gas the solvent is liquid. If the solution is solid-solid solution at hat time it is very difficult to decide “which is which?” He said that for easy cases the solute has generally lower concentration than solvent. The separation of the constituents of a solution is possible and easy for single salt solutions by evaporation but the separation of a mixture of salts is more problematic.
191
In the universal (isolated system) there is nothing lost but in an open system there could be something lost. Also he mentioned that energy may lose in the latter case. In contrast to his explanation about the concept of dissolving using the dispersal model he believes that dissolving is a chemically reversible change.
192
4.3 Evaluation of third interview He explained that solid, liquid and gas can dissolve any of the others. He emphasised that in the dissolving process the bonds of solute molecules or ions are broken. Also, the bonds of the solvent molecules are broken and the solute particles attach themselves to the molecules of solvent electrostatically. The solute and solvent particles come together and energy is released. So, the process is driven by energy. Single ions or single molecules of solute are surrounded by a sphere of solvent molecules, normally held together electrostatically. Salt in water is an example from daily life for dissolving. However, he emphasised that solid, liquid and gas can dissolve any of the others. He confused the concepts of dissolving and mixing. He said that liquid in another liquid is mixing rather than dissolving. He stated that dissolving is different from melting. In melting we do not have a solvent and the solute’s phase changes from a solid to liquid by melting but in dissolving we have solute and solvent. The answer to the third question was the dispersal model. He said that, after dissolving, the particles are distributed. The solute particles are distributed throughout the solvent. The difference between solute and solvent primarily depends on the their quantity, which can be in moles or by volumes. The minority substance is a solute and the majority substance is a solvent. Also, according to his opinion, a solvent always has weak internal bonding and solute often has very strong internal bonding.
193
He stressed that, because “the definition of solution states that no permanent change has taken place, we can get back the solute and solvent by distillation of solvent or by crystallisation. In crystallisation, we always lose a certain amount. We have to do crystallisation many times in order to get all the solute back. But, he added that getting back the pure solvent back is very hard. He agreed that dissolving is a chemically reversible change and he emphasised that, historically, "it was seen as a kind of mixing".
194
4. Evaluation of fourth interview Lecturer D believes that dissolving is a chemical process but he does not want to call it a chemical reaction because in his words “it is the process where by solid is taken into a liquid and a combination of the liquid and solid remains a liquid. It is a chemical process but the resulting liquid has the properties of the original liquid and solid rather than something totally new.” Also, gas dissolves in a liquid. The examples of dissolving are salt in water, salt on the ice, sugar in coffee. For third question his explanation was that some solids are hard to dissolve because of their molecular structure. Also, he believes that equilibrium and the energy levels of the bonds may affect dissolving. He determines that dissolving is not melting. The words which are related to melting are change of phase, latent energy, boiling, freezing, same material, molecular structure. On the other hand, ion, mobility, and chemical change are related to dissolving. He again stresses that chemical change is a very different concept from dissolving, and that melting is a physical change and dissolving is a chemical change. A solute is distributed through the liquid but he emphasised that it depends on the substances. They are not only distributed but they are divided into two or more parts and each of them remains associated with each other. It becomes mobile, moves around and associates with others. In simple explanation of solute is that it is the solid bit. Solvent is the liquid bit. By evaporation, condensation, crystallisation, and electrolysis it is possible to get all of the solute and solvent back but after electrolysis there is a need to bring the substances together.
195
APPENDIX 5
THE RESPONSES OF SCIENCE SPECIALIST STUDENTS
5.1 Ideas about dissolving in general
Students
S1 S2
The responses of students A substance is dispersed within and throughout another substance. A solute is dispersed in a solvent this can be either a chemical reaction or a natural effect of dispersion (entropy).
S3
It is where you put a solid sort of particles of solid into a solution into a liquid. They suspencially in the liquid.
S4
It is a physical process whereby a solvent surrounds a solid to make a solution. If the process is ‘hydration’ water molecules physically surround the atoms of the solid. If ‘hydrolysis’ occurs, the solvent actually reacts with the solid.
5.2 Ideas about the constituents of a solution
S1 S2
S3 S4
SOLVENT
SOLUTE
A solvent will allow a solute to dissolve unit. Water is a good solvent. Solvent is a substance into which a solute disperses. The solvent is the substance which ‘accepts’ the most of the other substance. Solvents are often liquids or gases. The solvent is the thing you put in the or you dissolving something into. i.e. solvent is tea. The solvent is the substance which actually does the dissolving.
The solute is the substance which is ‘accepted’. Solutes are often solids.
The solute what you put in. i.e. solute is sugar. The solute is the product from solid and solvent.
196
5.3 Ideas about dissolving and melting
S1 S2 S3
S4
DISSOLVING AND MELTING
WORDS ASSOCIATED WITH DISSOLVING
WORDS ASSOCIATED WITH MELTING
dissolving is similar to melting dissolving is not melting dissolving is completely different from melting
Energy
energy
-
-
different concentration does not need heat
same concentration need heat
Two species (Solvent + SolidSolution)
Only one substance. Physical change of state due to energy breaking bonds within a solid lattice
dissolving is not the same as melting
5.4 Ideas about the process of dissolving Chemical change
Physical Change
Reversible Change
Conservation of matter
S1
-
Yes
Yes
Yes, by crystallisation
S2
Yes
Yes
Yes
Yes, by evaporation
S3
-
Yes
Yes
S4
No
Yes
Yes
Yes, by boiling, but not always Yes, by distillation, evaporation and chromatography
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5.5 Examples
EXAMPLES
S2
Sugar in water, salt in water, ink in water, gold in platinum (gold ring), coffee in water, tea in water and jelly in water. Salt in water, detergent in water, and sugar in tea.
S3
Sugar in tea, dirt in a puddle in water
S4
Sugar cube in hot cup of tea, bath crystals in bath of water
S1
5.6 Drawings
DRAWINGS S1 S2 S3 S4
Non-chemical model- solute is particle, solvent is continuous- intermingled and Chemical model, particles- detailed Non-chemical (dispersal model) solute and solvent are particles intermingled and Chemical model, particles- general No drawing Chemical model Solute and solvent particles- detailed
198
APPENDIX 6
THE RESPONSES OF FIRST YEAR SCIENCE SPECIALIST STUDENTS
6.1 Ideas about dissolving in general S1- A soluble [SOLUTE] is mixed in a solution [SOLVENT], the solution [SOLVENT] molecules push between the molecules of soluble [SOLUTE], due to the comparable weakness of its bonds. S2- An ionic substance e.g. NaCl(s) is added to a solvent and changes to Na+(aq) and Cl-(aq). S3- The solid is suspended in the water. The solid molecules move in between the water molecules. S4- The water molecules and solid molecules mix together to form a solvent [SOLUTION]. The solid molecules get in between the water molecules. S5- Particles that make up the substance separate from each other into ions. These are negatively and positively charged and are attracted to different ends of the water molecule. S6- The molecules are broken down, the bonds are broken. They can be reformed with the solvent. S7- THIS ANSWER WAS GIVEN TWICE. S8- Atoms or molecules form bonds with solute molecules or atoms and the bonds in the solid break. S9- It is the complete absorption of a compound in a solvent.
199
6.2 Ideas about the constituents of a solution SOLUTE and SOLVENT: S1- Solute is a substance, solvent is the medium (usually fluid). S2- Solvent is liquid in which the solid-solute is added. e.g. water-salt S3- Solute is the substance that dissolves and solvent is the liquid that contains the solid. S4- Solute is what dissolves, solvent is the liquid that contains the solute. S5- Solute is the substance which dissolves, solvent is the substance in which the solute dissolves. S6- Solute is what is formed when a substance dissolves solvent is what dissolves the substance (Solvent = solute- substance). S7- Solute: When a substance dissolves in solution, solvent is what dissolves the substance. S8- Solute is the substance in solution, solvent is the liquid in which the solute is dissolved. Solute is ionic substance, solvent is water. S9- Solute is the matter to be dissolved (sugar), solvent is the medium in which the solute is to be dissolved (water).
200
6.3 Ideas about dissolving and melting DISSOLVING AND MELTING
WORDS ASSOCIATED WITH DISSOLVING
WORDS ASSOCIATED WITH MELTING
S1
Do not know
-
-
S2
Dissolving is not same as melting no response
ionic substance
-
liquid
liquid
dissolving is not melting no response
liquid
liquid
bond breaking
bond breaking
bond breaking
bond breaking
bond breaking
bond breaking
liquid
liquid
-
-
S3 S4 S5 S6 S7 S8 S9
dissolving is not the same as melting Dissolving is not the same as melting dissolving is different from melting dissolving is not same as melting
6.4 Ideas about the process of dissolving Chemical change
Physical change
Reversible changes
conservation of matter
S1 S2 S3 S4
not always Yes Yes
-
not always Yes Yes Yes
S5
Yes
-
Yes
S6
Yes
-
Yes
S7
Sometimes
-
Sometimes
S8 S9
Yes Yes
-
Yes Yes
Yes, by evaporation Yes, by evaporation Yes, by evaporation Yes, by boiling and condensation Yes, by evaporation and condensation Yes, by filtration and evaporation Not always but sometimes by evaporation, filtration and crystallisation Yes, by evaporation Yes, by evaporation
201
6.5 Examples Students S1 S2 S3 S4 S5 S6 S7 S8 S9
EXAMPLES sugar in tea, and salt in cooking sugar in tea, and salt in water for cooking sugar in tea or coffee, salt in cooking and bath salts sugar in tea, and salt in boiling water salt in water, sugar in water, and coffee granules in water sugar in tea, salt in water, bath salts and salt in sea sugar in tea, bath salts and salt in water sugar in tea or coffee, salt in water sugar in tea, washing powder in water salt in water
6.6 Drawings Students
DRAWINGS
S1
Non-chemical (dispersal) model, particles- Intermingled
S2
Non-chemical (dispersal) model, particles- Intermingled
S3
Non-chemical (dispersal) model, particles- breaking bits and Chemical model, solute and solvent particles- detailed
S4
Chemical model, solute and solvent particles- detailed
S5
Non-chemical (dispersal model), solute is particle, solvent is continuousdisappear
S6
Non-chemical (dispersal) model, solute is particle, solvent is continuous- disappear
S7
Non-chemical (dispersal) model, solute is particle solvent is continuous- disappear
S8
Chemical model, solute and solvent particles- detailed
S9
Non-chemical (dispersal) model, solute is particle solvent is continuous- disappear and Chemical model, solute and solvent particles - detailed
202
APPENDIX 7 THE RESPONSES OF NON-SPECIALIST STUDENT TEACHERS 7.1 Ideas about dissolving in general PHYSICAL CHANGE S1- Disappearing in a liquid. Solid form which turns to liquid.
CHEMICAL CHANGE S4- Particles from the solute split up and bond together.
S2- Making a powder turn to liquid.
S22- Break down of a solute in a
S3- Solid breaking down and being
solvent –to form a solution- can be
incorporated into liquid. S5- A solid form disappears into another
chemical change or an energy spread.
form. S6- Break down of substance in liquid. S7- Dissolving involves a solid combining with a liquid to make a different liquid. S8- A substance ‘disappears’-but is still in whatever it was dissolved in- you just cannot see it. S9- A substance like sugar disappears in hot water, its molecules are changed from solid to liquid, but remain in the liquid. S10- An item changing its matter, becoming invisible. S11- Solid into liquid form. S12- A substance joining with a liquid. S13- A solid /liquid or gas changes into another form. S14- There are two ideas: A substance is put into a liquid. The substance either combines with the liquid or the substance spreads out in the liquid. S15- A substance is absorbed within another. S16- Adding water.
203
S17- Soluble things disappear in water. S18- A soluble substance is disappeared into a solution. S19- Disappearing in water. S20- When solids dissolve into liquid. S21- A solid becomes part of a liquid. S23- Solid forms into a liquid. S24- A solid’s molecules disperse in a liquid. S25- A solid changes into a liquid. S26- A solid substance breaking up to form part of a liquid. S27- A solid is placed in a liquid and it dissolves and becomes a liquid. S28- Solid breaking down and being incorporated in a liquid. S29- A solute with disperse into a solvent until saturation point. S30- A particle breaking up in a liquid. S31- When something mixes in completely with other substances. So, it appears to have disappeared. S32- Dissolving is where something solid is made into something liquid. S33- Solid disintegrates into the liquid. S34- It melts into the solution. S35- Solid becomes liquid. S36- Dissolving is when the substance Disappears or melts in the liquid. S37- A solid changes in to a liquid. S38- A solid becomes part of a liquid. S39- When one solid item breaks down into a liquid until no longer visible.
204
7.2 Ideas about the constituents of a solution SOLVENT
SOLUTE S2 S3 S4 S6
the powder the solid that has dissolved a substance which dissolves in liquid
S6 S7 S8 S9 S14 S15 S18 S21 S22 S23 S24
liquid what it does the liquid, it has dissolved into the material that is of the most quantity a substance what it is dissolved in the occurring substance it dissolves ‘thing’ in which the solute dissolves solution solution not soluble the medium in which it dissolves solid smell
S7 S8 S9 S15 S18 S21 S22 S23 S24 S25 S26
S25
solid
S27
S26
a substance can be dissolved
S29
S27 S30
S30 S31 S33 S34
solute is the substance to be dissolved solute is the solid to be dissolved
S34
used to dissolve a solid a solvent is the liquid used to dissolve a solvent is the solution solvent is when it [solute] joins into the liquid solvent is the liquid
what is dissolved what we put in the chemical we put it in ingredient the substance soluble the substance that dissolves liquid liquid liquid something has been dissolved in a substance the end product when a solid is dissolved a solid or fluid which has equal polarity to the solvent to cause the breaking a solute is something which is soluble solute is something which is soluble
S35
S35
liquid used to dissolve a solid
S36
S36 S37
solvent is the water liquid before saturation
S37 S38
S38
the solvent is the liquid which causes it to dissolve solvent is the liquid
S39
solute is solution saturated with solid (salt water - sea) the substance to be dissolved is the solute solid to the dissolved the solute is the substance which dissolves solute is the hard substance
S1 S2 S3 S4
S31 S33
S39
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APPENDIX 8
INTERVIEWS WITH TECHNICIANS
8.1 INTERVIEW WITH T1
1. Can you explain the concept of dissolving? - The way I think of dissolving is to add solute to a liquid and usually stirring it would dissolve into the liquid and it is usually temperature dependent.
2. Please give some examples from daily life? - The usual one is that sugar in tea, stir the tea dissolve the sugar. Salt when we are cooking.
3. Can you fill in the gap with a suitable word: “Dissolving..............................melting.” - I cannot actually think a word relating these. Dissolving I think a connection with a liquid and a solid. Melting just needs heat. a. Is there any word that relates these two concepts? - A solid is involved for both but liquid is also involved in the dissolving.
4. After dissolving the substances where do they go? - They taken up by the liquid. The solute that would be the solid that you are going to dissolve in the solvent. 206
5. Can we get back the solute and solvent from the solution? How? - Let the liquid evaporate and collect the solid. a. Is there anything lost or added? - If you collect the evaporating vapour, you should be able to get both of them back exactly but you have to devise a system this is good enough to collect them.
6. Do you think dissolving is chemical reversible change? - It is physically reversible change.
7. Can you explain “dissolving” drawing pictures?
- No. I cannot.
207
8.2 INTERVIEW WITH T2
1. Can you explain the concept of dissolving? - When you take one substance and put into another, particles disappear because they spread out, getting smaller. - Particles? Molecules? - Not molecules but the particles what you are dissolving get smaller and disappear.
2. Please give some examples from daily life? - Sugar in tea, bubbles gas in lemonade drinks, using alcohol to get grease, to get ... stain.
3. Can you fill in the gap with a suitable word: “Dissolving..............................melting.” - I cannot think. I cannot really make sentence. If you heat something it will melt. If you want to dissolve something you can heat to dissolve it not always but ... They are not same. If you try to melt something like ice, you just one substance. It stays same that becomes liquid. It is changing from solid to liquid often. In dissolving I think the solid's particles are still there you cannot see them. a. Is there any word that relates these two concepts? - Heat and temperature. 4. After dissolving the substances where do they go? - They do not go anywhere. They are still there. They just spread out.
208
5. Can you explain the difference between solute and solvent? - The solute is the substance that you want to dissolve and the solvent is the substance in which you are dissolving. 6. Can we get back the solute and solvent from the solution? How? - By evaporating. a. Is there anything lost or added? - No. It should stay the same.
7. Do you think dissolving is chemical reversible change? - Yes.
8. Can you explain “dissolving” drawing pictures? - No. I cannot.
209
8.3 INTERVIEW WITH T3
1. Can you explain the concept of dissolving? - Basically, it is a solid in the liquid making the solution, simply.
2. Please give some examples from daily life? - Sugar in tea, salt in vegetables when you are cooking in the water.
3. Can you fill in the gap with a suitable word: “Dissolving..............................melting.” - Dissolving is similar to melting. Both need, heat dependent, temperature dependent. a. Is there any word that relates these two concepts? - I think temperature because solubility goes up with temperature mostly and melts quicker ... melt ... temperature ...
4. After dissolving the substances where do they go? - They diffuse through the liquid. Not everything will dissolve depend on the solubility.
5. Can you explain the difference between solute and solvent? - Solvent is the liquid. Solute which is the dissolving in.
210
6. Can we get back the solute and solvent from the solution? How? - Yes. By evaporating. a. Is there anything lost or added? - You will lose all the water. I think you get most of get back. You probably get it all of back I mean, water can, heat it and you evaporate you are going to the water will evaporate and leave the salt, all of it I think.
7. Do you think dissolving is chemical reversible change? - Yes. It must be reversible if you can get back. I think it must be physical.
8. Can you explain “dissolving” drawing pictures? - No, I cannot.
211
APPENDIX 9. TABLES OF TECHNICIANS’ INTERVIEWS
T1
T2
T3
Dissolving Ionic
-
-
-
Dissolving Covalent
-
-
-
Generalised Dissolving
Non-chemical (dispersal) model (The solute is taken up by the solvent.)
Non-chemical (dispersal) model (Particles disappear because they spread out, getting smaller.)
Non-chemical (dispersal) model (After dissolve the solute substances diffuse through the liquid.)
Solvent
Liquid
The solvent is the substance in which we are dissolving.
Solvent is the liquid.
Solute
Solid
The solute is the substance that we want to dissolve.
Solute which is dissolving in the solvent.
DissolvingMelting
-
They are not same.
Dissolving is similar to melting.
Words associated with Dissolving
Dissolving has a connection with a liquid and a solid
Heat and temperature In dissolving solute particles are still there but we can not see them.
Heat, and temperature.
Words associated with Melting
A solid is needed in Melting. It just needs heat.
Heat and temperature. Melting needs a heat. It has just one substance. It becomes a liquid. There is a change from solid to liquid.
Heat and temperature.
212
T1
T2
T3
Chemical Change
No
Yes
No
Physical Change
It is Physical Change.
-
It must be Physical Change.
Reversible change
Yes. Let the liquid evaporate and collect the solid
Yes, by evaporation
Yes, by evaporating
Conservation of matter
Yes, but we have to devise a system, this is good enough to collect them.
Yes. It should stay the same.
Yes, we probably get all of it back.
Examples
Sugar in a tea, and salt in water.
Sugar in Tea, Bubble gas in lemonade drinks, using alcohol to get grease, to get stain
Sugar in tea, salt in vegetables when we are cooking in the water.
Drawings
No drawing
No drawing
No drawing
213
REFERENCES
Ahtee. M., (1993) “Finnish Pupils’ Ideas of Dissolving Salt in Water”, in “Proceedings of the Second European Conference on Research in Chemical Education (ATTI)”, Editors: Bargellini. A. & Todesco. P.
E., Second European Conference on Research in
Chemical Education, Italy. Andrews. D. H., & Kokes. R. J., (1962), “Fundamental Chemistry”, Jonh Wiley & Sons, Inc., London. Atkins. P. W., (1989) “General Chemistry”, Scientific American Books, New York. Bargellini. A., Lardicci. L., Mannelli. M. & Raspi. G., (1993) “Middle School Students’ Alternative Frameworks about the Learning of Change of State, Solution and Chemical Reaction; A View from Italy”, in “Proceedings of the Second European Conference on Research in Chemical Education (ATTI)”,
Editors: Bargellini. A. &
Todesco. P. E., Second European Conference on Research in Chemical Education, Italy. Barnea. N., & Dori. Y. J., (1996), “Computerised Molecular Modelling as a Tool to Improve Chemistry Teaching”, Journal of Chem. Inf. Comput. Sci., 36, 629-636. Baum. S. J., Scaife. C. W. J., (1975) “Chemistry: A Life Science Approach”, Macmillan Publishing Co., Inc., New York.
214
Borges A. T. (1997), “Models of Magnetism”, in “Exploring Models and Modelling in Science and Technology Education”, MISTRE Group, The University of Reading, Reading. Brady. J. E., & Humiston. G. E., (1986), “General Chemistry-Principles and Structure”, John Wiley & Sons, New York. Brock. W. H., (1992), “The Fontana History of Chemistry”, Fontana Press, London. Brockington. J., Stamper. P., Browning. D. R., & Skinner. A. C., (1981), “Combined Chemistry”, Longman Press, Essex. Brown. W. H., & Rogers E. P., (1980), “General, Organic and Biochemistry”, Willard Grant Press, Boston. Burton. G., Holmon. J., Pilling. G., & Waddington. D., (1994), “Salters Advaced Chemistry- Chemical Ideas”, Heinemann, Oxford. Buttle. J. W., Daniels. D. J., & Beckett. P. J., (1981), “Chemistry: A Unified Approach”, Butterworth & Co. Ltd., London. Chiswell. B., & James. D. W., (1969), “Fundamental Aspects of Inorganic Chemistry”, John Wiley & Sons Australasia Pty Ltd., London. Garvie. D., Hughes. J., Reid. J., & Robertson. A., (1979), “Concise Chemistry”, Oxford University Press, Oxford. Gilbert. C., (1990), “Models for Primary Science and Technology Education”, Longman Group Resources Unit, York.
215
Gilbert. J. K., (ed.) (1993), “Models & Modelling in Science Education”, Association for Science Education, Hatfield. Gilbert. J. K., & Boulter. C., (1995a), “The role of models and modelling in some narratives of science learning”, Paper presented at the Annual Conference of the American Educational Research Association, San Fransisco, 18-22 April. Gilbert. J. K., Boulter. C., & Rutherford. M., (in press, 1997a), “Models in Explanations, Part 1: Horses for Courses?”, International Journal of Science Education. Gilbert. J. K., (1997c), “Models in Science and Science Education”, in “Exploring Models and Modelling in Science and Technology Education”, MISTRE Group, University of Reading, Reading. Gray. H. B., Simon. J. D., & Trogler W. C., (1995), “Braving the Elements”, University Science Books, Sausalito, California. Greenstone. A. W., & Harris. S. P., (1975), “Concepts in Chemistry”, Harcourt Brace Jovanovic, London. Hairison. R. M., de Mora. S. J., Rapsomanikis. S., Johnston. W. R., (1991), “Introductory Chemistry for the Environmental Sciences”, Cambridge University Press, Cambridge. Hägg. G., (1969), “General and Inorganic Chemistry”, John Wiley & Sons, Inc., London.
216
Holding. B., (1987), “Investigation of School Children’s Understanding of the Process
of
Dissolving
with
Special
Reference
to
the
Conservation of Matter and the Development of Atomistic Ideas”, Unpublished PhD Thesis, University of Leeds. Holum. J. R., (1975), “Elements of General and Biological Chemistry”, John Wiley and Sons, Inc., London. Hudson. J., (1992) “The History of Chemistry”, MacMillan, Basingstoke. Hughes. D. E. P., & Maloney. M. J., (1964), “Advanced Theoretical Chemistry”, Chatto & Windus Ltd., London. Johnstone. K. & Scott. P., (1991), “Diagnostic Teaching in the Science Classroom: Teaching/
Learning
strategies
to
promote
development
in
understanding about conservation of mass on dissolving”, Research in Science & Technology Education, 9 (2), 193-212. Kneen. W. R., Rogers. M. J. W., Simpson. P., Contributing ed. Millen. D. J., & Nyholm. S. R., (1985), “Chemistry Facts, Patterns and Principles”, Longman, Essex. Longden. K. A., (1984), “Understanding of Dissolving shown by Eleven to Twelve Year Old Children”, Unpublished MSc Thesis, University of Oxford. Longden. K., Black. P., & Solomon. J., (1991) “Children’s Interpretation of Dissolving”, International Journal of Science Education, 13 (1) 59-68. Mahan. B. H., (1980), “University Chemistry”, Addison-Wesley, London.
217
Millen. D. J., (1969), “Chemical Energetics and the Curriculum”, Wm. Collins Sons & Co. Ltd., London. Mortimer. C. E., (1988), “Modern Universite Kimyasi” (Translated by Altinata. T., Nisli. G., Akcay. H., Anil. H., Celebi. S., Avcibasi. H., Toprak. M., Balkose. D., Henden. E., Toscali. D., Yenigul. B., published by Caglayan Kitabevi, Istanbul). Nebergall. W. H., Schmidt. F. C., Holtzclaw. H. F, Bailar. J. C., (1968), “General Chemistry”, D. C. Heath and company, Lexington. Nusirjan & Fensham. P., (1987), “Descriptions and Frameworks of Solutions and Reactions in Solutions”, Research in Science Education, 17, 139148. O’Connor, R. F., (1974), “Chemical Principles and Their Biological Implications”, Hamilton Publishing Company, California. Ogborn. J., Kress. G., Martins. I., & McGillicuddy. K., (1996), “Explaining Science in the Classroom”, Open University Press, Buckingham. Oversby. J., (1997), “Explaining Dissolving- The concept of a satisfying explanation”, Paper presented to the Annual Conference of The Association of Science Education Tutors, Reading, April, 1997. Prieto. T., Blanco. A., & Rodriguez. A., (1989), “The Ideas of 11 to 14-Year-old Students about the Nature of Solutions”, International Journal of Science Education, 11 (4), 451- 463. Radel. S. R., & Navidi. M. H., (1990), “Chemistry”, West Publishing Company, New York.
218
Sharpe. A. G., (1964) “Solubility Explained”, in “Education In Chemistry”, 1 (2), 75-82. Silberberg. M., (1996), “Chemistry: The molecular Nature of Matter and Change”, Mosby-Year Book, Inc. Missouri.
219
