Hands-On Lab

Hands-On Lab Cohesion and Surface Tension of Water Timing: one 90-minute class session Objective(s): Students will investigate the properties of water including surface tension and cohesion. Safety Precautions: Remind students to wear closed-toe shoes and not to eat or drink anything in the lab (including the experimental materials). Tell students to report any broken glass and not to try and clean it up by themselves. Materials: Per group: • coin (penny, dime, or nickel) • 25 mL graduated cylinders, 2 • 200 mL beaker • eyedroppers, 4 • liquid soap or dish detergent in a small container • distilled water • cooking oil • food coloring • glass stirring rod • stopwatch or timer Teacher Preparation: Materials can be gathered one day prior to the lab. Prepare a copy of the Student Investigation Sheet for each student. Procedure: The Hands-On Labs include both Directed and Guided Inquiry approaches. If your students are new to the investigational methods being used in the Hands-On Lab, it is recommended that the Directed Inquiry approach be used to provide scaffolding that will ensure student safety and support the success of their investigations. Often, the Directed Inquiry approach involves modeling the basic laboratory techniques and methods to be used in the activity. A discussion of each step in the investigative process will also be included. In some cases, students may then be asked to create a procedure based on the one modeled for them. This may involve changing specific variables or adjusting the procedure to determine the effect on the outcome.

You may choose to use the Guided Inquiry path on its own or after completing the Directed Inquiry activity. During Guided Inquiry, students are allowed to conduct the investigations more independently. They will be given opportunities to formulate their own questions, develop their own procedures, and/or manipulate variables of their own choosing. It may be necessary to provide additional materials and supplies for students using Guided Inquiry. It will also be important to set clear limits on students’ activities to ensure their safety and the relevance of their inquiry experience to the content you are teaching. Directed Inquiry Briefly review the properties of water with students. You may want to draw a water molecule on the board and discuss the polarity of water molecules. Have students work in their lab groups to answer the following questions in their notes, and then review the answers as a class. Students may refer to the notes they recorded during the Explore section of the Water Model Lesson to help them with the answers: 1. What type of bond holds oxygen and hydrogen atoms together within a water molecule? 2. What types of charges are found on the oxygen and hydrogen atoms of a water molecule? 3. What type of bond holds one water molecule to another? 4. Is this bond stronger or weaker than the bonds within water molecules? Discuss the meanings of the terms “hydrophobic” and “hydrophilic.” Molecules of water are strongly attracted to one another because they are polar. On the other hand, oil molecules are nonpolar. They are considered hydrophobic, or “water fearing.” While water readily dissolves many substances, it does not dissolve oil or other nonpolar substances well. It cannot form hydrogen bonds with nonpolar substances. Also, discuss the chemical properties of soap, listing properties on the board that are relevant to the lab exercise: Detergent (soap) molecules have both polar and non-polar regions. One end of a molecule of soap is hydrophilic and is attracted to water. Another end is hydrophobic and is attracted to oil. Soap and other surfactants allow two liquids to mix that do not ordinarily mix. Soap and water clean oil from the skin because soap surrounds oil molecules, its nonpolar side facing the oil and its polar side facing the water. In this way, small oil droplets can be suspended in water and removed. A mixture such as this, of liquids that do not ordinarily mix is called an emulsion.

Students will work in groups of three or four for both lab exercises. Review the steps for each exercise before groups conduct the exercises, modeling each step if possible. Surface Tension Part 1:

1. Determine the volume of one drop of water by filling a graduated cylinder with 10 mL of water and counting the number of drops required to reach a volume of 11 mL. 2. Calculate the volume of one water drop by dividing 1 mL by the number of drops required to reach 11 mL. For example, if 5 drops are required to reach 11 mL, divide 1 by 5, which is 0.2 mL. Thus, there would be 0.2 mL in one drop of water. 3. Record this calculated volume in your notes. 4. Predict in your notes how many drops of water can fit on the surface of a coin without spilling over the sides of the coin. Write your prediction as “number of drops” and as milliliters. 5. Test your prediction by carefully placing drops of water, one at a time, onto the surface of the coin. Be sure not to touch the surface of the coin (or the water drops already on the coin surface) with the eyedropper. Also be sure to keep count of the water drops. Stop when the final drop of water placed on the coin runs over the sides of the coin. Do not count this drop in your results. 6. Record the results in a data table as “number of drops” and as milliliters. Next, have students predict whether or not (and how) their results might change in the presence of a detergent. They should predict how many drops can fit on a coin that has been covered with liquid soap. After students have formed their predictions, review the next set of procedures, which is similar to the previous set of procedures. Part 2: 1. Using a clean eyedropper, place a small drop of liquid soap onto the surface of a coin and use the eyedropper to spread the soap so that it covers the entire surface. 2. Carefully place drops of water, one at a time, onto the surface of the coin. Stop when the final drop of water placed on the coin runs over the sides of the coin. Do not count this drop in your results. 3. Record the results as “number of drops” and as milliliters.

Students will now study the cohesive properties of water in the presence of a hydrophobic substance— oil. Review the steps below: Cohesion Part 1: 1. Fill a graduated cylinder with 8 mL of distilled water. 2. Predict the effect of adding 2 mL of cooking oil to the water. 3. Use another graduated cylinder to measure 2 mL of oil. 4. Slowly add the cooking oil to the water, slightly tilting the cylinder holding the water. 5. Record your observations. 6. Fill the empty graduated cylinder with 8 mL oil. 7. Predict the outcome of adding 2 mL water. 8. Use a clean graduated cylinder to measure 2 mL of water. 9. Slowly add the water to the cooking oil, slightly tilting the cylinder holding the oil. 10. Record your observations.

Next, have students predict the effect of adding food coloring to the water + oil mixtures. Then review the steps below: Part 2: 1. Add three or four drops of food coloring to each cylinder of water + oil. 2. Stir vigorously, making sure none of the liquid spills out. 3. After waiting approximately two minutes, record your observations. Finally, have students predict the effect of adding drops of water and liquid soap to water and stirring the mixture. Then review the steps below: Part 3: 1. Fill a beaker with 150 mL water. 2. Using a clean eyedropper, place three or four drops of oil on top of the water. 3. Using another clean eyedropper, add three or four drops of liquid soap and stir to mix thoroughly. 4. Record your observations.

Guided Inquiry For a guided inquiry, explain to students that they will be investigating the surface tension and cohesion of water. They may design their experiments based on their knowledge of these properties of water and the materials used in the lab. Ask students some guiding questions to help them focus their inquiry: 1. What type of bond holds oxygen and hydrogen atoms together within a water molecule? 2. What type of bond holds one water molecule to another? What is the reason for this bond’s attraction? Is this bond stronger or weaker than the bonds within water molecules? 3. What are the main chemical properties of soap (detergent)? 4. How can you test the surface tension of water using a coin? 5. In the coin experiment, what will be the independent and dependent variables? 6. How can you demonstrate water cohesion using oil, dye, and detergent?

Students should work in their lab groups to design two experiments: one for surface tension and one for cohesion. Their experiments should utilize all of the materials provided. Allow students to conduct Internet research to determine the chemical properties of detergent (soap) related to its molecular polarity. Students should receive approval prior to beginning their investigations. Help students test their questions and predictions by making sure they use a logical progression of experiments. For instance, make sure they test the mixture of oil and water before adding a detergent. Have students record their data using tables. They can design their tables independently, but should make space to include their results as mostly observations. Analysis and Conclusions: In order to help students analyze and interpret their results, consider discussing some or all of the following questions or assigning them as homework:

1. How did your predictions compare with your results? What might explain discrepancies, if any? Answers will vary. 2. What type of chemical attraction is responsible for both the high surface tension and cohesion of water? With respect to this chemical attraction, why is oil unable to dissolve in water? Hydrogen bonds, resulting from the polarity of water molecules, are responsible for the high surface tension and cohesive properties of water. Oil cannot dissolve in pure water because it does not form hydrogen bonds with water molecules (since it is a non-polar molecule). 3. How and why did the liquid soap change the number of drops that could fit on the coin’s surface? How and why did it change the association between oil and water? Detergent molecules have both polar and non-polar regions, allowing it to bind to both water and oil. The detergent disrupts hydrogen bonding between water molecules, allowing fewer water molecules to stay bound together on the surface of the coin before spilling over. The detergent also brought water and oil together, forming an emulsion solution of water, detergent, and oil. 4. Using the Internet or other resources to learn about the relevant chemical properties of ethanol, answer the following question: Compared to water, how much ethanol would you expect to fit on the surface of a coin: more, less, or the same amount? Why? Ethanol molecules also have a polar O-H bond and can form hydrogen bonds with other ethanol molecules, but to a lesser degree than water. This is because ethanol is a larger molecule and can only form one hydrogen bond with another molecule (as opposed to two hydrogen bonds per water molecule). As a result of this difference, one

would expect the surface tension of ethanol to be lower than that of water and thus would expect a smaller volume of ethanol to fit on the surface of a coin compared to water.

Inquiry and Nature of Science Skills in this Lab: • Identify Questions o Develop a question that:  Asks a question about a specific science concept or process o Recognize and develop testable questions that  Specify a cause-effect relationship  Require the changing of one variable at a time  Can be answered with a science investigation or observational study. o Develop predictions/hypotheses that:  State what may happen in an investigation based on prior knowledge or experience (prediction) • Design Investigations o Design and conduct investigations using:  Fair test - changing only one variable at a time makes comparisons valid  Independent variable - the one variable the investigator chooses to change  Dependent variables - what changes as a result of, or in response to, the change in the independent variable • Gather Data o Use tools and the SI (metric) system to accurately measure:  Volume o Chooses appropriate tools to conduct an investigation  Other laboratory equipment o Uses senses to observe  Seeing o Uses the appropriate format to record data  Writing (journal, worksheet, electronic text) • Interpret Data o Identifies and interprets patterns  Based on an analysis of data collected during an investigation • Evaluate Evidence o Drawing and supporting a conclusion by  Using data to determine the cause effect relationship observed in the investigation  Comparing results to hypothesis  Answer the testable question • Scientific Investigation o Scientific Investigation:  Scientific investigation begins with a testable question  Scientific investigation results in things we know and things we don't know.

Science takes place in many locations including labs, offices, in the field, and under the ocean.  Scientific investigations lead to the development of scientific explanations. o Scientific Data and Outcomes  People are more likely to believe ideas if good reasons are given for them.  It is important in science to keep honest, clear, and accurate records. Scientific Endeavor o Characteristics of Science:  Science is based on factual knowledge  Scientists are curious about wanting to know how things work 

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