Edexcel GCSE Science Revision Guide - Foundation

Chemistry

The early atmosphere The Earth’s first atmosphere The Earth’s first atmosphere was formed by gases produced from volcanoes. Volcanoes gave out...

...lots of carbon dioxide

...lots of water vapour

...small amounts of other gases

The early atmosphere did not contain any oxygen.

The Earth was very hot to start with, and there were no oceans. As the Earth cooled down the water vapour in the atmosphere condensed to form liquid water. This liquid water became the oceans.

Explain why scientists cannot be certain about the composition of the early atmosphere.

There were no humans around to measure the atmosphere, so they have to use clues. There are different sources of information, but not all the evidence leads to the same conclusion. This means that it is difficult to know exactly what the early atmosphere was like and how it has changed.

The evidence includes: • the gases produced by volcanoes today which tells us what gases would have been produced by volcanoes in the past s • the atmospheres of other planet , tem and moons in the solar sys n where the atmosphere has not bee changed by living things • iron compounds found in very old rocks that could only form if there was no oxygen.

You don’t need to remember these different sources of information. You DO need to remember that there ARE different sources of information, some of which tell us different things.

1. (a)  Name the two gases present in the largest amounts in the Earth’s early atmosphere. (2 marks) Target (b) State where scientists think these gases came from. (1 mark) D−G 2. Describe how the Earth’s oceans formed.

(2 marks)

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Chemistry

A changing atmosphere Adding oxygen

The atmosphere of the early Earth was mainly carbon dioxide, with little or no oxygen. Today the atmosphere has much less than 1% carbon dioxide and has 21% oxygen. This means that: • carbon dioxide has been removed from the atmosphere • oxygen has been added to the atmosphere.

The first organisms that evolved on Earth did not use or release oxygen. About 1 billion years ago some forms of life evolved in the oceans that used photosynthesis. Photosynthesis uses carbon dioxide and produces oxygen as a waste product. Eventually plants evolved and some of them adapted to life on land, so gradually more and more oxygen was added to the atmosphere.

Removing carbon dioxide Carbon dioxide dissolved in the oceans.

The shells of dead marine organisms fall to the sea bed and become part of the sediment. Over millions of years the layers of sediment become squashed and form sedimentary rocks.

Marine organisms used the dissolved carbon dioxide to make calcium carbonate for shells.

Adding oxygen Earth cools and oceans form.

Oxygen builds up in the oceans. Forms of life evolve in the oceans and photosynthesis begins.

Oxygen builds up in the atmosphere.

There is a lot of limestone in the Earth’s crust. Explain how the formation of limestone removes carbon dioxide from the Earth’s atmosphere.

Some carbon dioxide in the atmosphere dissolves in the oceans. Some marine organisms use dissolved carbon dioxide to make calcium carbonate for their shells. When they die, these marine organisms sink to the sea bed and their shells eventually form limestone. The carbon locked up in the limestone originally came from the atmosphere.

1.  State how the percentage of carbon dioxide changed from the Earth’s early atmosphere Target to the amount in today’s atmosphere. D−G (1 mark)

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2. (a) State the main substance in the shells of marine organisms. (1 mark) (b)  Describe how the shells change into limestone. (3 marks)

You need to explain the link between limestone and carbon by saying that limestone is made from shells which are mostly calcium carb onate. In an exam you could pr esent the answer to this kind of question as a flow chart.

3.  Explain why the amount of oxygen in the atmosphere only started to increase Target C−D about 1 billion years ago. (2 marks)

Chemistry

The atmosphere today The amounts of different gases in the atmosphere are shown in the pie chart and table. The atmosphere also contains water vapour, but this is not usually included because the amount changes depending on the weather. nitrogen

The composition of the atmosphere Gas

% in dry air

nitrogen

78

oxygen

21

argon

0.9

carbon dioxide

0.04

other gases

traces other gases

oxygen

Changes in the atmosphere The composition of the atmosphere today is not constant. It can be changed by human activities. • Burning fossil fuels releases carbon dioxide into the atmosphere. Sulfur dioxide can also be released from burning coal. • Farming affects the atmosphere, as cattle and rice fields release methane. • Deforestation adds carbon dioxide to the atmosphere if the trees are burnt. There are also fewer trees to remove carbon dioxide from the atmosphere.

Explain one natural cause of changes in the atmosphere.

When volcanoes erupt they release a lot of sulfur dioxide into the atmosphere. They also release carbon dioxide.

xide is Remember that sulfur dio sphere when also added to the atmo al are burnt. fossil fuels such as co

1.  Name the gas that is present in the largest amount in air. (1 mark) 2. (a) State two human activities that add carbon dioxide to the atmosphere. (2 marks) Target D−G (b)  Name one other gas that human activities add to the atmosphere. (1 mark)

There are two marks for this question, so you need to mention two different natural changes. Exam questions similar to this have proved to be especialy tricky be prepared!

3.  What changes to the atmosphere can occur naturally? (2 marks)

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