Evolution of the Atmosphere
Revise Evolution of the Atmosphere for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Oxygen built up as algae and plants photosynthesised, while carbon dioxide fell as it dissolved in oceans and locked into rocks and fossil fuels.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Two major changes shaped the atmosphere: carbon dioxide decreased and oxygen increased.
Oxygen increased because algae and plants evolved and carried out photosynthesis, producing oxygen. Algae first produced oxygen about 2.7 billion years ago, and over the next billion years plants evolved and oxygen levels gradually increased until animals could evolve.
Carbon dioxide decreased for several reasons. Photosynthesis removed it. It dissolved in the oceans, and carbonates were precipitated to form sediments. And carbon was locked away in fossil fuels and sedimentary rocks formed from the remains of dead organisms.
Revision notes
Oxygen increasing
Algae evolved about 2.7 billion years ago and produced oxygen by photosynthesis.
Over the next billion years plants evolved, and the level of oxygen gradually increased until there was enough for animals to evolve.
Carbon dioxide decreasing
Photosynthesis by algae and plants removed carbon dioxide from the atmosphere.
Carbon dioxide also dissolved in the oceans, and carbonates were precipitated to form sediments, locking carbon away in rocks.
Carbon locked in rocks and fuels
Sedimentary rocks such as limestone formed from the shells and skeletons of marine organisms.
Fossil fuels — coal, crude oil and natural gas — formed from the remains of plants and plankton buried and compressed over millions of years. Both store carbon that was once atmospheric carbon dioxide.
Key points
- Algae produced the first oxygen 2.7 billion years ago.
- Photosynthesis increased oxygen levels.
- Photosynthesis also decreased carbon dioxide.
- Carbon dioxide dissolved in the oceans.
- Carbonates formed sediments locking away carbon.
- Fossil fuels store carbon from ancient organisms.
Worked examples
Example 1
Explain how oxygen was produced in the Earth's atmosphere. [2 marks]
Model answer
Algae evolved about 2.7 billion years ago and carried out photosynthesis (1 mark), which produces oxygen as a product, and levels increased further as plants evolved (1 mark).
Example 2
Give three ways carbon dioxide was removed from the early atmosphere. [3 marks]
Model answer
It was used up by algae and plants during photosynthesis (1 mark). It dissolved in the oceans, where carbonates were precipitated as sediments (1 mark). Carbon was locked away in fossil fuels and sedimentary rocks formed from dead organisms (1 mark).
Example 3
Explain why animals could not evolve until after algae and plants. [2 marks]
Model answer
Animals need oxygen for respiration (1 mark), and there was little or no oxygen in the atmosphere until algae and plants had produced it by photosynthesis (1 mark).
Common mistakes
Saying oxygen came from volcanoes.
It came from photosynthesis by algae and later plants.
Giving only one route for carbon dioxide removal.
Photosynthesis, dissolving in oceans, and locking into rocks and fuels are all required.
Getting the timescale wrong.
Algae first produced oxygen about 2.7 billion years ago.
Forgetting the ocean route.
Dissolving and precipitation as carbonates is a major removal mechanism.
Exam tips
- Learn all three carbon dioxide removal routes.
- Name algae and give the 2.7 billion year figure.
- Link oxygen production to photosynthesis explicitly.
- Explain animal evolution through the need for oxygen.
Key terms
- Photosynthesis
- The process producing oxygen and removing carbon dioxide.
- Algae
- The first organisms to produce oxygen.
- Sedimentary rock
- Rock formed from deposited sediments, storing carbon.
- Fossil fuel
- A fuel formed from ancient organisms, storing carbon.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.