# Explain why the concentration of oxygen in the Earth's atmosphere increased significantly about 2.7 billion years ago and describe one effect this had on other gases present at the time.

> Eduqas GCSE Chemistry — C9 Chemistry of the atmosphere · Explain · 4 marks

## Mark scheme (4 marks)

1. Photosynthetic organisms (algae / cyanobacteria / blue-green algae) appeared / evolved
2. These organisms produced oxygen through photosynthesis
3. The levels of carbon dioxide decreased / carbon dioxide was removed from the atmosphere
4. Ammonia and methane were removed / broken down / oxidised by the oxygen

## Key terms

- [atmosphere](https://www.gradenine.co.uk/glossary/atmosphere)
- [oxygen](https://www.gradenine.co.uk/glossary/oxygen)

## Related

- [Revision notes for Eduqas GCSE Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-concentration-of-oxygen-38c88ecb) · Published by Druglandscape Ltd.