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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Photosynthetic organisms such as cyanobacteria evolved and began producing oxygen via photosynthesis.
2. The oxygen released accumulated in the atmosphere, raising its concentration.
3. As oxygen rose, it reacted with and removed carbon dioxide, lowering CO₂ levels.
4. The increased oxygen also oxidised gases like ammonia and methane, reducing their atmospheric presence.

Examiner tips

  • Use the word ‘produced’ for photosynthesis; mention cyanobacteria. Show the chain: oxygen → CO₂ removal → oxidation of NH₃/CH₄. Keep each point short and directly linked to the marks.
  • common_mistakes
  • :
  • Confusing photosynthesis with respiration. Failing to mention the oxidation of ammonia or methane. Using vague terms like ‘more oxygen’ without explaining the cause.

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 in this question

atmosphere · oxygen

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