Describe how the Earth's early atmosphere changed over billions of years to produce an atmosphere that contains mainly nitrogen and oxygen.

Eduqas GCSE Chemistry — C9 Chemistry of the atmosphere · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The early atmosphere was dominated by carbon dioxide and water vapour, with trace amounts of methane or ammonia.
Water vapour condensed to form oceans, and CO₂ dissolved in the oceans, reducing atmospheric CO₂.
Photosynthetic organisms then removed CO₂ and released O₂, gradually increasing oxygen levels.
Nitrogen was released by volcanic activity and bacteria; being inert it accumulated, giving the present atmosphere a high N₂ content.

Examiner tips

  • Use the exact terms: carbon dioxide, water vapour, oceans, photosynthesis, oxygen, nitrogen, volcanic activity.
  • Show the sequence: CO₂ → oceans → photosynthesis → O₂, then nitrogen accumulation.
  • Mention that nitrogen is unreactive, so it builds up.

Common mistakes

  • Confusing the order of events (e.g., putting nitrogen release before CO₂ reduction).
  • Using vague terms like ‘gases’ instead of specifying CO₂, H₂O, N₂, O₂.

Mark scheme (4 marks)

  1. The early atmosphere was mainly carbon dioxide (and water vapour / small amounts of other gases such as methane or ammonia)
  2. Water vapour condensed to form the oceans, and carbon dioxide dissolved in the oceans (reducing CO₂ levels)
  3. Photosynthesis by plants/algae/microorganisms removed carbon dioxide and released oxygen
  4. Nitrogen levels increased over time as it was released by volcanic activity and/or bacteria, and nitrogen is unreactive so it accumulated

Key terms in this question

early atmosphere · nitrogen · oxygen

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