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9.2.2 How the atmosphere evolved

9.2.2 How the atmosphere evolved

Water vapour condensed to form the oceans

  1. The early atmosphere held a great deal of water vapour.
  2. As the Earth cooled, that vapour could no longer stay as a gas.
  3. It condensed into liquid water, which fell as rain.
  4. The water collected in hollows on the surface and formed the oceans.
  5. Condensation removed most of the water vapour from the atmosphere itself.
Key Idea

Cooling is what drove the change: condensation happens when a vapour is cooled below its boiling point.

Carbon dioxide dissolved in the new oceans

  1. Carbon dioxide is soluble in water.
  2. As the oceans formed, large amounts of it dissolved out of the atmosphere.
  3. Once dissolved, it reacted to form carbonates.
  4. Those carbonates were deposited as sediment on the sea floor.
  5. The amount of carbon dioxide in the atmosphere therefore decreased.
Note

The carbon was not destroyed but locked away, first in the oceans and then in rock.

Photosynthesis added oxygen

  1. Primitive plants and algae appeared in the oceans.
  2. They carried out photosynthesis: carbon dioxide+water→glucose+oxygen\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}carbon dioxide+water→glucose+oxygen
  3. Photosynthesis used up carbon dioxide and released oxygen.
  4. As these organisms spread, the amount of oxygen gradually increased.
  5. The increase took a very long time, because it depended on how fast the organisms spread.
Example
  • Carbon dioxide: high at the start, falling as it dissolved and as plants used it.
  • Oxygen: almost none at the start, rising as photosynthesis spread.

Two processes acting on carbon dioxide at once

  1. Dissolving in the oceans removed carbon dioxide from the atmosphere.
  2. Photosynthesis removed it as well, and added oxygen in its place.
  3. The two together explain why carbon dioxide fell so far from its early level.
  4. Only photosynthesis explains the rise in oxygen, since dissolving adds none.
  5. An account of the change names both processes and says what each one did.
Self review
  • How did the oceans form?
  • What happened to the carbon dioxide as the oceans formed?
  • Write the word equation for photosynthesis.
  • Which process added oxygen to the atmosphere?
  • Name the two processes that reduced the amount of carbon dioxide.
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Four-stage process diagram showing early atmospheric gases, ocean formation, carbon dioxide storage in carbonate rocks, and oxygen production by photosynthesis The early atmosphere contained a great deal of water vapour and carbon dioxide, but almost no oxygen. Cooling, ocean formation and the appearance of photosynthetic organisms gradually changed this composition.

Water vapour condensed to form the oceans. Carbon dioxide then dissolved in the oceans and became locked in carbonate rocks, while photosynthesis removed more carbon dioxide and released oxygen.

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Which gas was abundant in the early atmosphere before the oceans formed?

9.2.2 How the atmosphere evolved Revision Guide

  1. GCSE
  2. /Chemistry
  3. /9.2.2 How the atmosphere evolved

Revision notes for Edexcel GCSE Chemistry 9.2.2 How the atmosphere evolved: explanations and worked examples.

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