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Earth science - EdexcelThe modern atmosphere

There is evidence that the Earth's early atmosphere contained less oxygen but more carbon dioxide and water vapour than it does today. Increased emissions of greenhouse gases have led to climate change.

Part of Chemistry (Single Science)Fuels and Earth science

The modern atmosphere

Compared to the Earth's early , the modern atmosphere contains:

  • less water vapour
  • less carbon dioxide
  • more oxygen
Air is composed of 78% nitrogen, 21% oxygen and 1% other gases, including 0.9% argon and 0.04% carbon dioxide.
The composition of the modern atmosphere

Question

Name the most abundant gas in the modern atmosphere.

Water vapour

The Earth cooled after it formed. When it became cool enough, water vapour in the atmosphere to liquid water. It fell as rain, creating the oceans.

Carbon dioxide

Carbon dioxide is a very gas. It readily in water. As the oceans formed, carbon dioxide dissolved and the amount in the atmosphere decreased.

Oxygen

Plants make their own food by . In this process, carbon dioxide from the atmosphere reacts with water. Glucose is produced, with oxygen as a :

Carbon dioxide + water 鈫 glucose + oxygen

6CO2(g) + 6H2O(l) 鈫 C6H12O6(aq) + 6O2(g)

Photosynthesis by primitive plants and caused the removal of carbon dioxide from the air and the release of oxygen. This had two effects:

  • the amount of carbon dioxide decreased further
  • the amount of oxygen in the atmosphere gradually increased
Green algae cells, viewed through an electron microscope
Green algae cells, viewed through an electron microscope

Test for oxygen

Oxygen supports . If oxygen is present in a test tube, a glowing splint relights when it is held inside.

Question

Describe two reasons why the amount of carbon dioxide in the atmosphere decreased over time.

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The greenhouse effect
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The Earth's early atmosphere

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