C21 Composition Of Gasses In The Atmosphere Flashcards
The Earth’s atmosphere consists of the following gases:
78% nitrogen 21% oxygen 0.03% - 0.04% carbon dioxide About 1% argon Small proportions of other noble gases Varying proportions of water vapour
The early atmosphere probably contained:
Little or no oxygen
A large amount of carbon dioxide
Water vapour
Small amounts of other gases, such as ammonia and methane
Scientists cannot be sure about the early atmosphere since it formed so long ago. They can only analyse evidence from other sources.
Oxygen (early atmosphere)
Some ancient rocks contain iron compounds that break down in the presence of oxygen. They could only have formed if there was little or no oxygen at the time.
Carbon dioxide and water vapour (early atmosphere)
A volcano releases large volumes of carbon dioxide and water vapour. Since the early atmosphere was produced by volcanic activity, it was likely to have contained a large amount of carbon dioxide and water vapour.
What happened to water vapour?
The Earth cooled after it formed. When it became cool enough, water vapour in the atmosphere condensed to liquid water. It fell as rain, creating the oceans.
What happened to carbon dioxide?
Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved and the amount in the atmosphere decreased.
What happened to oxygen?
Plants make their own food by photosynthesis. In this process, carbon dioxide from the atmosphere reacts with water. Glucose is produced, with oxygen as a by-product:
Carbon dioxide + water → glucose + oxygen
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
Photosynthesis by primitive plants and algae 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
