Atmospheric Carbon Dioxide (GCSE Chemistry)

Atmospheric Carbon Dioxide

Carbon Dioxide Reduction

The Role of Plants and Algae

• Plants and algae reduced carbon dioxide. Using the process of photosynthesis, plants and algae reduced carbon dioxide in the atmosphere. This carbon dioxide was taken up by the plants and algae.
• Photosynthesis uses carbon dioxide and water. In the reaction of photosynthesis, carbon dioxide and water are the reactants. We saw this reaction previously in the tutorial on Atmospheric Oxygen.

The Role of Rock Formation 

• Sedimentary rocks and fossil fuels contain carbon. Both sedimentary rocks and fossil fuels contain carbon. As plants and animals evolved and increased in numbers on the plant, some died in anaerobic conditions, decomposing over millions of years to form coal, crude oil and methane.
• Forming rocks decreases atmospheric carbon dioxide. The formation of  sedimentary rocks and fossil fuels decreases the atmospheric carbon dioxide. This is because the carbon dioxide from the atmosphere is deposited and stored into these substances in the form of carbon.

The Causes of Atmospheric Changes

Formation of Deposits

• Amounts of atmospheric gases have changed. As we’ve seen, the amounts of carbon dioxide and oxygen in the atmosphere have changed over time. The amount of carbon dioxide has decreased, whilst the amount of oxygen has increased. The amount of nitrogen gas has gradually built up steadily.
• Carbon dioxide was absorbed from the atmosphere. As we’ve seen, carbon dioxide was removed from the atmosphere through photosynthesis in plants.
• Carbon can become trapped. We have also seen that carbon can become trapped in rocks. Some examples of this are deposits of limestone, coal, crude oil and natural gas.

Worked example: Explain how the atmosphere has changed in composition over time (6 marks)

Model answer worth 6 marks. This is a 6 mark answer because:

• The answer is logically structured correctly in order of time.
• All gases in the atmosphere have been correctly identified.
• There is a clear and correct explanation of how each gas has changed over time.
• The three ways that carbon dioxide levels have decreased have been explained.
• The timescales are correct.

4.6 billion years ago the composition of the atmosphere consisted of mostly carbon dioxide and water vapour as a result of intense volcanic activity over millions of years.

Nitrogen was also produced in volcanoes and the levels built up over time.

After 1 billion years, the earth cooled. The water vapour cooled and condensed to form oceans.

A large amount of  carbon dioxide dissolved in the ocean. This reduced the amount of carbon dioxide in the atmosphere. The carbon dioxide formed insoluble sediments which sank to the bottom of the ocean to form sedimentary rocks like limestone, over millions of years. The carbon was locked away in this form.

2.7 billion years ago, algae evolved in the oceans. They began to photosynthesise in the presence of sunlight.

Carbon dioxide + water   →     glucose + oxygen

The carbon dioxide levels decreased further. Oxygen was introduced into the atmosphere and the levels increased as plants and algae increased over time.

Over the next billion years, enough oxygen formed for animals to evolve. As plants and animals died over millions of years, the carbon they contained was locked away in the form of coal, crude oil and natural gas. This reduced the levels of carbon dioxide in the atmosphere.

Nowadays there is about 80% nitrogen and about 20 % oxygen with small amounts of carbon dioxide, noble gases and water vapour.