The proportions of different gases in the atmosphere — AQA GCSE Chemistry
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The proportions of different gases in the atmosphere explained
The atmosphere has been stable in composition for about 200 million years.
Read the full explanation
Today it is roughly four-fifths nitrogen, about 80%, and one-fifth oxygen, about 20%. The remaining small proportion contains other gases, including carbon dioxide, water vapour and noble gases such as argon. These fractions describe dry air; water vapour varies with location and weather. For example, in 100 cm³ of dry air, about 80 cm³ is nitrogen and about 20 cm³ is oxygen, leaving only a small volume for carbon dioxide, water vapour and noble gases. The phrase 'much the same' means the proportions have not changed significantly over that period, even though human activities now add carbon dioxide.
Your focus
- State the approximate proportions of nitrogen and oxygen in today's atmosphere.
- Name the other gases present in small proportions.
- Describe how long the atmosphere has had much the same composition.
The proportions of different gases in the atmosphere exam tips
Marking Points
- State that the atmosphere has had much the same proportions of gases for about 200 million years.
- Give nitrogen as about four-fifths, approximately 80%, of the atmosphere.
- Give oxygen as about one-fifth, approximately 20%, of the atmosphere.
- Name carbon dioxide, water vapour and noble gases as small proportions of the remaining gases.
- Use the fractions four-fifths and one-fifth correctly when describing the main gases.
- Recognise that the percentages describe dry air and that water vapour is variable.
Examiner Tips
- 💡Learn the two main fractions as four-fifths nitrogen and one-fifth oxygen, with the percentages.
- 💡When asked for other gases, name carbon dioxide, water vapour and noble gases rather than giving a single example.
- 💡Use the phrase 'much the same' to show stability over 200 million years, not exact constancy.
Common Mistakes
- Reversing the proportions and stating 80% oxygen; correction: nitrogen is about 80% and oxygen about 20%.
- Treating carbon dioxide as a major component; correction: it is a small proportion, along with water vapour and noble gases.
- Assuming the composition has never changed; correction: it has been much the same for about 200 million years, but the early atmosphere was different.