The proportions of different gases in the atmosphere — AQA GCSE Combined Science
Test yourself on The proportions of different gases in the atmosphere with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
The proportions of different gases in the atmosphere explained
For about 200 million years the atmosphere has been stable in composition, and it is still roughly the same today.
Read the full explanation
The main gases are nitrogen, about 78% of the atmosphere, and oxygen, about 21%. The remaining 1% is mostly argon, with small and variable amounts of carbon dioxide, water vapour and other noble gases. These proportions are approximate because water vapour varies with location and weather, and carbon dioxide has risen slightly since industrialisation. The early atmosphere was very different, formed mainly by volcanic activity and containing little or no oxygen, so the current stability is the result of long-term processes such as photosynthesis.
about four-fifths (approximately 80%) nitrogen
The modern atmosphere is a mixture of gases, and nitrogen is by far the most abundant. Roughly four out of every five molecules in dry air are nitrogen, so the proportion is about 80% or four-fifths. This value is a proportion by volume, which for gases is effectively the same as proportion by number of molecules. A useful method is to picture 100 gas molecules chosen at random from dry air: about 80 would be nitrogen, about 20 oxygen, and only a small number would be argon, carbon dioxide, water vapour and other trace gases. Nitrogen is unreactive under normal conditions, which helps explain why its proportion has remained stable for a very long time. When asked to describe the atmosphere, state the gas and its approximate proportion, and remember that the figures refer to dry air unless water vapour is mentioned.
about one-fifth (approximately 20%) oxygen
Oxygen is the second most abundant gas in the modern atmosphere, making up about one-fifth of dry air, which is approximately 20%. Like the other atmospheric proportions, this is a proportion by volume, and for gases it is effectively the same as the proportion by number of molecules. A helpful method is to imagine 100 molecules of dry air: about 20 are oxygen and about 80 are nitrogen, with the remaining small fraction made up of argon, carbon dioxide and other trace gases. Oxygen is the gas that supports combustion and is used in respiration by most living organisms, so its proportion matters for life on Earth. When describing the atmosphere, always pair the gas with its approximate proportion and keep the values for nitrogen and oxygen the correct way round.
small proportions of various other gases, including carbon dioxide, water vapour and noble gases.
The atmosphere is a mixture, not a compound, so each gas keeps its own properties and can be separated by physical means. About four-fifths of the air is nitrogen and about one-fifth is oxygen, leaving only small proportions of other gases. These trace gases include carbon dioxide, water vapour and the noble gases, mainly argon. Carbon dioxide is released by combustion and respiration and is used by photosynthesis, so its concentration varies slightly with location, season and time of day. Water vapour varies greatly, condensing to form clouds and rain. Noble gases such as argon are chemically inert because they have full outer electron shells, so they are used in lamps and as shielding gases. A useful method is to treat dry air as roughly 78% nitrogen, 21% oxygen and about 1% argon plus other trace gases, then explain why the percentages are approximate rather than fixed.
Your focus
- State the approximate proportions of nitrogen and oxygen in the atmosphere.
- Identify argon, carbon dioxide and water vapour as minor components.
- Describe how the present atmosphere differs from the early atmosphere.
Show all 12 objectives
- Recall that nitrogen makes up about four-fifths, or approximately 80%, of the atmosphere.
- Represent the proportion of nitrogen as a fraction, a percentage or a simple particle model.
- Compare the proportion of nitrogen with that of oxygen and the trace gases.
- Recall that oxygen makes up about one-fifth, or approximately 20%, of the atmosphere.
- Represent the proportion of oxygen as a fraction, a percentage or a simple particle model.
- Compare the proportion of oxygen with that of nitrogen and the trace gases.
- Recall the approximate proportions of nitrogen and oxygen in the atmosphere.
- Identify carbon dioxide, water vapour and noble gases as minor components of the atmosphere.
- Explain why the proportions of carbon dioxide and water vapour in air are variable rather than fixed.
The proportions of different gases in the atmosphere exam tips
Marking Points
- Nitrogen makes up about 78% of the atmosphere.
- Oxygen makes up about 21% of the atmosphere.
- Argon is the next most abundant gas at roughly 0.9%, so the remaining 1% is mainly argon.
- Carbon dioxide is a trace gas, currently around 0.04%, and water vapour is variable.
- The composition has been approximately constant for 200 million years, but the early atmosphere was different.
- Percentages are approximate and should be quoted to sensible values, not as exact fixed numbers.
- States that nitrogen makes up about four-fifths, or approximately 80%, of the atmosphere.
- Recognises that the proportion is a fraction or percentage of the total volume of dry air.
- Links the value to a simple model, such as 80 out of every 100 molecules being nitrogen.
- Distinguishes nitrogen from oxygen, which is the second most abundant gas at about one-fifth.
- Uses the correct gas name, nitrogen, rather than referring vaguely to 'air' or 'the main gas'.
- Applies the figure when comparing the abundances of atmospheric gases.
- States that oxygen makes up about one-fifth, or approximately 20%, of the atmosphere.
- Recognises that the proportion is a fraction or percentage of the total volume of dry air.
- Links the value to a simple model, such as 20 out of every 100 molecules being oxygen.
- Distinguishes oxygen from nitrogen, which is the most abundant gas at about four-fifths.
- Uses the correct gas name, oxygen, rather than a formula or a vague description.
- Applies the figure when comparing the abundances of atmospheric gases.
- State that nitrogen makes up about four-fifths (roughly 78%) of the atmosphere and oxygen about one-fifth (roughly 21%).
- Identify carbon dioxide, water vapour and noble gases as present in small proportions, with argon as the most common noble gas.
- Explain that the atmosphere is a mixture of gases, so the proportions can be described as percentages by volume.
- Recognise that carbon dioxide and water vapour concentrations vary with location, weather, season and human activity, so quoted values are approximate.
- Link the chemical inertness of noble gases to their full outer electron shells and their uses, such as argon in filament lamps.
Examiner Tips
- 💡Learn the two main percentages as 78% nitrogen and 21% oxygen, and remember the remaining 1% is mostly argon.
- 💡If asked to compare with the early atmosphere, mention high carbon dioxide and little or no oxygen.
- 💡Use approximate language such as about or roughly, because the values are not exact.
- 💡Check whether the question asks for percentages or fractions before writing your answer.
- 💡Quote the proportion as a fraction and a percentage, for example four-fifths or about 80%, to show the value clearly.
- 💡If a question asks you to compare gases, give both the gas name and its proportion rather than the proportion alone.
- 💡Check whether the question says dry air; if water vapour is included, the percentages of the other gases are slightly lower, so read the wording carefully.
- 💡Give the proportion as both a fraction and a percentage, for example one-fifth or about 20%, to make the value unambiguous.
- 💡When comparing nitrogen and oxygen, state which gas is larger and by how much, for example nitrogen is about four times as abundant as oxygen.
- 💡Read the question carefully to check whether it refers to dry air or to air including water vapour, because the stated percentages change slightly.
- 💡Quote approximate proportions as fractions or percentages, for example nitrogen about four-fifths and oxygen about one-fifth, and make clear they are approximate.
- 💡When asked to name trace gases, give carbon dioxide, water vapour and a named noble gas such as argon rather than saying only 'other gases'.
- 💡Link any question about varying carbon dioxide or water vapour to a process such as combustion, respiration, photosynthesis or condensation to gain explanation credit.
Common Mistakes
- Reversing nitrogen and oxygen percentages; correction: nitrogen is about 78% and oxygen about 21%.
- Saying carbon dioxide is a major component; correction: it is a trace gas, roughly 0.04%.
- Treating the 1% as entirely carbon dioxide; correction: it is mostly argon, with carbon dioxide and other trace gases.
- Describing the atmosphere as unchanging since Earth formed; correction: it has been stable for about 200 million years, but the early atmosphere was very different.
- Writing that nitrogen is about 20% of the atmosphere: this confuses nitrogen with oxygen, so learn the pair as nitrogen 80% and oxygen 20%.
- Stating that nitrogen is 78% exactly and treating that as the required value: the specification asks for approximately 80% or four-fifths, so use the rounded proportion unless a more precise figure is requested.
- Describing nitrogen as a compound or as a reactive gas: nitrogen is an element and is relatively unreactive, which is why its proportion is stable.
- Writing that oxygen is about 80% of the atmosphere: this swaps the values for nitrogen and oxygen, so remember oxygen is the smaller share at about 20%.
- Confusing the proportion of oxygen with the proportion of carbon dioxide, which is a trace gas at roughly 0.04%: keep trace gases clearly separate from the main two.
- Treating 21% as the required value: the specification asks for approximately 20% or one-fifth, so use the rounded proportion unless a more precise figure is requested.
- Writing that oxygen is the most abundant gas in the atmosphere; the correction is that nitrogen is most abundant at about four-fifths, with oxygen at about one-fifth.
- Treating carbon dioxide as a major component rather than a trace gas; the correction is that it is present in small, variable proportions, even though it is important for climate and photosynthesis.
- Describing air as a compound of nitrogen and oxygen; the correction is that air is a mixture, so the gases are not chemically bonded and can be separated by fractional distillation.