AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemistry of the atmosphere › Composition and evolution of the atmosphere
Practise Proportions of gases in the atmosphere. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What the air around you is made of today, and how long it has been this way. Expect 1-mark recall questions on Paper 2, and short calculations that use fractions, percentages and ratios of the gases in air.
Key facts
Nitrogen ≈ four-fifths ≈ 80% of the air
Oxygen ≈ one-fifth ≈ 20% of the air
Small proportions: carbon dioxide (about 0.04%), water vapour, noble gases (mainly argon)
Proportions much the same as today for about 200 million years
Nitrogen : oxygen ≈ 4 : 1
Volume of a gas = percentage ÷ 100 × total volume
Percentage = part ÷ total × 100
Notes
The atmosphere today
diagram
For about the last 200 million years, the proportions of the gases in the atmosphere have been much the same as they are today.
About four-fifths (about 80%) of the air is nitrogen, N2.
About one-fifth (about 20%) of the air is oxygen, O2.
The rest is small proportions of other gases: noble gases (mainly argon, just under 1%), carbon dioxide (about 0.04%) and water vapour (the amount changes with the weather).
Dry air is about four-fifths nitrogen and one-fifth oxygen.
Some questions give more exact values for dry air, such as 78% nitrogen, 21% oxygen and 0.9% argon. Always use the numbers the question gives you.
Air is a mixture: the gases are not chemically bonded to each other.
Fractions, percentages and ratios
Volume of a gas in a sample of air = percentage ÷ 100 × total volume.
Example: 250 cm3 of air contains about 80 ÷ 100 × 250 = 200 cm3 of nitrogen and 20 ÷ 100 × 250 = 50 cm3 of oxygen.
To turn a fraction into a percentage, multiply by 100: four-fifths = 4 ÷ 5 × 100 = 80%.
Nitrogen : oxygen ≈ 80 : 20 = 4 : 1, so there is about four times as much nitrogen as oxygen in air.
To find a percentage from data: percentage = part ÷ total × 100.
How to answer each type of question
Recall: name a gas or give its proportion
1 mark eachGrade 3
Learn the two big numbers: nitrogen about 80% (four-fifths), oxygen about 20% (one-fifth).
Give one clear answer. If you give two answers and one is wrong, you can lose the mark.
Example. (a) Which gas makes up about four-fifths of the Earth's atmosphere? (b) Give the approximate percentage of oxygen in the atmosphere. (c) Name one other gas that is present in the atmosphere in a small proportion.
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(a) Nitrogen (1) (b) 20% (allow 21%) (1) (c) Any one from: carbon dioxide / water vapour / argon (or another noble gas) (1)
Calculate the volume of a gas in a sample of air
2 marksGrade 5
Write down the percentage of the gas (use the value in the question if one is given).
Divide by 100 and multiply by the total volume.
Give the unit, the same as the unit of the total volume.
Example. A party balloon contains 3.0 dm3 of air. Calculate the approximate volume of oxygen in the balloon.
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20 ÷ 100 × 3.0 (1) = 0.60 dm3 (1) (Using 21% gives 0.63 dm3, which also scores 2 marks.)
Don’t lose marks
Saying the air is mostly oxygen. It is mostly nitrogen.
Saying carbon dioxide is a major gas in today's air. It is only about 0.04%.
Dividing by the final volume instead of the starting volume when working out the percentage of oxygen from an experiment.
Writing '200 years' or '2 billion years' instead of about 200 million years.
Leaving the unit (cm3 or dm3) off a calculated volume.
More tips
Memory tricks
Four out of every five parts of the air you breathe in are nitrogen.
Quick estimate of oxygen: divide the volume of air by 5.
Calculator: 20% of a number is × 0.2 and 80% is × 0.8.
Sense check: the percentages of all the gases in air must add up to about 100%.
Exam technique
If the question gives exact percentages (e.g. 78% and 21%), use those numbers, not 80% and 20%.
Show your working (e.g. 20 ÷ 100 × 250) so you can still get a method mark if you slip.
'Give' and 'Name' questions want a short answer, not an explanation.
In experiment questions, the decrease in volume is the oxygen that reacted, and the gas left is mainly nitrogen.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the two main gases in airNitrogen makes up about four-fifths of the air and oxygen about one-fifth.
Grade 3
Give the percentages of nitrogen and oxygenAir is about 80% nitrogen and about 20% oxygen.
Grade 4
Name the gases present in small proportionsCarbon dioxide, water vapour and noble gases such as argon are present in small proportions.
Grade 4
State how long the proportions have been similarThe proportions of the gases have been much the same as today for about 200 million years.
Grade 5
Calculate the volume of a gas in airMultiply the total volume by the percentage divided by 100, e.g. 20 ÷ 100 × 500 cm3 = 100 cm3 of oxygen.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What is the approximate percentage of oxygen in the Earth’s atmosphere?
20%
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Which gas makes up about four-fifths of the Earth’s atmosphere? Tick (✓) one box.[1]
Argon
Carbon dioxide
Nitrogen
Oxygen
(b) What is the approximate percentage of oxygen in the Earth’s atmosphere?[1]
(c) Name two other gases found in small proportions in the atmosphere.[2]
(d) For about how long have the proportions of gases in the atmosphere been much the same as they are today?[1]
Show the answer and mark scheme
(a)Answer: Nitrogen
(b)Answer: 20%
20 (%)
(c)
carbon dioxide
water vapour
argon / (other) noble gases
(d)
(about) 200 million years
Question 2Medium5 marks
A classroom contains 150 m3 of air.
(a) Calculate the approximate volume of nitrogen in the classroom.[2]
(b) Air contains about 0.04% carbon dioxide. Calculate the volume of carbon dioxide in the classroom.[2]
(c) The percentage of carbon dioxide in the classroom air increased during a lesson. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: 120 m3
80 ÷ 100 × 150
120 (m3)
(b)Answer: 0.06 m3
0.04 ÷ 100 × 150
0.06 (m3)
(c)
the people in the room breathe out carbon dioxide (produced by respiration)