Composition of clean, dry air
Key idea: Clean, dry air is about 78% nitrogen and 21% oxygen, with most of the remainder argon. Water vapour is left out so gas percentages can be compared on the same basis.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Read the approximate composition of clean, dry air.
- Compare the major gases and identify the small remainder.
- Calculate both gas volumes to the nearest cubic centimetre.
- Identify the main minor gas and carbon dioxide percentage.
- Correct the claim about the last one per cent.
- Find the remaining volume, main gas and mixture status.
- Calculate the total and gap to two decimal places.
- Explain how rounding and trace gases affect the total.
- Identify the dry-air reading and humid sample’s extra gas.
- Explain why water vapour lowers nitrogen’s percentage.
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ContinueAir is a mixture: each gas in it keeps its own properties, and the amounts can change without any reaction. This lesson gives the composition of clean, dry air and shows how to read and compare the values in a composition table.
What clean, dry air contains
The composition of air is given as a percentage by volume. In 100 cm³ of clean, dry air, about 78 cm³ is nitrogen and about 21 cm³ is oxygen. Compare gas volumes at the same temperature and pressure.
| Gas | Formula | Approximate percentage by volume |
|---|---|---|
| nitrogen | N₂ | 78% |
| oxygen | O₂ | 21% |
| argon (the main noble gas) | Ar | 0.9% |
| carbon dioxide | CO₂ | 0.04% |
| other noble gases, such as neon and helium | — | traces |
Data table
| Gas | Clean, dry air |
|---|---|
| Nitrogen | 78 |
| Oxygen | 21 |
| Argon | 0.9 |
| Carbon dioxide | 0.04 |
These values are approximate. They are rounded, so they need not add up to exactly 100%.
Why “dry” air?
Air also contains water vapour, but its amount changes with the weather, the place and the temperature. It can be almost none in a cold, dry place and a few per cent in hot, humid air. Leaving water vapour out lets us compare the other gases on the same basis. Their major proportions are fairly stable in well-mixed outdoor air, but dry air is not an exact, unchanging recipe: carbon dioxide, for example, varies with place and time.
“Dry air” does not mean that ordinary outdoor air has no water vapour. It means the water vapour has been removed, or left out of the calculation, before the other gases are compared.
Worked example 1
Compare the gases in a dry-air record
Problem
A dry-air record gives nitrogen 78.1%, oxygen 20.9%, argon 0.9% and carbon dioxide 0.04%. Compare nitrogen with oxygen, and state what the small remainder contains.
Show solution step by step
Compare the two main gases
Method
Divide the nitrogen value by the oxygen value.Reason
A ratio turns “more” into “how many times more”.Working
Answer
78.1 ÷ 20.9 ≈ 3.7, so there is almost four times as much nitrogen as oxygen by volume.
Account for the remainder
Method
Add the two main gases and subtract from 100%.Reason
The table lists the smaller gases separately, so the remainder is not a single gas.
Working
Answer
78.1% + 20.9% = 99.0%; 100% − 99.0% = 1.0%. This remainder is mostly argon, with carbon dioxide and traces of other gases. The listed 0.9% and 0.04% do not fill it exactly because the values are rounded; do not treat the difference as a measured amount of an unidentified gas.
Answer
Working
Answer
Nitrogen is the main gas, almost four times as abundant as oxygen. The remainder is mostly argon, with a little carbon dioxide and other noble gases.
Quick check
Use “about” or “approximately”: the values are rounded, so a total slightly above or below 100% does not mean a gas is missing.
Guided practice 2
Find the oxygen in a sample of air
Problem
A syringe holds 200 cm³ of clean, dry air. Use the composition of dry air to find the volumes of oxygen and nitrogen in it, to the nearest cm³.
Use the percentages by volume
Hints
Hint 1
percentage
21% means 21 cm³ in every 100 cm³.
Hint 2
scale up
The sample is twice 100 cm³.
Show solution step by step
Oxygen
Method
Multiply the sample volume by the percentage of oxygen.
Reason
Percentage by volume is the fraction of the total volume.
Working
Answer
21/100 × 200 cm³ = 42 cm³
Nitrogen
Working
Answer
78/100 × 200 cm³ = 156 cm³. Check: 42 + 156 = 198 cm³; the other 2 cm³ is mostly argon.
Quick check
The check step uses the remainder: about 1% of the sample should be left over for argon and the other minor gases.
Check your understanding 1 Identify the remainder before correcting the claim
Use the dry-air table: which gas makes up most of the small remainder, and what approximate percentage of the whole sample is carbon dioxide?
Show answer
The remainder is mostly argon. Carbon dioxide is about 0.04% of the whole dry sample; subtracting nitrogen and oxygen does not identify the remainder as one gas.
Spot the mistake 3
“The last 1% of air is carbon dioxide”
Learner claim
A learner writes: “Air is 78% nitrogen and 21% oxygen, so the other 1% is carbon dioxide.” Find the error and correct it on paper or in the optional notes, then compare your complete statement.
Correct the statement
Show solution step by step
Find the error
Method
Compare the claim with the data table.Reason
Subtracting from 100% gives the size of the remainder, not what it is made of.
Working
Answer
Carbon dioxide is about 0.04%, not 1%.Correct it
Working
Answer
The remaining 1% is mostly noble gases, mainly argon (about 0.9%). Carbon dioxide is only about 0.04%.
Common mistake
Carbon dioxide matters for the carbon cycle and climate, but it is a very small part of air by volume.
Check your understanding 2 Air over heated copper
100 cm³ of dry air is passed over heated copper, which removes all the oxygen. The remaining gas is measured at the original temperature and pressure. What volume remains, which gas is its main constituent, and is the remaining gas pure nitrogen?
Show answer
Oxygen accounts for about 21 cm³, so about 100-21 = 79 cm³ remains. It is mainly nitrogen, with noble gases and a little carbon dioxide. It is not pure nitrogen; removing oxygen leaves the other gases too.
Check your understanding 3 Calculate the total before interpreting it
A dry-air table gives oxygen 20.9%, nitrogen 78.1%, argon 0.93% and carbon dioxide 0.04%. Find their total and the gap from 100%, to 2 decimal places. Use the listed numbers without further rounding.
Show the steps Optional working
Show answer
The total is 99.97%; the gap is 0.03 percentage points. This subtraction alone does not measure an exact amount of unidentified gas. Explain the data limitations in the next activity.
Check your understanding 4 Why the total is not exactly 100%
A dried-air table gives oxygen 20.9%, nitrogen 78.1%, argon 0.93% and carbon dioxide 0.04%. Find the total and explain why the difference from 100% does not measure an exact amount of missing gas.
Work on paper or in the optional notes. Compare rounding, unlisted trace gases and what the small gap cannot measure.
Show answer
The total is 20.9 + 78.1 + 0.93 + 0.04 = 99.97%. The values are rounded, and trace gases are unlisted. Both affect the small difference of 0.03 percentage points, so it cannot be assigned exactly to an unidentified gas.
Check your understanding 5 Identify what changed in the comparison
P dries air from a park; Q measures humid air from the same park and reports a lower nitrogen percentage. Choose which reading has the dry-air basis and what Q includes in its total.
Show answer
P has the dry-air basis. Q includes water vapour in its total, so its nitrogen percentage has a different denominator. Explain the lower percentage without inventing a reaction in the next activity.
Check your understanding 6 Humid air gives a lower nitrogen reading
Two instruments sample the same park. Instrument P dries the air first; instrument Q measures humid air directly. Q reports a lower percentage of nitrogen than P. Explain why this does not mean that some nitrogen has reacted or disappeared.
Work on paper or in the optional notes. Compare the total volume used as the denominator and the presence of water vapour.
Show answer
Q’s sample includes water vapour, so its total volume contains an extra gas. Nitrogen’s share is calculated against a total that includes water, so its percentage is lower than on a dry-air basis. No nitrogen has reacted: the change comes from what is included in the total. Only P’s result should be compared with the dry-air values.
Common mistakes
- Swapping the values: air is about 78% nitrogen and 21% oxygen, not the other way round.
- Calling air a compound. It is a mixture.
- Including water vapour when a question asks about dry air.
- Treating rounded values as exact.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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