Air Composition and Data
Interpret dry-air composition data, distinguish major and minor constituents, and handle variable water vapour without treating rounded values as exact.
Continue where you stopped
The core idea
On this page
Learning objectives
- describe the volume composition of gases present in dry air as being approximately 78% nitrogen, 21% oxygen and the remainder being noble gases (with argon as the main constituent) and carbon dioxide
1. Outcome and prerequisites
By the end, you should be able to read a composition table or chart, compare major and minor constituents of air, and explain why water vapour is treated separately from clean, dry air.
The official course values are approximate percentages by volume. They describe a mixture: the gases keep their identities and their rounded values need not add to exactly 100%.
Water vapour varies with place, weather and temperature. Excluding it gives a stable basis for comparing the other gases; it does not mean ordinary outdoor air contains no water vapour.
2. Modelled example
Data table
| Gas | Clean, dry air |
|---|---|
| Nitrogen | 78 |
| Oxygen | 21 |
| Argon | 1 |
| Carbon dioxide | 0 |
Modelled example 1
Read the values before recalling a rule
Problem
A dry-air record gives nitrogen 78.0%, oxygen 20.9%, argon 0.9% and carbon dioxide 0.04%. Compare nitrogen with oxygen and explain what the small remainder contains.
View solution step by step
Make the comparison
Method
State that nitrogen is the largest constituent and is nearly four times as abundant as oxygen.
Reason
The conclusion comes from 78.0 ÷ 20.9 ≈ 3.7, not from a memorised label.
Working
78.0% > 20.9%; both are major constituents.
Interpret the remainder
Method
Identify argon and carbon dioxide as minor constituents represented in the remainder.
Reason
The table lists them separately, so the missing fraction cannot be assigned to one gas.
Working
minor remainder includes argon, carbon dioxide and other trace gases.
3. Guided practice
Guided practice 2
Change the sampling condition
Problem
Two instruments sample the same park. Instrument P dries the air first; instrument Q measures humid air directly. Q reports a lower percentage for nitrogen. Explain why this does not show that nitrogen disappeared.
Use the changed denominator
Hints
Hint 1: compare conditions
Only one instrument removes water vapour.
Hint 2: percentage meaning
Adding a variable constituent changes each gas’s fraction of the total even if its amount has not reacted away.
View solution step by step
Identify the changed constituent
Method
Q includes water vapour while P reports dry-air composition.
Reason
Water vapour is part of Q’s total volume.Working
humid total = dry gases + variable water vapour.
Explain the percentage
Method
The larger total can give nitrogen a smaller percentage without a chemical loss of nitrogen.
Reason
A percentage is a part divided by the stated total.
Working
changed denominator ≠ nitrogen reaction.
4. Plausible error contrast
A learner reads 78% nitrogen and 21% oxygen and says, “The final 1% must be carbon dioxide.” The tempting step is subtracting from 100%, but the conclusion ignores other minor gases and rounding. Most of the small remainder is noble gases, mainly argon; carbon dioxide is only a small fraction.
5. Changed-context transfer
Mind stretcher 1: Compare two unfamiliar samplesExtension
Sample A contains 77.9% nitrogen, 20.8% oxygen, 0.9% argon and 0.04% carbon dioxide after drying. Sample B was not dried and contains 2.0% water vapour. Which sample should be compared with the course dry-air reference, and why?
Show feedback
Use Sample A because its stated condition matches the dry-air reference. Sample B can still be valid humid-air data, but its water vapour changes the total against which every percentage is calculated.
6. Independent evidence
Without using the lesson title as a cue, a table gives 20.7% oxygen, 78.1% nitrogen, 0.93% argon and 0.05% carbon dioxide for a dried sample. Identify the two major constituents, compare their abundances, account for the minor remainder and state one reason the values should be described as approximate.
Continue to Air Pollutants: Sources, Effects and Controls.