Air Composition and Evidence
Interpret evidence for the approximate composition of clean, dry air and distinguish normal components from pollutants.
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The core idea
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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
Outcome: use a table or graph to state the approximate composition of clean, dry air and explain the small rounded remainder. Prerequisite: read percentages and compare bar heights.
1. Definition
Dry air is air considered without its variable water vapour. Its major gases are measured by percentage by volume.
2. Key Ideas
- approximately 78% nitrogen and 21% oxygen;
- argon, the main noble-gas constituent, and other minor gases such as carbon dioxide make up the small remainder;
- normal components are not automatically pollutants.
3. Detailed Explanations
Data table
| Gas | Dry air |
|---|---|
| Nitrogen | 78 |
| Oxygen | 21 |
| Gas group | Approximate percentage by volume |
|---|---|
| nitrogen | 78% |
| oxygen | 21% |
| argon (the main noble gas), other noble gases and carbon dioxide | small remainder |
The two displayed percentages need not total exactly 100% because they are rounded and the minor gases are grouped. Argon is the main noble-gas constituent of air. Water vapour is excluded by the word dry.
4. Common Mistakes
- Reversed: 78% oxygen and 21% nitrogen. Use the graph: nitrogen is the taller bar.
- Forced total: calling the remainder exactly 1% carbon dioxide. It also contains noble gases, and the figures are approximate.
- Pollutant confusion: nitrogen in clean air is not a pollutant merely because some nitrogen oxides are pollutants.
5. Exam Tips
Read the representation before recalling a value: name the gas, quote the approximate percentage and state the evidence comparison.
6. Worked Examples
Modelled example 1
Model: interpret the composition graph
Problem
Study the worked solution
Read the bars
Method
Identify nitrogen at 78% and oxygen at 21%.Reason
The labels and bar heights provide direct evidence.Working
N₂ ≈ 78%; O₂ ≈ 21%.
Guided practice 2
Guided: a changed sample
Problem
Try this before viewing the solution
Hints
Hint 1: compare the values
View solution step by step
Compare, then conclude
Method
Compare 77.9 with 78 and 21.0 with 21.Reason
Both agree within the precision shown.Working
The sample is consistent with the approximate model.
Challenge 3
Transfer: explain the remainder
Problem
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Hints
Hint 1: check the scope
View complete solution
Name the model limit
Method
It displays only the two major gases.Reason
Noble gases and carbon dioxide occupy most of the small rounded remainder.Working
Two main bars ≠ complete ingredient list.
7. Mind Stretchers
Independent: A learner says “the chart totals 99%, so the experiment lost 1% of the air”. Evaluate the claim.