Composition of Air — Gases in the Atmosphere | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Air composition, greenhouse effect and climate implications appear in most series.

Approximate proportions

Dry air is a mixture, not a compound
It is made up of:
- Nitrogen, N₂ — about four-fifths of the air, roughly 78–80%
- Oxygen, O₂ — about one-fifth, roughly 20–21%
- Argon and other noble gases — about 1% in total
- Carbon dioxide, CO₂ — about 0.04% (small but a key )
- Water vapour — a variable amount depending on humidity
These proportions have stayed roughly constant for the last 200 million years; only the trace gases (especially CO₂) have shifted noticeably over the last two centuries

Exam tip

Naming the gases in dry air

What comes up: a pie chart of dry air is shown and you are asked to identify the largest segment (about four-fifths), or to name the trace gas with the highest percentage.

Write (two marks): (1) The largest segment is nitrogen — it makes up about 78–80% of the atmosphere. (2) The trace gas present in the greatest amount is argon (approximately 1%).

Watch out: oxygen is only about one-fifth (roughly 21%), not four-fifths. If the question uses the MCQ option "gas Z" as the largest segment, the answer is nitrogen, not argon or neon.