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Detailed notes on Air and Water for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Air is a mixture of gases — mainly nitrogen and oxygen. Cambridge tests the composition of clean air, reactions of air with metals, and how fractional distillation of liquid air separates its components.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Clean dry air is ~78% N₂ and ~21% O₂. Fractional distillation of liquid air separates the components by their different boiling points.
Composition of clean, dry air (by volume):
Separation by fractional distillation of liquid air:
Uses:
| Gas | Uses |
|---|---|
| O₂ | Hospitals (breathing), steel making (Bessemer converter), burning fuels efficiently, welding |
| N₂ | Inert atmosphere for food packaging (extends shelf life), fertiliser production (Haber process), liquid N₂ as cryogen |
| Ar | Inert atmosphere in light bulbs and welding (prevents oxidation), filling double-glazed windows |
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'State the approximate percentage of nitrogen and oxygen in clean dry air' (1 mark — ~78% N₂, ~21% O₂). 'Explain how nitrogen is obtained from liquid air' (3 marks — air liquefied, nitrogen has lower BP than oxygen, boils off first, collected). 'State ONE use of nitrogen and explain why nitrogen is suitable' (2 marks — food packaging; nitrogen is unreactive, prevents oxidation/food spoilage).
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C11); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on air, written exactly the way a tutor would explain them at the board.
Question
State the approximate percentage by volume of nitrogen, oxygen, and noble gases in dry air.
Step-by-step solution
Step 1
Nitrogen (N₂): approximately 78% by volume.
Step 2
Oxygen (O₂): approximately 21% by volume.
Step 3
Noble gases (mainly argon, Ar): approximately 1% by volume. CO₂: approximately 0.04%.
Answer
N₂: ~78%; O₂: ~21%; Ar/noble gases: ~1%; CO₂: ~0.04%.
Examiner tip
Cambridge mark schemes accept N₂: 78–80%; O₂: 20–21%. Round numbers are acceptable.
Question
Describe how oxygen is obtained from air by fractional distillation.
Step-by-step solution
Step 1
Air is filtered to remove dust and other particles.
Step 2
The clean air is compressed and then cooled until it liquefies (temperature reduced to below −183 °C, the boiling point of oxygen).
Step 3
The liquid air is fed into a fractionating column where temperature increases from bottom to top.
Step 4
Nitrogen (b.p. −196 °C) evaporates first and is collected from the top of the column. Oxygen (b.p. −183 °C) has a higher boiling point and remains as liquid longer; it is collected from the lower part of the column.
Answer
Filter → liquefy → fractional distillation: N₂ (b.p. −196 °C) evaporates first; O₂ (b.p. −183 °C) collected from below.
Question
State TWO large-scale industrial uses of nitrogen gas obtained from fractional distillation of air.
Step-by-step solution
Step 1
Use 1: manufacture of ammonia (Haber process) — N₂ + 3H₂ ⇌ 2NH₃; ammonia is used to make fertilisers.
Step 2
Use 2: inert atmosphere for food packaging — N₂ is unreactive; displaces O₂ in packaging to prevent food oxidation and extend shelf life.
Step 3
Additional use: liquid nitrogen for quick-freezing food and cryogenic storage.
Answer
N₂ uses: (1) Haber process (making NH₃/fertilisers); (2) inert atmosphere for food packaging; (3) liquid nitrogen for freezing.
Definitions to memorise and the exact keywords mark schemes credit for air answers — sharpened from recent examiner reports for the 2026 0654 sitting.
Dry air is approximately 78% nitrogen, 21% oxygen, 1% noble gases (mainly argon), and 0.04% carbon dioxide by volume; plus variable amounts of water vapour.
An industrial process that separates the components of liquefied air by exploiting their different boiling points: N₂ (−196 °C) evaporates before Ar and O₂ (−183 °C).
A glowing (not burning) wooden splint relights in a gas sample if the gas is oxygen.
Argon (Ar) makes up approximately 0.93% of dry air by volume; it is chemically inert and used in electric light bulbs and welding shields.
The traps other students keep falling into on air questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2022
Why it happens
Students remember the two main gases but transpose the values.
How to avoid it
N₂ ≈ 78% (the majority — nearly four-fifths). O₂ ≈ 21% (about one-fifth). Use the phrase 'air is four-fifths nitrogen, one-fifth oxygen' as a memory aid.
Why it happens
Students think the more abundant gas (N₂) distils off later.
How to avoid it
The gas with the LOWEST boiling point distils first. N₂ b.p. = −196 °C; O₂ b.p. = −183 °C. N₂ boils off first (lower b.p.) from the fractionating column.
Why it happens
Students overestimate CO₂ because of its prominence in climate discussions.
How to avoid it
CO₂ is only ~0.04% (400 ppm) of dry air — a trace component. It is not a major component alongside N₂ and O₂.
The things students keep getting wrong in this sub-topic, answered.