What is the composition of air according to the Cambridge IGCSE Coordinated Science syllabus?

Air is primarily composed of nitrogen (approximately 78%) and oxygen (about 21%). The remaining 1% consists of argon, carbon dioxide, water vapor, and trace gases. Understanding the composition of air is crucial for studying various environmental and biological processes.

How does air pressure affect weather patterns?

Air pressure is the force exerted by the weight of air in the atmosphere. It plays a significant role in weather patterns. High air pressure typically leads to clear, calm weather, while low air pressure is associated with clouds and precipitation. This concept is essential in the Cambridge IGCSE Coordinated Science curriculum for understanding meteorological phenomena.

Why is oxygen important for living organisms?

Oxygen is vital for the survival of most living organisms as it is required for cellular respiration, a process that releases energy from food. This energy is essential for growth, repair, and maintaining bodily functions. The study of oxygen's role in respiration is a key component of the Cambridge IGCSE Coordinated Science syllabus.

How does human activity impact air quality?

Human activities, such as burning fossil fuels, industrial processes, and deforestation, release pollutants like carbon dioxide, sulfur dioxide, and nitrogen oxides into the air. These pollutants can degrade air quality, leading to health problems and environmental issues like acid rain and global warming. Understanding these impacts is crucial for students studying the Cambridge IGCSE Coordinated Science course.

What methods are used to measure air pollution?

Air pollution is measured using various methods, including air quality monitoring stations that detect pollutants like particulate matter, nitrogen dioxide, and ozone. These measurements help assess the air quality index (AQI) and inform policies to improve air quality. This topic is covered in the Cambridge IGCSE Coordinated Science curriculum to help students understand environmental science and public health.

Why is "Stating that air is 20% N₂ and 80% O₂ (reversing the percentages)" a common mistake in Air?

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. Source: 0654 Examiner Report 2022

Why is "Confusing which gas comes off first in fractional distillation of air" a common mistake in Air?

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 is "Listing CO₂ as a major component of air" a common mistake in Air?

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.

Air and WaterCambridge IGCSE Coordinated Sciences 0654

Air

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Air — Cambridge IGCSE Coordinated Science 0654

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.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C11.4 — State the composition of clean air.
C11.5 — Describe the separation of air by fractional distillation.
C11.6 — Describe the uses of nitrogen, oxygen, and argon.

Composition and Separation of Air

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):

Nitrogen (N₂): ~78%
Oxygen (O₂): ~21%
Argon (Ar): ~1%
Carbon dioxide (CO₂): ~0.04%
Water vapour and other noble gases: trace amounts

Separation by fractional distillation of liquid air:

Air filtered to remove dust
CO₂ and water vapour removed (absorbed by KOH pellets and silica gel respectively)
Air is compressed and cooled repeatedly → liquefied (liquid air, BP ≈ −196°C)
Liquid air enters a fractionating column; warmed gradually
N₂ (BP −196°C) boils off FIRST from the top
Ar (BP −186°C) boils next
O₂ (BP −183°C) remains liquid longest; collected from bottom

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

Nitrogen and oxygen make up 99% of dry air; argon, CO₂ and water vapour account for the rest.

Air: 78% N₂, 21% O₂, 1% Ar, 0.04% CO₂.
Fractional distillation of liquid air: N₂ (−196°C) boils first, then Ar, then O₂ (−183°C).
Lower BP = collected first/at top of column.

How it’s examined

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.

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Step-by-step worked examples — Air

Step-by-step solutions to past-paper-style questions on air, written exactly the way a tutor would explain them at the board.

1Composition of dry air
Core
Question
State the approximate percentage by volume of nitrogen, oxygen, and noble gases in dry air.
Step-by-step solution
1. Step 1
  Nitrogen (N₂): approximately 78% by volume.
2. Step 2
  Oxygen (O₂): approximately 21% by volume.
3. 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.
2Obtaining oxygen from air by fractional distillation
Extended
Question
Describe how oxygen is obtained from air by fractional distillation.
Step-by-step solution
1. Step 1
  Air is filtered to remove dust and other particles.
2. Step 2
  The clean air is compressed and then cooled until it liquefies (temperature reduced to below −183 °C, the boiling point of oxygen).
3. Step 3
  The liquid air is fed into a fractionating column where temperature increases from bottom to top.
4. 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.
3Industrial uses of nitrogen
Core
Question
State TWO large-scale industrial uses of nitrogen gas obtained from fractional distillation of air.
Step-by-step solution
1. Step 1
  Use 1: manufacture of ammonia (Haber process) — N₂ + 3H₂ ⇌ 2NH₃; ammonia is used to make fertilisers.
2. Step 2
  Use 2: inert atmosphere for food packaging — N₂ is unreactive; displaces O₂ in packaging to prevent food oxidation and extend shelf life.
3. 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.

Key Definitions and Keywords — Air

Definitions to memorise and the exact keywords mark schemes credit for air answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Composition of air
Examiner keyword
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.
Fractional distillation of liquid air
Examiner keyword
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).
Glowing splint test (O₂)
Examiner keyword
A glowing (not burning) wooden splint relights in a gas sample if the gas is oxygen.
Argon in air
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.

Common Mistakes and Misconceptions — Air

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.

✕Stating that air is 20% N₂ and 80% O₂ (reversing the percentages)
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.
✕Confusing which gas comes off first in fractional distillation of air
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.
✕Listing CO₂ as a major component of air
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.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

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Air — frequently asked questions

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Air

Short Study Notes in the form of Slides

Short Study Notes — Air

Detailed Notes

What you’ll learn

How it’s examined

1Composition of dry air

2Obtaining oxygen from air by fractional distillation

3Industrial uses of nitrogen

Composition of air

Fractional distillation of liquid air

Glowing splint test (O₂)

Argon in air

✕Stating that air is 20% N₂ and 80% O₂ (reversing the percentages)

✕Confusing which gas comes off first in fractional distillation of air

✕Listing CO₂ as a major component of air

Practice questions

Past paper style quiz

Assess your understanding

Air — frequently asked questions