Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want air — composition, combustion and atmospheric change — to become reliable marks instead of approximate percentages remembered once.
What query it owns: how to understand and revise air in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the air revision-guide angle, while Tutopiya’s Air subtopic page owns the learning resource and the free Air quiz owns the practice.

Air is a mixture of gases that supports life, combustion and countless chemical processes. Cambridge IGCSE Coordinated Science (0654) expects you to state the composition of clean dry air by volume, explain how combustion and photosynthesis change gas proportions, and describe air pollution and its effects. This guide covers the syllabus percentages, reaction equations and the question types that appear every year.

Key takeaways

• Clean dry air by volume: ~78% nitrogen, ~21% oxygen, ~0.9% argon, ~0.04% carbon dioxide, plus trace gases.
• Combustion uses oxygen and produces carbon dioxide (and water vapour); oxygen level decreases in enclosed spaces.
• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — removes CO₂, releases O₂.
• Respiration and combustion increase CO₂; photosynthesis decreases it.
• Air pollution: CO (incomplete combustion), SO₂ (acid rain), oxides of nitrogen (acid rain, photochemical smog), particulates.

What is air in Cambridge IGCSE Coordinated Science?

The air topic covers the composition of the atmosphere, how biological and chemical processes alter gas levels, experiments to investigate air composition, and pollutants with their sources and effects. Examiners test exact percentages, balanced equations, and explain links between human activity and atmospheric change.

You can read the full explanation, diagrams and notes on Tutopiya’s Air subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Composition by volume	Percentages of each gas in clean dry air	”State the percentage of oxygen in air.”
Combustion	Fuel + O₂ → CO₂ + H₂O	”Write the equation for complete combustion of methane.”
Photosynthesis	Plants remove CO₂, release O₂	”Explain how photosynthesis affects air composition.”
Rusting	Iron reacts with O₂ and water	”Describe what is needed for iron to rust.”
Pollution	Harmful gases and particulates	”Describe the effects of sulfur dioxide.”

Composition of clean dry air

Gas	Approximate % by volume	Role / note
Nitrogen (N₂)	78%	Mostly unreactive; dilutes oxygen
Oxygen (O₂)	21%	Needed for respiration and combustion
Argon (Ar)	0.9%	Noble gas; unreactive
Carbon dioxide (CO₂)	0.04%	Used in photosynthesis; greenhouse gas
Other (Ne, He, CH₄, etc.)	Trace amounts	Various minor roles

Processes that change air composition

Process	Effect on O₂	Effect on CO₂	Where it occurs
Combustion (complete)	Decreases	Increases	Engines, fires, power stations
Respiration	Decreases	Increases	Living organisms
Photosynthesis	Increases	Decreases	Green plants (in light)
Rusting	Decreases	—	Iron + O₂ + water
Decomposition	Decreases	Increases	Decaying organic matter

Air pollution — sources and effects

Pollutant	Source	Effect
Carbon monoxide (CO)	Incomplete combustion of fuels	Toxic; binds to haemoglobin
Sulfur dioxide (SO₂)	Burning fossil fuels containing sulfur	Acid rain; respiratory problems
Oxides of nitrogen (NOₓ)	High-temperature engines	Acid rain; photochemical smog
Particulates (soot)	Diesel engines, coal burning	Respiratory disease; reduced visibility
Carbon dioxide (CO₂)	Combustion of all carbon fuels	Enhanced greenhouse effect / climate change
Methane (CH₄)	Agriculture, landfill, natural gas leaks	Greenhouse gas

Air in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical air stem
State	Percentage or fact	”State the percentage of nitrogen in clean dry air.”
Write an equation	Balanced combustion/photosynthesis	”Write the equation for complete combustion of propane.”
Explain	Cause and effect	”Explain why the percentage of CO₂ in air is increasing.”
Describe	Process or pollution effect	”Describe how sulfur dioxide causes acid rain.”
Compare	Two processes	”Compare the effect of photosynthesis and respiration on air.”

Worked exam-style stems (how to answer the wording)

1. “State the approximate percentage by volume of oxygen and nitrogen in clean dry air.” Oxygen: 21%; nitrogen: 78%. Mark-scheme reward: both correct to nearest whole number.
2. “Explain why a candle stops burning when covered with a jar.” The flame uses up oxygen inside the jar; when oxygen falls below the level needed for combustion, the flame goes out. Reward: oxygen used up + insufficient oxygen for combustion.
3. “Describe how sulfur dioxide causes acid rain.” Sulfur dioxide dissolves in rainwater and reacts to form sulfuric acid, which falls as acid rain and damages buildings, lakes and forests. Reward: dissolves in rain + forms acid + environmental damage.

Practise on the Air quiz.

How air connects to the rest of Coordinated Science

Air links to Water, Noble Gases (argon in air) and biology topics including Plant Nutrition (photosynthesis) and Human Influences on Ecosystems. The Cambridge IGCSE Coordinated Science resource hub links every subtopic.

Common mistakes students make

• Stating air is 21% nitrogen (it is 78% nitrogen, 21% oxygen).
• Forgetting water vapour is removed when describing clean dry air.
• Writing incomplete combustion products as CO₂ only (CO and C may also form).
• Confusing photosynthesis (removes CO₂) with respiration (produces CO₂).
• Stating rusting needs oxygen only (needs oxygen and water).

When you need more support

If air questions keep costing marks, work through the Air quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

What is the composition of clean dry air? Approximately 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide by volume, plus trace gases.

Why is the percentage of carbon dioxide increasing? Combustion of fossil fuels and deforestation increase CO₂ release while reducing the number of plants removing it by photosynthesis.

What is complete combustion? A fuel reacts with sufficient oxygen to produce carbon dioxide and water only, releasing maximum energy.

How do I revise air effectively? Learn the composition percentages, combustion and photosynthesis equations, pollution table, then take the Air quiz.

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