Incomplete Combustion – IGCSE Chemistry Definition
IGCSE Chemistry definition of incomplete combustion: burning a fuel in limited oxygen, producing CO and/or soot. Covers dangers, equations, and exam tips for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Incomplete combustion is tested primarily as a safety and environmental concern. The examiner wants you to know the products (CO and/or C), why CO is dangerous, and the visual observation of a yellow/sooty flame. This topic regularly appears alongside boiler and gas heater safety questions.
The 0620 definition
Incomplete combustion is the burning of a substance in a limited supply of oxygen, producing carbon monoxide (CO) and/or carbon (soot, C) instead of carbon dioxide.
Water is still produced.
Possible products
Depending on how limited the oxygen supply is:
| Oxygen supply | Carbon product | Equation example |
|---|---|---|
| Slightly limited | CO (carbon monoxide) | 2CH₄ + 3O₂ → 2CO + 4H₂O |
| Very limited | C (soot/carbon) | CH₄ + O₂ → C + 2H₂O |
In practice, a mixture of CO₂, CO, and C is often produced.
The dangers of carbon monoxide
Carbon monoxide is:
- Colourless — you cannot see it
- Odourless — you cannot smell it
- Toxic — it binds to haemoglobin in blood 200 times more strongly than oxygen
- Causes oxygen starvation — organs are deprived of oxygen, leading to headache, drowsiness, unconsciousness, and death
This is why carbon monoxide detectors are fitted in homes with gas boilers.
Observations comparing complete and incomplete combustion
| Feature | Complete combustion | Incomplete combustion |
|---|---|---|
| Flame colour | Blue | Yellow / orange |
| Smoke | None | Black smoke / soot |
| Products | CO₂ + H₂O only | CO and/or C + H₂O |
| Energy released | Maximum | Less |
Practical example: Bunsen burner
- Air hole open: plenty of oxygen → blue flame (complete combustion)
- Air hole closed: limited oxygen → yellow flame (incomplete combustion, soot deposited)
Worked exam question
A gas heater in a poorly ventilated room produces a yellow flame. (a) Explain why the flame is yellow. [1] (b) Name the toxic gas produced. [1] (c) Explain why this gas is dangerous to health. [2]
Mark scheme
(a) Incomplete combustion occurs due to insufficient / limited oxygen supply [1]
(b) Carbon monoxide / CO [1]
(c) Carbon monoxide binds to haemoglobin [1]; prevents oxygen being carried in the blood / causes oxygen starvation [1]
Common exam mistakes
- Saying carbon monoxide has a smell — it is odourless. This is precisely why it is so dangerous; you cannot detect it without a monitor.
- Writing “carbon dioxide” as the toxic product — CO₂ is not the concern here; carbon monoxide (CO) is the toxic gas.
- Confusing soot (carbon, C) with smoke — soot is solid carbon particles deposited from the flame.
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