Complete Combustion – IGCSE Chemistry Definition
IGCSE Chemistry definition of complete combustion: burning a fuel in excess oxygen to produce CO₂ and H₂O. Covers equations, observations, 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.
Combustion equations are tested across the entire organic chemistry topic. Every time you write a balanced equation for burning a fuel — in questions about alkanes, alkenes, alcohols, or fuels — you are writing a complete combustion equation. Balancing these equations correctly is a core skill.
The 0620 definition
Complete combustion is the burning of a substance in a plentiful supply of oxygen, producing carbon dioxide and water as the only products.
General equation
Hydrocarbon + oxygen → carbon dioxide + water
Key examples
| Fuel | Equation |
|---|---|
| Methane | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Ethane | 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O |
| Propane | C₃H₈ + 5O₂ → 3CO₂ + 4H₂O |
| Ethene | C₂H₄ + 3O₂ → 2CO₂ + 2H₂O |
| Ethanol | C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O |
How to balance combustion equations
- Count the carbons in the fuel → same number of CO₂ molecules
- Count the hydrogens in the fuel → half that number of H₂O molecules
- Count all oxygen atoms on the right side → adjust O₂ on the left
- If you get a fraction of O₂, multiply the entire equation by 2
Observations during complete combustion
- Blue flame (clean, no soot)
- No smoke
- Heat and light produced
- Products: CO₂ (turns limewater milky) and H₂O (condenses on cool surfaces)
Environmental impact
Complete combustion is “better” than incomplete combustion because it does not produce toxic CO or soot. However, it still produces CO₂, a greenhouse gas that contributes to the greenhouse effect and climate change.
Worked exam question
Write the balanced equation for the complete combustion of butane, C₄H₁₀. [2]
Mark scheme
2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O [1 for correct formulae, 1 for balancing]
OR: C₄H₁₀ + 6½O₂ → 4CO₂ + 5H₂O (half coefficients are sometimes accepted — check with your teacher)
Common exam mistakes
- Leaving the equation unbalanced — the oxygen coefficient is usually the hardest to calculate. Count all O atoms on the right first.
- Writing CO instead of CO₂ — that would be incomplete combustion.
- Forgetting that alcohols already contain oxygen in their formula — this affects the O₂ needed.
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