Alkane – IGCSE Chemistry Definition
IGCSE Chemistry definition of alkane: a saturated hydrocarbon with general formula CₙH₂ₙ₊₂. Covers naming, reactions, and exam tips for Cambridge 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.
Alkanes are the simplest homologous series and the foundation of organic chemistry at IGCSE. You must know their naming, general formula, combustion reactions, and how they differ from alkenes. They appear on every IGCSE Chemistry paper.
The 0620 definition
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only single covalent bonds between carbon atoms.
The first four alkanes
| Name | Formula | Structural formula | State at 25 °C |
|---|---|---|---|
| Methane | CH₄ | CH₄ | Gas |
| Ethane | C₂H₆ | CH₃CH₃ | Gas |
| Propane | C₃H₈ | CH₃CH₂CH₃ | Gas |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ | Gas |
Boiling points increase with chain length because intermolecular forces increase as molecules get larger.
Reactions of alkanes
1. Combustion (burning in air)
Complete combustion (excess O₂): CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion (limited O₂): 2CH₄ + 3O₂ → 2CO + 4H₂O
Methane is natural gas — the most common fuel for domestic heating and cooking.
2. Substitution with halogens (UV light)
CH₄ + Cl₂ → CH₃Cl + HCl
Conditions: UV light required. See substitution reaction.
3. Cracking (thermal decomposition)
Long-chain alkanes are broken into shorter, more useful molecules. See cracking.
Alkanes vs alkenes
| Property | Alkanes | Alkenes |
|---|---|---|
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Saturation | Saturated | Unsaturated |
| Bromine water test | No reaction | Decolourises |
| Characteristic reaction | Substitution | Addition |
Worked exam question
Write the balanced equation for the complete combustion of propane, C₃H₈. [1]
Mark scheme
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [1]
Common exam mistakes
- Writing the general formula wrong — alkanes are CₙH₂ₙ₊₂, not CₙH₂ₙ (that is alkenes).
- Saying alkanes react with bromine water — they do not. Only alkenes decolourise bromine water.
- Forgetting to balance combustion equations — count all C, H, and O atoms carefully.
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