Homologous Series – IGCSE Chemistry Definition
IGCSE Chemistry definition of homologous series: a family of compounds with the same general formula and similar properties. Covers key features and 0620 tips.
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.
The concept of a homologous series is the organising principle of organic chemistry at IGCSE. Every organic topic — naming, reactions, properties — makes more sense when you understand that compounds within a series share a pattern. Paper 2 and Paper 4 both test the definition and features.
The 0620 definition
A homologous series is a family of organic compounds that have the same functional group, the same general formula, and similar chemical properties. Each successive member differs by CH₂.
Features of a homologous series
- Same general formula (e.g. alkanes: CₙH₂ₙ₊₂)
- Same functional group (e.g. alkenes: C=C; alcohols: −OH)
- Similar chemical properties (because they share the same functional group)
- Gradual change in physical properties (boiling point increases as chain length increases)
- Each member differs from the next by CH₂ (e.g. CH₄, C₂H₆, C₃H₈…)
- Members can be represented by the same general formula
The homologous series you must know
| Series | General formula | Functional group | First member |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | None (all C−C and C−H) | Methane, CH₄ |
| Alkenes | CₙH₂ₙ | C=C double bond | Ethene, C₂H₄ |
| Alcohols | CₙH₂ₙ₊₁OH | −OH (hydroxyl) | Methanol, CH₃OH |
| Carboxylic acids | CₙH₂ₙ₊₁COOH | −COOH (carboxyl) | Methanoic acid, HCOOH |
Trend in physical properties
As chain length increases within any homologous series:
- Boiling point increases — longer molecules have stronger intermolecular forces
- Viscosity increases — longer chains tangle more
- Flammability decreases — harder to vaporise, so harder to ignite
Worked exam question
State three features of a homologous series. [3]
Mark scheme
Any three from: same general formula [1]; same functional group [1]; similar chemical properties [1]; gradual change in physical properties [1]; successive members differ by CH₂ [1]
Common exam mistakes
- Saying members have “the same properties” — chemical properties are similar, but physical properties change gradually. The answer must distinguish between the two.
- Forgetting the CH₂ difference between successive members — this is a commonly tested point.
- Writing “same formula” instead of “same general formula” — each member has a different molecular formula, but they all fit the same general formula.
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