Methane (CH₄) is a hydrocarbon and the simplest member of the alkane homologous series. Explain what is meant by the term homologous series and describe two features that all alkanes share as members of the same homologous series.

WJEC GCSE Chemistry (Wales) — 2.6 Carbon chemistry / organic chemistry · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A homologous series is a group of compounds that have the same general formula and differ by a constant unit, usually a CH₂ group.

All alkanes share the general formula CₙH₂ₙ₊₂.

All alkanes contain only single C–C and C–H bonds – they are saturated.

All alkanes have similar chemical properties and their physical properties, such as boiling point, increase gradually with increasing chain length.

Examiner tips

  • Use the definition of a homologous series – mention the CH₂ difference. Include the general formula CₙH₂ₙ₊₂. State the saturation (single bonds) and the trend in physical properties. Keep the answer concise and use exam‑specific terminology.

Common mistakes

  • Confusing alkanes with alkenes or alkynes. Forgetting to mention the CH₂ difference. Using vague terms like "similar" without specifying saturation or the formula.

Mark scheme (4 marks)

  1. A homologous series is a family/group of compounds that share the same general formula (and differ by CH₂ each time).
  2. All alkanes have the same general formula (CₙH₂ₙ₊₂).
  3. All alkanes contain only single (C–C and C–H) bonds / are saturated.
  4. All alkanes have similar chemical properties (and their physical properties show a gradual trend / boiling point increases with increasing chain length).

Key terms in this question

homologous series · alkane

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