Methanol, ethanol, and propanol are all members of the alcohol homologous series. Describe three features that show these substances belong to the same homologous series, and state the functional group present in all alcohols.

WJEC A-Level Chemistry (Wales) — 2.7 Alcohols and carboxylic acids · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

1. All three contain the same functional group – the hydroxyl group (–OH).
2. They share the general formula CₙH₂ₙ₊₁OH, where n = 1, 2, 3.
3. Each successive member differs by one –CH₂– unit, giving a gradual increase in boiling point.

Functional group present in all alcohols: –OH (hydroxyl group).

Examiner tips

  • Use the word ‘contain’ for the functional group; mention the general formula; show the CH₂ difference; give a physical trend example; keep it concise.
  • Include the exact formula CₙH₂ₙ₊₁OH to earn full marks for the general formula.
  • Mention the hydroxyl group explicitly as –OH to satisfy the functional group requirement.

Common mistakes

  • Writing the formula as CₙH₂ₙ₊₂OH (incorrect hydrogen count).
  • Omitting the –CH₂– difference between members.

Mark scheme (5 marks)

  1. All alcohols contain the same functional group (–OH / hydroxyl group)
  2. Members share the same general formula (CₙH₂ₙ₊₁OH)
  3. Each successive member differs by one CH₂ unit / members differ by CH₂
  4. Members show a gradual trend / gradual change in physical properties (e.g. boiling point increases with chain length)
  5. Members have similar chemical properties / react in the same way

Key terms in this question

homologous series · functional group

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