Methanol, ethanol and propan-1-ol are the first three members of the alcohol homologous series. Describe the physical properties of these three alcohols and explain how and why these properties change as the carbon chain length increases.

Eduqas A-Level Chemistry — 4.3 Alcohols and phenols · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

All three alcohols are liquids at room temperature.
All three are miscible with water.
Boiling point rises from methanol (64.7 °C) to ethanol (78.4 °C) to propan‑1‑ol (97.4 °C).
The increase is due to larger molecules having more surface area, giving stronger London dispersion forces and more hydrogen‑bonding opportunities.
Consequently, solubility in water falls: methanol > ethanol > propan‑1‑ol.

Examiner tips

  • Use the exact terms ‘liquids at room temperature’, ‘miscible with water’, ‘boiling point increases’, ‘stronger intermolecular forces’, ‘solubility decreases’.
  • Show the trend in the correct order and give a brief reason for each change.
  • Keep the answer concise – one sentence per point.
  • Mention the specific boiling points if space allows to demonstrate depth of knowledge.

Common mistakes

  • Saying the alcohols are gases or solids at room temperature.
  • Forgetting to mention miscibility with water.
  • Confusing the trend of solubility (writing that it increases instead of decreases).

Mark scheme (5 marks)

  1. All three alcohols are liquids at room temperature (or have relatively low boiling points)
  2. All three alcohols are miscible with water / dissolve completely in water
  3. Boiling point increases as the carbon chain length increases (from methanol to propan-1-ol)
  4. Larger molecules have stronger intermolecular forces / more energy needed to separate the molecules
  5. Solubility in water decreases as chain length increases / propan-1-ol is less soluble than methanol

Key terms in this question

homologous series · carbon chain length

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