A student wants to separate ethanol from a mixture of ethanol and water. Describe how simple distillation can be used to obtain a sample of ethanol from the mixture, and explain why this technique works for this separation.

Eduqas A-Level Chemistry — 1.5 Energetics · Describe · 5 marks · View as Markdown

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

Ethanol has a boiling point of 78 °C. Water has a boiling point of 100 °C.

Model answer (5 marks)

1. Heat the mixture in a distillation flask.
2. Ethanol, having the lower boiling point (78 °C), evaporates first.
3. The ethanol vapour rises into the condenser.
4. In the condenser the vapour is cooled and condenses back into liquid ethanol.
5. Simple distillation works because the different boiling points mean only ethanol vapourises at 78 °C, leaving water behind in the flask.

Examiner tips

  • Use the word "evaporates" for the first step. Mention the condenser explicitly. Explain the temperature difference as the reason for separation. Keep the answer concise and point‑wise.

Common mistakes

  • Forgetting to mention the condenser. Saying both liquids evaporate together. Using the wrong boiling point for ethanol.

Mark scheme (5 marks)

  1. The mixture is heated
  2. Ethanol evaporates / turns to vapour first because it has the lower boiling point (78 °C)
  3. The ethanol vapour moves into / through a condenser
  4. The vapour is cooled and condenses back into a liquid
  5. Simple distillation works because ethanol and water have different boiling points, so only the ethanol vaporises at 78 °C, leaving water behind in the flask

Key terms in this question

simple distillation

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