A student uses simple distillation to separate ethanol from a mixture of ethanol and water. Explain why simple distillation is able to separate these two liquids, and describe what happens to the mixture during the process.

WJEC A-Level Chemistry (Wales) — 1.5 Energetics · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Ethanol and water have different boiling points – ethanol 78.4 °C, water 100 °C. When the mixture is heated, the component with the lower boiling point, ethanol, evaporates first. The ethanol vapour rises through the column and enters the condenser, where it is cooled and condenses back into liquid, which is collected. As ethanol leaves the flask, the remaining solution becomes richer in water; the ethanol concentration in the flask decreases as distillation continues.

Examiner tips

  • Mention the boiling point difference first to justify separation.
  • Show the sequence: heating → ethanol vapour → condenser → collection.
  • Explain that the flask becomes more water‑rich as ethanol is removed.

Common mistakes

  • Failing to state the boiling point difference.
  • Confusing the order of evaporation and condensation.
  • Not noting that the flask becomes more concentrated in water.

Mark scheme (5 marks)

  1. Ethanol and water have different boiling points
  2. When heated, ethanol (the liquid with the lower boiling point) evaporates / turns to vapour first
  3. The ethanol vapour moves away from the solution / travels into the condenser
  4. The vapour is cooled / condensed and collected as a liquid
  5. The remaining solution becomes more concentrated in water / the amount of ethanol in the flask decreases as distillation continues

Key terms in this question

simple distillation

Related

More Energetics questions

▶ Try answering this question with AI marking (free) →