A student is trying to separate two miscible liquids collected from a chemical reaction — ethanol and water. Explain how fractional distillation could be used to separate these two liquids.

OCR GCSE Chemistry A: Gateway Science (J248) — C2.1 Purity and separating mixtures · Explain · 4 marks · View as Markdown

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

Ethanol boils at 78 °C and water boils at 100 °C. When mixed together, ethanol and water form a mixture that cannot be separated by simple distillation alone.

Model answer (4 marks)

1. Heat the ethanol‑water mixture in a fractionating column.
2. Because ethanol has a lower boiling point (78 °C) it evaporates first.
3. The vapour rises through the column, condenses and re‑vapours repeatedly, enriching the ethanol in the vapour phase.
4. The vapour is cooled and condensed, collecting ethanol as the first fraction; the remaining liquid, richer in water, is collected as the second fraction.

Examiner tips

  • Use the word "evaporates" for the lower‑boiling component. Show the sequence: heating → vapour rise → condensation → collection. Mention that the column provides multiple condensations to separate the two liquids.

Common mistakes

  • Confusing the order of collection (collecting water first). Failing to state that the column allows repeated condensation and re‑vapourisation. Using vague terms like "separate by heating" without explaining the role of boiling points.

Mark scheme (4 marks)

  1. The mixture is heated in a fractionating column
  2. Ethanol has a lower boiling point so it evaporates / vaporises first
  3. The two liquids are collected as separate fractions because they have different boiling points
  4. The vapour is cooled / condensed to collect each liquid

Key terms in this question

fractional distillation

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