A student has a mixture of two liquids, ethanol and water. Explain how fractional distillation could be used to obtain a pure sample of ethanol from this mixture.

WJEC GCSE Chemistry (Wales) — 1.3 Water · Explain · 4 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 (4 marks)

1. The mixture is heated in a fractionating column.
2. Ethanol, having a lower boiling point (78 °C) than water (100 °C), evaporates first.
3. The ethanol vapour rises through the column and is collected as a separate fraction.
4. The vapour is cooled and condensed to give a pure liquid ethanol.

Examiner tips

  • Use the word "evaporates" to show understanding of phase change. Mention the column as the key separation device. Show the sequence: heating → evaporation → rise → collection → condensation. Use the correct boiling points to justify the order.

Common mistakes

  • Confusing boiling point with melting point. Failing to mention the fractionating column. Saying the vapour is "collected" without noting condensation. Using vague terms like "separated" without explaining the mechanism.

Mark scheme (4 marks)

  1. The mixture is heated in a fractionating column
  2. Ethanol has a lower boiling point than water, so it evaporates / boils first
  3. The ethanol vapour rises up the fractionating column and is collected as a separate fraction
  4. The ethanol vapour is cooled and condensed to give a pure liquid

Key terms in this question

fractional distillation · fraction

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