A student wants to obtain a sample of pure water from a salt solution. Describe how simple distillation could be used to do this, and explain why this method is suitable for separating water from the salt solution.

WJEC GCSE Chemistry (Wales) — 1.3 Water · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Heat the salt solution.
2. Water evaporates, leaving the salt behind.
3. The vapour is cooled and condenses into liquid water.
4. This works because water boils at 100 °C, while the salt has a much higher boiling point and remains in the residue.

Examiner tips

  • Use the sequence of heating, evaporation, condensation and residue. Mention the boiling point difference to justify the method.

Common mistakes

  • Forgetting to mention the salt stays in the residue. Confusing the boiling point of water with that of the salt.

Mark scheme (4 marks)

  1. The salt solution is heated
  2. Water evaporates / turns to vapour / steam and leaves the solution
  3. The water vapour is cooled and condensed to give liquid water
  4. This method is suitable because water has a much lower boiling point than the dissolved salt / the solute has a much higher boiling point than the solvent

Key terms in this question

simple distillation

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