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

Eduqas GCSE Chemistry — 1.3 Water · Describe · 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 until it boils.
2. The water evaporates, forming vapour.
3. The vapour is carried into a condenser where it cools and condenses into pure liquid water.
4. The salt remains in the original vessel because its boiling point is far higher than that of water, so only the water evaporates and is collected.

Examiner tips

  • Use the word "boils" to show heating to 100 °C. Mention the condenser explicitly. Explain the temperature difference between water and salt.
  • Use the exact phrase "higher boiling point" to match the mark scheme.

Common mistakes

  • Failing to mention the condenser. Confusing boiling point with melting point. Saying the salt evaporates instead of remaining behind.

Mark scheme (4 marks)

  1. The solution is heated
  2. The water evaporates / turns to vapour / becomes a gas
  3. The water vapour is cooled and condensed (collected as liquid water)
  4. The technique is suitable because the salt has a much higher boiling point than water, so only the water evaporates

Key terms in this question

simple distillation

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