A student has a sample of rock salt, which contains salt (sodium chloride) dissolved in water along with insoluble mud and grit. The student wants to obtain a pure, dry sample of salt from the mixture. Describe how the student could do this, naming the techniques used at each stage.

Edexcel GCSE Chemistry (1CH0) — 1.2 Methods of separating and purifying substances · Describe · 4 marks · View as Markdown

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

Rock salt is a naturally occurring mixture of salt and insoluble rock particles. It can be processed to obtain pure sodium chloride.

Model answer (4 marks)

1. Filter the mixture to remove the insoluble mud and grit (filtration). 2. Collect the clear salt solution that passes through the filter paper. 3. Heat the filtrate to evaporate most of the water (evaporation). 4. Cool the remaining solution so that sodium chloride crystals form (crystallisation) and then dry the crystals.

Examiner tips

  • Use the command word ‘describe’ – give a short, clear sequence of steps. Include the names of the techniques: filtration, evaporation, crystallisation, drying. Keep each step to one sentence. Mention that the filtrate is the clear solution.
  • Show the logical order: remove solids first, then remove water, then form crystals, then dry. Examiners reward the correct sequence and technique names.

Common mistakes

  • Confusing filtration with decanting or not naming the technique. Forgetting to mention evaporation before crystallisation. Writing a long paragraph instead of concise steps.

Mark scheme (4 marks)

  1. Filter the mixture to remove the insoluble mud and grit (filtration)
  2. The salt solution (filtrate) passes through the filter paper and is collected
  3. Heat / evaporate the filtrate to remove most of the water
  4. Allow the remaining solution to cool so that salt crystals form (crystallisation), then dry the crystals

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