A student has a mixture of sand and salt dissolved in water. Describe how the student could obtain a dry sample of pure salt from this mixture.

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).

Model answer (4 marks)

Filter the mixture through filter paper to remove the sand, leaving a clear salt solution (filtrate). Heat the filtrate to evaporate most of the water, concentrating the salt. Allow the remaining solution to cool; salt crystals will precipitate. Collect and dry the crystals to obtain a dry sample of pure salt.

Examiner tips

  • Use the word ‘filter’ to show the separation step; mention ‘filtrate’ to indicate the liquid that passes through.
  • Show the evaporation step and explain that cooling allows crystallisation.
  • Mention that the crystals are then dried to give a dry product.
  • Use correct terminology: ‘sand (insoluble residue)’, ‘salt solution’, ‘filtrate’, ‘evaporation’, ‘crystallisation’, ‘dry crystals’.

Common mistakes

  • Not specifying that the sand is the insoluble residue.
  • Failing to mention the evaporation step or that cooling causes crystallisation.
  • Leaving out the final drying step, or describing the product as ‘wet’ instead of ‘dry’.

Mark scheme (4 marks)

  1. Filter the mixture to remove the sand (insoluble residue)
  2. The salt solution (filtrate) passes through the filter paper
  3. Evaporate the salt solution / heat the filtrate to remove most of the water
  4. Allow the remaining solution to cool so that salt crystals form, then dry the crystals

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