Crystallisation can be used to obtain a pure salt from a solution. Describe the steps involved in obtaining pure copper sulfate crystals from a copper sulfate solution.

WJEC GCSE Chemistry (Wales) — 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 copper(II) sulfate solution in an open container until the water evaporates and the solution becomes saturated.
2. Continue heating until crystals begin to appear and the volume of liquid has reduced.
3. Allow the hot solution to cool; as it cools, the crystals grow.
4. Filter the crystals from the remaining liquid and let them dry to give pure copper(II) sulfate crystals.

Examiner tips

  • Use the word "heat" and mention an open container to show evaporation. Show the sequence: heat → saturation → cooling → filtering → drying. Mention that crystals form when the solution becomes saturated and then cools.
  • Include the key terms: saturated solution, crystals, filter, dry.

Mark scheme (4 marks)

  1. Heat / warm the copper sulfate solution (in an open container) to allow the solvent (water) to evaporate
  2. Continue heating until a saturated solution is formed / until crystals start to appear / until the volume of solution has reduced
  3. Allow the solution to cool so that crystals form / grow
  4. Filter the crystals from the remaining solution and allow them to dry

Key terms in this question

crystallisation

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