A student uses crystallisation to obtain pure copper sulfate crystals from a copper sulfate solution. Describe the steps the student should follow to carry out this process successfully.

Eduqas A-Level Chemistry — 1.5 Energetics · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

1. Heat the copper sulfate solution in an open container until it is warm.
2. Allow the solvent to evaporate until the solution becomes saturated or until crystals first appear.
3. Cool the saturated solution.
4. As the solution cools, copper sulfate crystals form.
5. Filter the crystals from the remaining liquid and let them dry.

Examiner tips

  • Use the exact sequence of steps; examiners look for the order of heating, evaporation, cooling, crystallisation and filtration.
  • Mention that the container should be open to allow evaporation.
  • Show that crystals are collected by filtration and then dried to remove residual solvent.

Common mistakes

  • Omitting the step of cooling before filtration.
  • Using a closed container, preventing evaporation.
  • Failing to mention that the crystals are dried after filtration.

Mark scheme (5 marks)

  1. Heat/warm the copper sulfate solution in an open container
  2. Allow the solvent to evaporate until a saturated solution is formed / until crystals begin to appear
  3. Allow the solution to cool
  4. Crystals form as the solution cools
  5. Filter the crystals from the remaining solution and allow them to dry

Key terms in this question

crystallisation

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