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

WJEC A-Level Chemistry (Wales) — 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 gently in an open container until it is warm.
2. Allow the solvent to evaporate slowly until the solution becomes saturated and crystals begin to appear.
3. Cool the saturated solution to encourage further crystal growth.
4. Filter the mixture to separate the crystals from the remaining liquid.
5. Dry the collected crystals, for example by patting them with filter paper or leaving them to air‑dry.

Examiner tips

  • Use the exact sequence of heating, evaporation, cooling, filtering, drying; each step is worth one mark.
  • Mention that the container should be open so evaporation can occur.
  • Show that the crystals are collected by filtration, not by decanting.
  • Include that the crystals are dried after filtration to give a pure product.

Common mistakes

  • Omitting the cooling step, which is essential for crystal growth.
  • Failing to specify that the container is open, leading to incomplete evaporation.
  • Not stating that filtration is used to separate crystals from the solution.

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 obtained / until crystals start to form
  3. Allow the saturated solution to cool
  4. Filter the mixture to collect the crystals / separate the crystals from the remaining solution
  5. Allow the crystals to dry / pat dry with filter paper

Key terms in this question

crystallisation

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