A student carries out crystallisation to obtain a pure sample of copper sulfate from a solution of copper sulfate. Describe the steps the student should follow to obtain dry copper sulfate crystals from the solution.

WJEC GCSE Chemistry (Wales) — 1.5 Rate of chemical change · 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 sulfate solution in an open container to evaporate water.
2. Continue heating until the solution becomes saturated and crystals begin to form.
3. Allow the hot solution to cool slowly so that larger crystals grow.
4. Filter the crystals from the remaining liquid and dry them to obtain a pure sample.

Examiner tips

  • Use the word "heat" and mention an open container to show evaporation. State that crystals form when the solution becomes saturated. Explain that cooling promotes crystal growth. Mention filtering and drying to remove residual solvent.

Common mistakes

  • Failing to note that the container must be open to allow evaporation. Omitting the cooling step, which is essential for crystal growth. Not specifying that the crystals should be filtered and dried to remove water.

Mark scheme (4 marks)

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

Key terms in this question

crystallisation

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