A student has prepared a sample of copper sulfate crystals from a reaction mixture that also contains some unreacted solid copper oxide. Explain how the student could obtain a pure, dry sample of copper sulfate crystals from this mixture.

Edexcel GCSE Chemistry (1CH0) — 1.2 Methods of separating and purifying substances · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Filter the mixture to remove the insoluble copper oxide.
2. Collect the filtrate, which contains copper(II) sulfate in solution, and heat it to evaporate some water.
3. Allow the concentrated solution to cool; copper(II) sulfate will crystallise.
4. Separate the crystals and dry them, e.g. by patting with filter paper or placing them in a warm oven.

Examiner tips

  • Use the word ‘filter’ to show the separation step. Mention that the filtrate contains CuSO₄. Explain that cooling causes crystallisation. Show that drying removes residual water.

Common mistakes

  • Failing to state that the filtrate contains CuSO₄. Not mentioning the cooling step to form crystals. Leaving out the drying step or describing it too vaguely.

Mark scheme (4 marks)

  1. Filtration is used to remove the insoluble copper oxide from the mixture
  2. The filtrate (copper sulfate solution) is collected and heated/evaporated to remove some of the water
  3. Leave the concentrated solution to cool, allowing copper sulfate crystals to form by crystallisation
  4. Remove the crystals and dry them, for example by patting with filter paper or leaving in a warm oven

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