A student has prepared a sample of copper(II) sulfate crystals that are contaminated with a small amount of an insoluble black solid. Describe how the student could obtain a pure, dry sample of copper(II) sulfate crystals from this contaminated mixture.

Cambridge International IGCSE Chemistry (0620) — 12.4 Separation and purification · 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. Dissolve the contaminated copper(II) sulfate in hot water to form a clear solution.
2. Filter the solution through filter paper to remove the insoluble black solid.
3. Heat the filtrate until it becomes saturated and crystals begin to form.
4. Cool the solution, filter off the copper(II) sulfate crystals, and dry them (e.g. between filter paper or in a warm oven).

Examiner tips

  • Use the word ‘filter’ to show removal of the solid; mention heating to saturation before cooling.
  • Show the sequence: dissolve → filter → evaporate/heat → cool → filter crystals → dry.
  • Use UK spelling (e.g. ‘filter’ not ‘filtrate’).

Common mistakes

  • Forgetting to heat the filtrate to saturation before cooling.
  • Describing the drying step as ‘let it sit’ without specifying a method or temperature.
  • Using the wrong solvent (e.g. cold water) which may not dissolve all CuSO4.

Mark scheme (4 marks)

  1. Dissolve the contaminated copper(II) sulfate in hot water to make a solution
  2. Filter the solution to remove the insoluble black solid (residue), collecting the filtrate
  3. Heat / evaporate the filtrate until the point of crystallisation (until saturated), then allow to cool
  4. Filter off the crystals and pat/dry them (e.g. between filter paper or leave to dry in a warm oven)

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