A student dissolves a soluble salt in water to make a solution. The student then uses crystallisation to obtain the solid salt from the solution. Describe the process of crystallisation the student should carry out.

Eduqas GCSE Chemistry — 1.3 Water · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Heat the solution in an open container until the water evaporates and a saturated solution remains.
Allow the saturated solution to cool; as it cools the solubility decreases and crystals begin to form.
Let the crystals grow slowly, ensuring they are well‑formed.
Filter the crystals from the remaining liquid and allow them to dry to give the solid salt.

Examiner tips

  • Use the exact sequence: heat, evaporate, cool, filter, dry.
  • Mention that the container should be open to allow evaporation.
  • Show that cooling is necessary for crystal growth.
  • State that the crystals are filtered and then dried.

Common mistakes

  • Omitting the cooling step, or saying the solution is simply left to evaporate.
  • Failing to mention that the container must be open to allow evaporation.

Mark scheme (4 marks)

  1. Heat the solution (in an open container / evaporating basin)
  2. Allow the solvent to evaporate until a saturated solution is formed / until crystals start to form
  3. Allow the solution to cool so that crystals grow
  4. Filter the crystals from the remaining solution (and allow to dry)

Key terms in this question

crystallisation

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