A student dissolves a soluble salt in water to produce a solution. The student wants to obtain dry crystals of the salt. Describe how the student would carry out crystallisation to obtain dry crystals of the salt from the solution.

Eduqas GCSE Chemistry — 1.1 The nature of substances and chemical reactions · 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 solvent evaporates and a saturated solution is obtained.\nContinue heating until small crystals begin to appear.\nRemove the heat and allow the solution to cool slowly so that larger crystals grow.\nOnce crystals have formed, filter the mixture to separate the crystals from the remaining liquid, then dry the crystals in a clean, dry place.

Examiner tips

  • Use the word "heat" and "cool" to show the stages of crystallisation.
  • Mention that the solution must become saturated before crystals form.
  • Show that filtration removes the crystals from the liquid.
  • State that the crystals are dried after filtration.

Common mistakes

  • Failing to mention that the solution must be saturated before crystals appear.
  • Leaving out the step of cooling to grow larger crystals.
  • Not specifying that the crystals are filtered and then dried.

Mark scheme (4 marks)

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

Key terms in this question

crystallisation

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