Zinc chloride can be prepared by reacting zinc carbonate with dilute hydrochloric acid. Describe how a pure, dry sample of zinc chloride could be obtained from this reaction.

Cambridge International IGCSE Chemistry (0620) — 7.3 Preparation of salts · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Add excess zinc carbonate to dilute hydrochloric acid to ensure all zinc chloride is formed.
Filter the mixture to remove the unreacted zinc carbonate.
Evaporate the filtrate to reduce the volume and concentrate the zinc chloride solution.
Allow the concentrated solution to cool so zinc chloride crystallises, then filter and dry the crystals to obtain a pure, dry sample.

Examiner tips

  • Use the word "add" to show the reagent addition, "filter" to remove solids, "evaporate" to concentrate, and "crystallise" to describe crystal formation.
  • Mention that excess zinc carbonate guarantees complete reaction and that drying removes water of crystallisation.
  • Show the sequence clearly: addition → filtration → evaporation → crystallisation → drying.
  • Use UK spelling (e.g., "crystallise").

Common mistakes

  • Failing to mention the use of excess zinc carbonate to drive the reaction to completion.
  • Omitting the step of evaporating the filtrate before crystallisation.
  • Not specifying that the crystals should be dried to remove any remaining water.

Mark scheme (4 marks)

  1. Add excess zinc carbonate to the dilute hydrochloric acid
  2. Filter to remove excess zinc carbonate
  3. Evaporate the filtrate / solution to reduce volume / concentrate the solution
  4. Allow to cool / crystallise, then filter and dry the crystals

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