A student has two unlabelled solutions. One solution contains sulfate ions and the other contains chloride ions. Describe how the student could identify which solution contains sulfate ions and which contains chloride ions. Include the expected observations for each test.

Eduqas GCSE Chemistry — C8 Chemical analysis · 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. Acidify the first solution with dilute hydrochloric acid to remove any carbonate or hydroxide ions.
2. Add a few drops of barium chloride solution. A white precipitate of BaSO₄ indicates the presence of sulfate ions.
3. Acidify the second solution with dilute nitric acid to keep the medium acidic.
4. Add a few drops of silver nitrate solution. A white or cream precipitate of AgCl confirms chloride ions.

Examiner tips

  • Use the correct acid for each test (HCl for BaCl₂, HNO₃ for AgNO₃).
  • Show the sequence of steps and the expected precipitate colour for each ion.
  • Mention that the precipitate is white for both tests but the reagents differ.

Common mistakes

  • Using the wrong acid (e.g. H₂SO₄) which can give a false positive for sulfate.
  • Failing to acidify before adding AgNO₃, leading to no precipitate.

Mark scheme (4 marks)

  1. Add dilute hydrochloric acid / acidify with dilute hydrochloric acid (to remove interfering ions / carbonates) before adding barium chloride solution
  2. Add barium chloride solution to test for sulfate ions; a white precipitate forms in the solution containing sulfate ions
  3. Add silver nitrate solution (in acidic conditions / after acidifying with dilute nitric acid) to test for chloride ions
  4. A white / cream precipitate forms in the solution containing chloride ions when silver nitrate is added

Key terms in this question

sulfate ions · chloride ions

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