A student carries out a series of tests to identify the ions present in an unknown solution. The student adds sodium hydroxide solution to the unknown solution and observes a white precipitate. When excess sodium hydroxide solution is added, the white precipitate dissolves. The student then tests the original solution with dilute hydrochloric acid followed by barium chloride solution and observes a white precipitate. Explain what these observations tell the student about the ions present in the unknown solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student has an unknown solution that may contain one or more of the following ions: Al³⁺, Ca²⁺, Cu²⁺, SO₄²⁻, Cl⁻. The student performs two separate tests on samples of the unknown solution.
Model answer (5 marks)
The white precipitate formed on adding NaOH shows that a metal cation that gives an insoluble hydroxide is present. The fact that the precipitate dissolves when excess NaOH is added identifies the cation as Al³⁺, because Al(OH)₃ is amphoteric and dissolves in excess NaOH, whereas Ca(OH)₂ does not.
The student then adds dilute HCl to the original solution before treating it with BaCl₂. The acid removes any carbonate or sulfite that might give a false positive, and the subsequent white precipitate with BaCl₂ confirms the presence of SO₄²⁻.
Hence the unknown solution contains Al³⁺ and SO₄²⁻.
The student then adds dilute HCl to the original solution before treating it with BaCl₂. The acid removes any carbonate or sulfite that might give a false positive, and the subsequent white precipitate with BaCl₂ confirms the presence of SO₄²⁻.
Hence the unknown solution contains Al³⁺ and SO₄²⁻.
Examiner tips
- State the precipitate with NaOH and explain its dissolution in excess NaOH to identify Al³⁺. Show the purpose of adding HCl before BaCl₂. Mention the BaCl₂ test for sulfate and the resulting white precipitate. Conclude with the two ions present.
- common_mistakes
- :
- Confusing Ca²⁺ with Al³⁺ because both give a white precipitate with NaOH. Forgetting to explain why the precipitate dissolves only for Al(OH)₃. Failing to note that HCl removes interfering ions before the sulfate test.
Mark scheme (5 marks)
- The white precipitate with sodium hydroxide indicates the presence of a metal cation that forms an insoluble hydroxide
- The white precipitate dissolving in excess sodium hydroxide identifies the cation as aluminium ions (Al³⁺), because aluminium hydroxide dissolves in excess NaOH whereas calcium hydroxide does not
- Adding dilute hydrochloric acid before barium chloride solution rules out carbonate or sulfite ions / ensures any interfering ions are removed before the sulfate test
- The white precipitate with barium chloride solution indicates the presence of sulfate ions (SO₄²⁻)
- Therefore the unknown solution contains aluminium ions (Al³⁺) and sulfate ions (SO₄²⁻)
Key terms in this question
precipitate · sodium hydroxide · barium chloride
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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