A water company suspects that a sample of borehole water contains dissolved metal ions and negative ions (anions). A technician carries out qualitative analysis on the water sample and observes a brick-red flame colour during a flame test. When dilute hydrochloric acid and then barium chloride solution are added to a separate portion of the water sample, a white precipitate forms. Identify the metal ion and the negative ion present in the water sample, and describe what each result tells the technician.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A water company suspects that a sample of borehole water contains dissolved metal ions and negative ions (anions). A technician carries out qualitative analysis on the water sample. During a flame test, a brick-red colour is observed. When dilute hydrochloric acid followed by barium chloride solution is added to a separate portion of the sample, a white precipitate forms.
Model answer (4 marks)
The brick‑red flame colour shows that the metal ion present is Ca²⁺.
The flame test is a qualitative test that identifies cations by the colour they emit when heated.
The white precipitate that appears after adding dilute HCl and then BaCl₂ shows that the anion present is SO₄²⁻.
The precipitate is BaSO₄, which is insoluble and therefore falls out of solution when Ba²⁺ reacts with sulfate ions.
The flame test is a qualitative test that identifies cations by the colour they emit when heated.
The white precipitate that appears after adding dilute HCl and then BaCl₂ shows that the anion present is SO₄²⁻.
The precipitate is BaSO₄, which is insoluble and therefore falls out of solution when Ba²⁺ reacts with sulfate ions.
Examiner tips
- Use the exact flame colour – brick‑red for Ca²⁺. Explain that the flame test is a cation identification method. State that the white precipitate is BaSO₄. Mention that BaSO₄ is insoluble, causing the precipitate.
Common mistakes
- Confusing the flame colour with sodium (yellow) or potassium (purple). Failing to recognise that the precipitate is BaSO₄ and not BaCl₂. Not explaining why the precipitate forms (solubility rule).
Mark scheme (4 marks)
- The brick-red flame colour identifies the metal ion as calcium (Ca²⁺)
- The flame test is a qualitative analysis test used to identify metal ions (cations) by observing the colour produced in the flame
- The white precipitate formed with barium chloride solution (after acidification with dilute hydrochloric acid) identifies the negative ion as sulfate (SO₄²⁻)
- The white precipitate forms because barium sulfate is insoluble and precipitates out of solution when barium ions react with sulfate ions
Key terms in this question
qualitative analysis · flame test · precipitate
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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