A geologist discovers a mineral sample and suspects it contains iron(III) ions, calcium ions and sulfate ions. Describe the tests the geologist would carry out to confirm the presence of each of these three ions, giving the expected observation for each test.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The geologist dissolves the mineral sample in dilute hydrochloric acid to produce a solution for testing.
Model answer (5 marks)
1. Add a few drops of sodium hydroxide solution to the test solution. A rust‑brown / reddish‑brown precipitate of Fe(OH)₃ forms, confirming Fe³⁺.
2. Add dilute hydrochloric acid to the solution, then add barium chloride solution. A white precipitate of BaSO₄ appears, confirming SO₄²⁻.
3. For Ca²⁺:
a. Flame test – hold a clean nichrome wire in the solution and place it in a Bunsen flame. A brick‑red / orange‑red flame is observed, confirming Ca²⁺.
b. (Alternative) Add excess sodium hydroxide to the solution. A white precipitate of Ca(OH)₂ forms, confirming Ca²⁺.
2. Add dilute hydrochloric acid to the solution, then add barium chloride solution. A white precipitate of BaSO₄ appears, confirming SO₄²⁻.
3. For Ca²⁺:
a. Flame test – hold a clean nichrome wire in the solution and place it in a Bunsen flame. A brick‑red / orange‑red flame is observed, confirming Ca²⁺.
b. (Alternative) Add excess sodium hydroxide to the solution. A white precipitate of Ca(OH)₂ forms, confirming Ca²⁺.
Examiner tips
- Use the exact test names (e.g. sodium hydroxide for Fe³⁺, BaCl₂ for SO₄²⁻, flame test or NaOH for Ca²⁺).
- State the colour or appearance of the precipitate/flame clearly.
- Mention the order of adding reagents to avoid interference.
Common mistakes
- Mixing reagents in the wrong order, e.g. adding BaCl₂ before acidising, which can give a false negative for sulfate.
- Failing to specify the colour of the precipitate or flame, or using the wrong test for Ca²⁺ (e.g. not recognising the brick‑red flame).
Mark scheme (5 marks)
- Add sodium hydroxide solution to test for iron(III) ions
- Observation for iron(III) ions: a rust-brown / reddish-brown precipitate forms
- Add dilute hydrochloric acid followed by barium chloride solution to test for sulfate ions
- Observation for sulfate ions: a white precipitate forms
- Add dilute hydrochloric acid followed by sodium hydroxide solution (or use a flame test showing a brick-red / orange-red flame) to confirm calcium ions — award the mark for either: [flame test] hold a nichrome wire in the solution and place in a Bunsen flame, observation: brick-red / orange-red flame; OR [precipitation] add excess sodium hydroxide, observation: white precipitate
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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