A student carries out tests on four unlabelled aqueous solutions, each containing a different metal ion. The solutions contain copper(II) ions, iron(III) ions, calcium ions, and zinc ions respectively. Describe how the student could use aqueous sodium hydroxide solution to identify which solution contains copper(II) ions, which contains iron(III) ions, and which contains zinc ions. Include the observations the student would make for each ion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Aqueous sodium hydroxide can be used to identify metal ions in solution by the colour of the precipitate formed.
Model answer (5 marks)
1. Add a few drops of aqueous NaOH to the first unknown solution. A blue precipitate forms – this indicates Cu²⁺.
2. Add NaOH to the second unknown. A brown or rust‑coloured precipitate appears – this is Fe(OH)₃, so the solution contains Fe³⁺.
3. Add NaOH to the third unknown. A white precipitate forms – this could be either Ca(OH)₂ or Zn(OH)₂.
4. Add a little more NaOH to the white precipitate. If it dissolves, the ion is Zn²⁺ (forming soluble [Zn(OH)₄]²⁻). If it remains insoluble, the ion is Ca²⁺.
5. The fourth unknown, after the above tests, will be the remaining ion (the one not identified as Cu²⁺, Fe³⁺ or Zn²⁺).
2. Add NaOH to the second unknown. A brown or rust‑coloured precipitate appears – this is Fe(OH)₃, so the solution contains Fe³⁺.
3. Add NaOH to the third unknown. A white precipitate forms – this could be either Ca(OH)₂ or Zn(OH)₂.
4. Add a little more NaOH to the white precipitate. If it dissolves, the ion is Zn²⁺ (forming soluble [Zn(OH)₄]²⁻). If it remains insoluble, the ion is Ca²⁺.
5. The fourth unknown, after the above tests, will be the remaining ion (the one not identified as Cu²⁺, Fe³⁺ or Zn²⁺).
Examiner tips
- Use the colour of the precipitate first to separate Cu²⁺, Fe³⁺ and the white‑precipitate group. Then test solubility in excess NaOH to distinguish Zn²⁺ from Ca²⁺.
- Mention the specific colours (blue, brown/rust, white) and the dissolution step for zinc.
- Keep the answer concise – 5 marks are for a short description, not a long essay.
Mark scheme (5 marks)
- Adding aqueous sodium hydroxide to the copper(II) ion solution produces a blue precipitate
- Adding aqueous sodium hydroxide to the iron(III) ion solution produces a brown/rust-coloured precipitate
- Adding aqueous sodium hydroxide to the zinc ion solution produces a white precipitate
- The white precipitate from the zinc ion solution dissolves when excess sodium hydroxide is added
- The calcium ion solution also produces a white precipitate with sodium hydroxide, but this precipitate does NOT dissolve in excess sodium hydroxide, allowing calcium to be distinguished from zinc
Key terms in this question
aqueous sodium hydroxide · copper(II) ions · iron(III) ions · zinc ions
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Reactions of ions in aqueous solution (A-Level only) questions
- A student has three unlabelled solutions: copper(II) sulfate, iron(III) chloride…
- A student adds dilute hydrochloric acid to separate aqueous solutions of silver …
- A student investigates the reactions of aqueous ammonium ions and aqueous carbon…
- A student adds a few drops of dilute sodium hydroxide solution to separate aqueo…