The general idea
- Aqueous sodium hydroxide is dripped into a solution of the unknown cation
- If the metal hydroxide is insoluble, it appears as a coloured precipitate that identifies the metal
- The colour of the precipitate is the diagnostic feature
Iron(II), iron(III) and copper(II)
| Cation | Ionic equation | Precipitate colour |
|---|
| Fe²⁺ (iron(II)) | Fe²⁺(aq) + 2 OH⁻(aq) → Fe(OH)2(s) | Pale green |
| Fe³⁺ (iron(III)) | Fe³⁺(aq) + 3 OH⁻(aq) → Fe(OH)3(s) | Orange-brown (rust colour) |
| Cu²⁺ (copper(II)) | Cu²⁺(aq) + 2 OH⁻(aq) → Cu(OH)2(s) | Light blue |
- Use the exam wording "light blue" for copper(II); just "blue" can lose marks
- Adding excess NaOH does not redissolve any of these three precipitates, which is why a few drops are enough
Ammonium ion, NH4+
- Add aqueous sodium hydroxide to the unknown sample and warm the mixture gently
- If ammonium ions are present, ammonia gas is released:
NH4+(aq) + OH⁻(aq) → NH3(g) + H2O(l)
- Identify the gas with damp red litmus paper held at the mouth of the tube: the paper turns blue, confirming ammonia and therefore ammonium ions
- Keep the test specifically clear in your answers: NH4+ is the ion in solution; NH3 is the gas released