Identifying ions with sodium hydroxide and other tests (HT)
A more systematic way to identify metal cations and common anions in solution uses sodium hydroxide (for cations) and various reagents (for anions).
Cation tests with NaOH
Add a few drops of NaOH(aq) to the test solution. Most metal cations form insoluble hydroxide precipitates of distinctive colours.
| Cation | Colour of precipitate | Notes |
|---|---|---|
| Aluminium Al³⁺ | White | Dissolves in excess NaOH (forms aluminate) |
| Calcium Ca²⁺ | White | Stays as solid in excess |
| Magnesium Mg²⁺ | White | Stays in excess |
| Copper(II) Cu²⁺ | Blue | |
| Iron(II) Fe²⁺ | Green | (turns brown on standing — oxidises) |
| Iron(III) Fe³⁺ | Brown |
Distinguishing white precipitates — add excess NaOH:
- Al(OH)₃ dissolves → clear (Al³⁺ confirmed).
- Ca(OH)₂ and Mg(OH)₂ stay solid.
To distinguish Ca²⁺ from Mg²⁺, do a flame test (Ca²⁺ gives orange-red).
Equation example
Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) (blue precipitate) Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) (brown precipitate)
Anion tests
Carbonates (CO₃²⁻)
Add dilute HCl. Effervescence occurs (CO₂ given off). Bubble through limewater → cloudy. CO₃²⁻ + 2H⁺ → H₂O + CO₂.
Sulfates (SO₄²⁻)
Add dilute HCl, then barium chloride solution. White precipitate of BaSO₄ forms. Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
(The HCl removes any carbonate first, which would also give a precipitate.)
Halides (Cl⁻, Br⁻, I⁻)
Add dilute nitric acid (HNO₃), then silver nitrate (AgNO₃) solution.
- Cl⁻: white precipitate of AgCl.
- Br⁻: cream precipitate of AgBr.
- I⁻: yellow precipitate of AgI.
(Nitric acid removes carbonates that would otherwise interfere.)
✦Worked example
A solution gives a brown precipitate with NaOH and a white precipitate when acidified with HCl + BaCl₂. Identify the salt.
- Brown ppt with NaOH → Fe³⁺.
- White ppt with HCl + BaCl₂ → SO₄²⁻.
The salt is iron(III) sulfate (Fe₂(SO₄)₃).
⚠Common mistakes
- Forgetting "excess NaOH" for Al³⁺ — that's the diagnostic step (it dissolves).
- Skipping HCl/HNO₃ before sulfate/halide tests — gives false positives.
- Confusing precipitate colours — Cu blue, Fe(II) green, Fe(III) brown is the classic trio to memorise.
- Saying Fe(II) ppt stays green — it slowly oxidises to brown Fe(III).
Links
Builds on C8.4 (flame tests). Combined with flame and gas tests for full ion identification. Used in real lab analysis.
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