Chemistry · 12.5 Identification of cations · Paper 5/6 practical
Cation Tests. Read the precipitate.
Add aqueous sodium hydroxide or aqueous ammonia to a solution — first dropwise, then in excess. The colour of the hydroxide precipitate and whether it dissolves in excess identifies the metal cation.
0620 Topic 12.5 — Tests for cations
NaOH(aq) · NH₃(aq)
Paper 5/6 — Practical
no reagent added
Sample
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This test
Solution colour
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Reagent
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Observation
add a reagent…
Observation log
Run tests to build your results.
📊 Full results table (0620 syllabus)
| Cation | + NaOH(aq) | + NH₃(aq) |
|---|
📋 Method (Cambridge practical procedure)
- Put about 1 cm³ of the unknown solution in a test tube; note its colour.
- Add aqueous sodium hydroxide a few drops at a time and note the colour of any precipitate.
- Continue adding NaOH until in excess; note whether the precipitate dissolves.
- Repeat with a fresh sample using aqueous ammonia, dropwise then in excess.
- For ammonium ions, warm the mixture with NaOH and test the gas with damp red litmus.
- Compare with the standard results to identify the cation.
⚠ Sources of error & precautions
- Add slowly — adding too much reagent at once hides the "soluble in excess" behaviour that distinguishes Al³⁺/Zn²⁺ from Ca²⁺.
- Iron(II) hydroxide turns brown near the surface on standing as it is oxidised — observe quickly.
- NaOH and ammonia are irritant/corrosive — wear eye protection.
- Use a clean tube each time to avoid contamination.
🎯 Syllabus reference (0620)
- 12.5 Identification of ions — describe tests using aqueous NaOH and aqueous NH₃ to identify Al³⁺, NH₄⁺, Ca²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺.