Topic 1 of 4
Distinguish aqueous cations
Observe a few drops first, then add the reagent in excess.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Distinguish aqueous cations
Observe a few drops first, then add the reagent in excess.
Use a fresh small portion of the unknown for each reagent. Add aqueous sodium hydroxide or aqueous ammonia a little at a time, mix, and record any precipitate. Then add excess reagent and record whether that precipitate dissolves. A precipitate is an insoluble solid, not simply a coloured solution.
| Cation | Aqueous NaOH | Aqueous ammonia |
|---|---|---|
| Al3+ | White precipitate; dissolves in excess to a colourless solution | White precipitate; insoluble in excess |
| Ca2+ | White precipitate; insoluble in excess | No precipitate |
| Cu2+ | Light blue precipitate; insoluble in excess | Light blue precipitate; dissolves in excess to a dark blue solution |
| Fe2+ | Green precipitate; insoluble in excess | Green precipitate; insoluble in excess |
| Fe3+ | Red-brown precipitate; insoluble in excess | Red-brown precipitate; insoluble in excess |
| Zn2+ | White precipitate; dissolves in excess to a colourless solution | White precipitate; dissolves in excess to a colourless solution |
| NH4+ | On warming, ammonia evolves; test with damp red litmus | Not a useful ammonium test: the reagent already contains ammonia |
Most precipitates in this table are metal hydroxides. For example, Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s). The copper ion can give a clear blue solution before reaction and a cloudy light-blue precipitate afterwards; describe both colour and physical change. Complex-ion formulae for reactions with excess ammonia are not required.
Worked example
Distinguish aluminium from zinc
An unknown forms a white precipitate soluble in excess NaOH. What next?
- This result fits both Al3+ and Zn2+; it is not enough to identify either.
- Test a fresh portion with aqueous ammonia, then excess.
- A white precipitate remaining in excess supports aluminium; dissolution to a colourless solution supports zinc.
Use a second discriminating test, rather than repeating the same ambiguous observation.