Tests for Cations (Edexcel IGCSE Science (Double Award)) : Revision Note
Tests for cations
Testing for metal cations
Metal cations in aqueous solution can be identified by the colour of the precipitate formed when sodium hydroxide (NaOH) is added
A few drops of NaOH is added at first and any colour changes or precipitates formed are noted
The addition of sodium hydroxide to the metal ions forms precipitates of different colours
A metal hydroxide precipitate typically forms if the hydroxide is insoluble in water
If no precipitate forms, the hydroxide may be soluble or there may not be enough of the metal ion present
A summary of the reactions is shown in the table below
Table showing the colour of the precipitates formed
Metal ion | Reaction | Colour |
|---|---|---|
Fe2+ | Fe2+ (aq) + 2OH– (aq) → Fe(OH)2 (s) | Pale green precipitate |
Fe3+ | Fe3+ (aq) + 3OH– (aq) → Fe(OH)3 (s) | Orange / brown precipitate |
Cu2+ | Cu2+ (aq) + 2OH– (aq) → Cu(OH)2 (s) | Blue precipitate |
Examiner Tips and Tricks
Examiners are often quite specific about the copper(II) ions, Cu2+ (aq). You need to specifically state light blue for the precipitate colour to be sure of the mark
Test for ammonium ion, NH4+
Add NaOH and warm the mixture
Observation with NaOH: Ammonia gas is released
Ionic equation:
NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
Test for ammonia gas: Ammonia turns damp red litmus paper blue
Examiner Tips and Tricks
Sometimes the cation may be present in very small amounts, so the precipitate might appear only as a slight cloudiness or faint colour change - this can still indicate a positive test result.
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