Tests for Cations (Edexcel GCSE Chemistry)

• Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide
• If only a small amount of NaOH is used then normally the metal hydroxide precipitates
• In excess NaOH some of the precipitates may dissolve
• For this reason just a few drops of NaOH are added at first and very slowly
• If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate which dissolves as quickly as it forms if excess solution is added
• A small amount is thus added, very gradually and any colour changes or precipitates formed are noted
• Then the NaOH is added in excess and the reaction is observed again
• Ca²⁺ ions can be distinguished from Al³⁺ as calcium hydroxide does not dissolve in excess NaOH but aluminium hydroxide does
• Most transition metals produce hydroxides with distinctive colours

Testing for ammonium ions,  NH₄⁺

• Ammonium ions are also tested with sodium hydroxide, but not by a precipitation reaction
• To test for the ammonium ion, gentle heating is required after adding the NaOH solution
• If the ammonium ion is present, ammonia gas is produced which can be tested with red litmus paper:

NH₄⁺ (aq) + OH^– (aq)→ NH₃ (g) + H₂O (l)