Chemical tests: aqueous ions

Some metal ions form coloured precipitates in a reaction with sodium hydroxide solution. Ammonium ions can be identified by heating with NaOH and using damp red litmus paper, which turns blue if positive. A dilute acid is usually added before testing for carbonates, sulfates or halides to avoid false positives. Halide ions produce different coloured precipitates when silver nitrate is added to a solution containing them. A white precipitate is formed when barium chloride is added to an acidified solution containing sulfate ions.