Metal ions in solution can be identified by adding a few drops of sodium hydroxide solution. Most metal ions make an insoluble metal hydroxide, which appears as a precipitate. The colour of the precipitate shows which ion is present. Calcium (Ca2+) and zinc (Zn2+) ions give a white precipitate, copper (Cu2+) ions give a blue precipitate, iron(II) (Fe2+) ions give a green precipitate and iron(III) (Fe3+) ions give an orange-brown precipitate. Calcium and zinc are told apart by adding excess sodium hydroxide. The zinc hydroxide dissolves in excess to give a colourless solution, but the calcium hydroxide does not.
Negative ions are tested for separately. Carbonate ions (CO32-) react with dilute acid and fizz, giving off carbon dioxide that turns limewater cloudy. Halide ions are tested with acidified silver nitrate solution. Chloride gives a white precipitate, bromide gives a cream precipitate and iodide gives a yellow precipitate.
Sulfate ions (SO42-) are tested with acidified barium chloride solution, or acidified barium nitrate. A white precipitate of barium sulfate forms. The solutions are acidified first so that carbonate ions cannot also give a white precipitate and cause a false result.