Precipitation Tests
Many compounds form as COLOURED PRECIPITATES when sodium hydroxide solution is added.
This is because the METAL IONS in the compounds form METAL ION HYDROXIDES which are INSOLUBLE.
To perform the test, ADD SODIUM HYDROXIDE to the compound and observe the colour of the precipitate formed.
CALCIUM IONS (Ca2+): White precipitate of calcium hydroxide [Ca(OH)2]
COPPER(II) IONS (Cu2+): Blue precipitate of copper(II) hydroxide [Cu(OH)2]
IRON(II) IONS (Fe2+): Green precipitate of iron(II) hydroxide [Fe(OH)2]
IRON(III) IONS (Fe3+): Brown precipitate of iron(III) hydroxide [Fe(OH)3]
ALUMINIUM IONS (Al3+): White precipitate of aluminium hydroxide [Al(OH)3] at first, but RE-DISSOLVES in excess NaOH to form a colourless solution
MAGNESIUM IONS (Mg2+): White precipitate of magnesium hydroxide [Mg(OH)2]