Solutions containing transition metal ions often have characteristic colours. You are expected to know the colours of solutions containing the following ions:
Fe2+(aq) → pale green
Fe3+(aq) → brown
Cu2+(aq) → blue
The test for the presence of Fe2+ ions, Fe3+ ions and Cu2+ ions is the addition of dilute sodium hydroxide to a solution of each ion to form a precipitate of the metal hydroxide.
Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s) (a dark/dirty green precipitate)
Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s) (an orange/brown precipitate)
Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) (a blue precipitate)