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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

[GCSE Chemistry only]

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)