Copper(II) sulfate forms a pale blue aqueous solution. When aqueous ammonia is added to this, initially a pale blue precipitate forms; this precipitate then dissolves in excess ammonia to form a deep blue solution. Explain why these solutions are coloured and the colour with excess ammonia is a deeper blue.

electron transitions between (split, partially filled) d orbitals;

absorption depends on energy difference between the split d orbitals;

waters replaced by ammonias;

ammonia (ligands) increase the splitting between the d orbitals/larger energy difference;

absorption moves to shorter wavelength/higher frequency/towards blue end of spectrum;

Part (b) lacked proper understanding of the chemical principles and was poorly answered by most of the candidates.