When excess ammonia solution is added to a solution of copper(II) sulfate the oxidation number of the copper ion does not change but there is a noticeable colour change. Outline the reasons for this change in colour.

the \({{\text{H}}_{\text{2}}}{\text{O}}\) ligand is exchanged for \({\text{N}}{{\text{H}}_{\text{3}}}\);

Accept suitable equation with co-ordination numbers of 4 and 6.

colour is due to electron transitions between the split d orbitals;

\({\text{N}}{{\text{H}}_{\text{3}}}\) causes a greater/different splitting than \({{\text{H}}_{\text{2}}}{\text{O}}\);

Quite a few candidates referred to electron transition between split d-orbitals, but they generally did not realize that the exchange of the ligand was the reason for the change in colour, and so lost 2 marks.