`Cu^(2+)` ions are coloured but `Zn^(2+)` ions are colourless. Give reason.

Explanation

`Cu^(2+)` ions contain one unpaired electron in its d-subshell and can undergo d-d transition. Therefore it is coloured.
In `Zn^(2+)`, there is no unpaired electrons, hence it does not undergo d-d transition. Hence it is colourless