Explain why `CuSO_4` is blue while `ZnSO_4` is white.

Explanation

In `CuSO-4` the `Cu^2+` ion has 3d^9 configuration with one unpaired electron, so it is blue. But in `ZnSO_4` the `Zn_2+` ion has `3d^10` configuration with no unpaired electron. So it is white.