Question

$CuS{O}_4.5H_{2}O$ is blue in colour while $CuS{O}_4$ is colourless. Why?

Answer 1

In hydrated $CuS{O}_4.5H_{2}O$ the water molecules which surround the central metal $C{u}^{2+}$ function as ligands. These ligands cause crystal field splitting which makes $d-d$ transitions possible. As a result of these $d-d$ transitions energy is released in visible region and $CuS{O}_4.5H_{2}O$ appears blue.

Anhydrous $CuS{O}_4$ doesn't have any water molecules i.e., it has no ligands. So, in this case no crystal field splitting occurs, hence $d-d$ transition does not take place. That’s why anhydrous $CuS{O}_4$ is colourless.