Question

Oxidation number of Cu?

Answer 1

Usually `+2`, sometimes `+1`.

Explanation:

Copper tends to exist at its `+2` oxidation state, and gives rise to blue compounds in that state. In its rarer `+1` state, it tends to be reddish-brown, as seen in copper(I) oxide `(CuO)`.

Copper(II) sulfate is a beautiful blue compound when hydrated, and becomes `CuSO_4*5H_2O` in most cases. When it's boiled to remove all the water, it becomes white, but will still have some light blue stains. This is one compound that can be used to test the presence of water.