Salt copper II sulfate, also known as copper(II) sulfate pentahydrate (CuSO₄·5H₂O), is blue.

Here's why:

* Copper(II) ions (Cu²⁺) are responsible for the blue color.

* In the hydrated form, the copper ions are surrounded by water molecules, which further enhance the blue color.

When the water molecules are removed, the compound becomes anhydrous copper(II) sulfate (CuSO₄) and changes color to white.