Why is copper sulphate blue?
Copper has two oxidation states, 2+ and 1+, the 2+ oxidation state Cu is Blue and 1+ oxidation state Cu is Green. These different colors are most likely due to the D–orbitals and there excitation states that occur when light in the wavelength that human can see excite the D–orbital electron into the their higher states and the wavelength of light that they give off when the D–orbitals fall back to their natural state.
