The color of the copper II sulfate aqueous solution and the solid copper II sulfate pentahydrate CuSO45H2O is blue. Both of these compounds exhibit a blue color due to the presence of the copper II ion Cu in the complex.The blue color arises from the electronic transitions within the d-orbitals of the copper II ion. In the copper II sulfate complex, the copper II ion has a d9 electronic configuration. When white light which contains all colors of visible light passes through the solution or reflects off the solid, the copper II ion absorbs some of the light in the red region of the spectrum. This absorption corresponds to the electronic transition of an electron from a lower energy d-orbital to a higher energy d-orbital.Since the red light is absorbed, the complementary color, blue, is transmitted or reflected, and that is the color we perceive. This phenomenon is known as the ligand field theory or crystal field theory, which explains the color of transition metal complexes based on the interaction between the metal ion and the surrounding ligands in this case, sulfate and water molecules .