The colour of a compound is determined by the way in which its electrons interact with light. When light strikes a compound, some of the light energy is absorbed by the electrons, and the rest is reflected. The colour of the compound depends on the wavelengths of light that are reflected.
In the case of copper sulfate, the copper(II) ion has an incomplete d-electron shell. This means that there are some electrons in the d-orbitals that are not paired. When light strikes the copper sulfate compound, these unpaired electrons can absorb energy from the light and become excited. The energy that is absorbed corresponds to the wavelength of light in the blue region of the visible spectrum. The other wavelengths of light are reflected, which is why copper sulfate appears blue.
The colour of a compound can also be affected by the presence of other ions or molecules. For example, if a small amount of ammonia is added to a copper sulfate solution, the solution will turn a deep blue colour. This is because the ammonia molecules react with the copper(II) ions to form a complex ion called copper(II) tetraammine. This complex ion has a different electronic structure than the copper(II) ion, and it absorbs light at a different wavelength. This is why the solution turns a deep blue colour.
