Final answer:
Aqueous copper(II) ions are blue due to electronic transitions that absorb certain wavelengths of light; the remaining light is perceived as blue. The color change to blue-violet upon the addition of ammonia occurs because ammonia alters the electronic structure of the copper ion, creating a complex with different light absorption properties.
Step-by-step explanation:
Aqueous copper(II) ions are blue because of the electronic transitions that occur within the metal ion. When light passes through a copper(II) solution, electrons in the d-orbital of copper absorb a portion of the light in order to move to a higher energy state. This absorption corresponds to certain wavelengths of light, and the color we see is the complementary color of the light absorbed, which is blue in the case of copper(II) ions. The energy change associated with this transition is described by the equation ΔE= hv, where ΔE represents the change in energy, h is Planck's constant, and v is the frequency of the absorbed light.
As electrons return to their ground state, they release energy in the form of light, which is typically not visible as it is in the infrared range, contributing to the blue color observed. In the presence of ammonia, water molecules are replaced in the coordination sphere of copper(II) by ammonia molecules, forming a complex ion which is blue-violet in color due to the different field strength of ammonia, which further alters the electronic structure of the copper ion.