A solution containing Cu2+ ions generally appears blue. This coloration is due to the d-d transition of the copper ions in the solution. When Cu2+ ions are dissolved in water, they form a coordination complex with water molecules, resulting in the characteristic blue color. The absorption of light in the visible spectrum by the d-d transitions of the copper ions leads to the observed color. The exact shade of blue can vary depending on the concentration of the solution and the presence of other ions or ligands. For instance, a more dilute solution may appear lighter in color, while a more concentrated solution may appear darker. The color change is a result of the electronic structure of copper ions and their interaction with the solvent molecules.