* Copper sulfate in solid form: Solid copper sulfate (CuSO₄) is typically white or pale blue. The copper ions (Cu²⁺) are tightly bound within the crystal lattice.
* Dissolving in water: When copper sulfate dissolves in water, the water molecules surround the copper ions, forming what's called a hydration shell. This shell consists of water molecules attracted to the positively charged copper ions.
* Absorption of light: The hydrated copper(II) ions, [Cu(H₂O)₆]²⁺, absorb light in the red and orange regions of the visible spectrum. This absorption is due to the d-d electronic transitions within the copper(II) ion.
* Complementary color: Because the red and orange light is absorbed, the remaining light we see is the complementary color, which is blue.
In essence, the blue color arises from the interaction between light and the hydrated copper(II) ions. The water molecules in the hydration shell play a crucial role in enabling these electronic transitions that lead to light absorption.
