Why is hydrated copper sulfate bright blue, but it turns white when you heat it dry?
In the hydrated salt, water molecules act as ligands around the Cu²⁺ ion, splitting its d-orbitals so it absorbs red/orange light and reflects blue (crystal field theory).
In the hydrated salt, water molecules act as ligands around the Cu²⁺ ion, splitting its d-orbitals so it absorbs red/orange light and reflects blue (crystal field theory). Heat it and the water ligands leave; with no ligand field to split the d-orbitals in the same way, anhydrous CuSO₄ no longer absorbs visible light like that and looks white. Add water back and the blue returns.
Key point
Ligands split the metal's d-orbitals; d–d electron jumps absorb visible light, giving the colour.
Common mistake
thinking the colour is a fixed property of copper regardless of its ligands.
Memory tip
no ligands, no d-orbital splitting → often colourless/white.
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