Question

Why Zn²⁺ ions are colourless while Cu²⁺ ions are blue in colour?

Answer 1

1. Zn²⁺ has a completely filled 3d¹⁰ shell, so there are no vacant d‑levels for d-d electronic transitions.
2. Cu²⁺ is 3d⁹, with an unpaired electron in the d‑orbitals; d-d transitions can absorb visible light.
3. Ligands split the d‑orbital energies (crystal‑field splitting); the absorbed wavelength depends on that splitting.
4. Hydrated Cu²⁺ absorbs red light and therefore appears blue/greenish‑blue, while Zn²⁺ remains colourless.