Which one of the following when dissolved in water gives coloured solution in nitrogen atmosphere?

Concept: Coloured solutions require partially filled d orbitals

| Compound | Ion | d config | Coloured? | |----------|-----|----------|----------| | $\text{Cu}_2\text{Cl}_2$ | $\text{Cu}^+$ | $3d^{10}$ | Colourless ✗ | | $\text{ZnCl}_2$ | $\text{Zn}^{2+}$ | $3d^{10}$ | Colourless ✗ | | $\text{CuCl}_2$ | $\text{Cu}^{2+}$ | $3d^9$ | Blue-green ✓ | | AgCl | $\text{Ag}^+$ | $4d^{10}$ | Colourless ✗ |

$\text{CuCl}_2$ dissolves in water to give $\text{Cu}^{2+}$ ($3d^9$) — blue-green coloured solution due to $d \rightarrow d$ transition.

Note: Nitrogen atmosphere prevents oxidation of Cu(I) to Cu(II), but $\text{CuCl}_2$ already contains Cu(II).

Answer: Option (3) — $\text{CuCl}_2$