Question

Which of the following molecules is colorless?

• A. $CuS{O}_4\cdot 5H_{2}O$ (crystal)
• B. $CuS{O}_4$ (anhydrous)
• C. ${\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}$
• D. ${\left[CuC{l}_4\right]}^{2-}$

Answer 1

Answer: (B)

Hydrated copper sulphate $CuS{O}_4\cdot 5H_{2}O$ is blue in colour. The ligand (water) molecules causes splitting of $d$-orbitals.
This facilitated $d-d$ transition and colour.
Anhydrous copper sulphate $CuS{O}_4$ is colourless. In the absence of ligand (water) molecules, splitting of $d$-orbitals is not possible.
Hence, $d-d$ transition is not possible.