Question

Explain the reason behind the difference in color of the two compounds ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{CuSO}}_4$.

Answer 1

Reason for the color of ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$ :-

1. Here the ligand which is the water molecules are the reason for splitting of d – orbitals.
2. Hence because of this factor the $\mathrm{d}–\mathrm{d}$ transition process and the color transition are facilitated.
3. In ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$ the oxidation state of Copper is$$+2$$, and by this we can understand that it has unpaired electrons.
4. As a result, $\mathrm{d}–\mathrm{d}$ transition is possible and hence we can see the blue color of hydrated Copper sulphate.

Reason for the colour of ${\mathrm{CuSO}}_4$:-

1. When we see anhydrous Copper sulphate ${\mathrm{CuSO}}_4$ it has no ligand present.
2. As a result the splitting of d – orbitals is not possible.
3. Hence due to this factor, $\mathrm{d}–\mathrm{d}$ transition is also not possible, which ultimately results in the colorlessness of ${\mathrm{CuSO}}_4$.

Therefore, due to the absence of ligand in ${\mathrm{CuSO}}_4$splitting of d – orbital does not occur and hence ${\mathrm{CuSO}}_4$appears colorless, but ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$is blue in color.