Question

What is the color of \[CuS{O_4} \cdot 5{H_2}O\]?
(A) Blue
(B) Black
(C) Green
(D) Brown

Answer 1

Hint: Compounds having water of crystallisation shows color whereas Anhydrous compounds are colorless. It is so because of the presence of water of crystallisation which facilitates the transition which transmits color to the compound.

Complete step by step answer:
The hydrated copper sulphate \[CuS{O_4} \cdot 5{H_2}O\] is blue in color. The ligand (water molecules) cause splitting of d – orbitals. We know that, due to this, the process of d – d transition and color transition is facilitated.
Whereas, on the other hand the 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.
We can understand the above from the following statements.
\[CuS{O_4} \cdot 5{H_2}O\] is blue in colour because its oxidation state \[ + 2\], thus it has unpaired electrons in it. So, d –d transition is possible so it exhibits color where on the other hand the anhydrous copper sulphate complex, d – d transition is not possible that’s why it is colorless.
Additional information: Copper sulphate pentahydrate is an odorless blue crystal that readily dissolves in water. It is also soluble in methanol, glycerol and slightly soluble in ethanol. Copper sulphate, sometimes referred to as blue vitriol or blue stone, is readily available and an economic source of power.
 \[CuS{O_4} + 5{H_2}O \to CuS{O_4} \cdot 5{H_2}O\] the reversible reaction is also possible \[CuS{O_4} \cdot 5{H_2}O\underrightarrow {\,\,\Delta }\,\,CuS{O_4} + 5{H_2}O\]
The copper has mass percent of $25.450\% $, Hydrogen has mass percent of \[4.037\% \], Oxygen has mass percent of \[57.671\% \], and Sulphur has mass percent of \[12.842\% \] in copper (II) sulphate pentahydrate.

So, the correct answer is Option A .

Note:
A copper sulphate solution is blue or bluish – green in color. Copper sulphate has also been called blue vitriol, blue – stone and roman vitriol.