Question

When crystals of copper sulphate are strongly heated, the residue obtained is
A.Red in colour
B.Blue in colour
C.Green in colour
D.Colourless

Answer 1

To solve this question, we must first understand the original colour of copper sulphate crystals and the reason behind the colour. Then we must understand the effect of heating on the crystal and then co-relate it to the relation between the conditions before and after heating.

Complete step by step answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
Copper sulphate is a compound which is formed by adding one mole of copper ion to one mole of sulphate ion. The chemical formula of copper sulphate is given as $CuS{O_4}$ . But this compound does not exist in the free form. Copper sulphate exists in the form of crystals.
Water of crystallization can be understood as the water molecules that are used in the crystalline framework of a metal complex or a salt, such that these water molecules are not directly bonded to the metal ion. When crystallization from water takes place, many compounds incorporate water molecules in their framework. In the case of copper sulphate, 5 water molecules are used in the formation of the crystal, hence the chemical formula can be given as $CuS{O_4}.5{H_2}O$ .
The blue colour of the copper sulphate colour is only because of this water of crystallization. When we heat this crystal of copper sulphate strongly, the crystal loses its water of crystallization and results in the formation of anhydrous copper sulphate solution. Because the compound has lost its water of crystallization, it has hence lost its colour. Hence after heating, the residue of the compound becomes colourless.

Hence, Option D is the correct option.

Note:
Because of the blue colour of the copper sulphate crystals, this compound is also known as blue stone or blue vitriol. One of the most major applications of copper sulphate is that it is used as a raw material for producing other salts of copper.