Question

When copper sulphate crystals are mixed in water, they _______.
(A) Disappears.
(B) Started floating.
(C) Make the solution colourless.
(D) None of the above.

Answer 1

This question is based on the concept of solubility of copper ions in the water. The reaction between copper sulphate crystals, and water is exothermic. This experiment is performed often in the experimental labs.

Complete step by step answer:
First, let us discuss the reaction between the copper sulphate crystals, and the water.
When the copper sulphate crystals are mixed in water, it starts dissolving. The chemical reaction is
CuSO$_4$.5H$_2$O $\rightleftharpoons$ CuSO$_4$ + 5H$_2$O
From the reaction, we can say that it is a reversible reaction, and the amount of energy is released during the dissolution, making it an exothermic reaction.
Thus, when dissolved in water, crystals start losing the attraction in between, and start moving randomly, and disappear in the water.
Hence, there is impairment of blue colour to the solution.
So, in the end we can conclude that when copper crystals are mixed in water, they disappear. The correct option is (A).

Additional information: Copper sulphate crystals hydrated with water are also known as blue vitriol, and also known as pentahydrate molecules. There are 5 molecules of water attached to the copper sulphate, they can be dehydrated into the anhydrous copper sulphate on heating.
Note: Don’t get confused about how to notice the change in colour after the addition of water. Anhydrous copper sulphate are of dirty white colour, thus when hydrated water molecules surround the Cu metal, and emits the blue colour. The hydrated copper sulphate molecules are referred to as water of crystallisation.