Question

Crystals of a substance changed their colour on heating in a closed test tube but regained it after some time when they were allowed to cool down. Name the substance and write its formula and explain the phenomenon involved.

Answer 1

Hint:The crystals which change their colours on heating in a close tube but regain after some time is because they hardly contain any contaminants in it. They can easily be affected by the heating. These crystals are generally formed from the aqueous solution and contain some water molecules in it.

Complete answer:
So the process by which the crystals change their colour on heating in a close test tube but regains its colour on cooling down is due to the phenomenon of water of crystallization. The phenomenon of water of crystallization means that the substance contains a fixed number of the water molecules present in it in one formula unit of the salt. The other name for this process is water of hydration. As the water molecules hydrate the substance.
The substance which undergoes this phenomenon is the hydrated copper sulphate. The formula for it is \[CuS{{O}_{4}}.5{{H}_{2}}O\]. It has five water molecules. When we heat the blue crystal of hydrated copper sulphate the five water molecules get removed and the salt changes into white colour. But on cooling down it tends to regain its water of crystallization and its blue colour too.

Note:This water of crystallization refers to the water which we found in the crystalline framework of the crystal or a metal complex or a salt but it is not directly attached or bonded to the metal cation. The other example of water of crystallization is hydrated calcium sulphate which contains two crystallizations. The formula for it is \[CaS{{O}_{4}}.2{{H}_{2}}O\].it is useful in the manufacturing of plaster of paris and is used as a coagulant.