Crystals of a substance changed their colour on heating in a closed test tube but regained it after sometime when they were allowed to cool down.

Question

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

(b) Name the compound whose one formula unit is associated with 10 water molecules. How is it prepared? Give equations of related reactions. Give two uses of the compound.

Answer 1

a) Crystals of a substance which changed their colour on heating in a closed test tube but regained it after sometime when they were allowed to cool down are copper sulphate or more specifically hydrated copper sulphate. 

Its formula is – CuSO₄.5H₂O 

The phenomena involved is due to the presence of molecules of water of crystallization in the molecule. Water of crystallization is the fixed number of water molecules that are present in one formula unit of a salt. When we heat the crystals of hydrated copper sulphate this water is removed and the salt turns white. But when it is allowed to cool down in open air then it regains its lost water of crystallization and hence regains its colour also. 

b) The compound whose one formula unit is associated with 10 water molecules is sodium carbonate (Na₂CO₃. 10H₂O) commonly known as washing soda. 

It is prepared by heating baking soda and then recrystallizing the product so obtained. 

The equations related to the above reaction is: - 

2NaHCO₃ Na₂CO₃ + H₂O + CO₂ Na₂CO₃ + 10H₂O → Na₂CO₃. 10H₂O 

Two uses of washing soda are: - 

1. It is used in glass, soap and paper industries 

2. It is used for removing permanent hardness of water.