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) CuSO_4.5H₂O is a blue crystalline solid. It becomes dirty white on heating due to loss of water molecules and it becomes amorphous

It regains its colour by absorbing water from atmosphere and becomes blue in colour.

(b)Na₂CO₃. 10H₂O. It is called sodium carbonate decahydrate or washing soda. It is prepared by passing CO₂ gas through saturated solution of ammonical brine. 

Uses: 

(i) It is used in the production of washing powder. 

(ii) It is used for the manufacture of glass.