(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.
(a) CuSO4.5H2O is a blue crystalline solid. It becomes dirty white on heating due to loss of water molecules and it becomes amorphous.
CuSO4.5H2O(Copper sulphate pentahydrate (Blue)) + heat → CuSO4 + 5H2O
It regains its colour by absorbing water from atmosphere and becomes blue in colour.
CuSO4 + 5H2O → CuSO4.5H2O
(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.
NH3 + H2O + CO2 + NaCl → NaHCO3 + NH4Cl
2NaHCO3 + heat → Na2CO3 + H2O + CO2
Na2CO3 + 10H2O → Na2CO3.10H2O
Uses:
(i) It is used in the production of washing powder.
(ii) It is used for the manufacture of glass.