CBSE Class 10 Science 2020 · Paper 31/4/1 · Question 25(b)

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Examples: copper sulphate pentahydrate, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$, and washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$ (gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, is another). Proof: heat a few crystals of hydrated copper sulphate in a dry boiling tube. Water droplets appear on the cooler part of the tube. Colour: the blue copper sulphate turns white. State: the blue crystals change into a white powder (anhydrous copper sulphate), ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$. On adding a few drops of water to the cooled powder the blue colour reappears, so the difference is due to the water of crystallisation.