(a) The blue colour of crystals of a substance changed on heating in a closed test tube but the colour was regained after sometime on cooling. Name the substance and write irts chemical formula. Explain the phenomenon invilved.
(b) Write name and chemical formula of two such compounds whose one formula unit is associated with 10 and 2 water molecules respectively.

Explanation

(a) The available informatin suggests that the blue crystals are of hydrated copper sulphate `(CuSO_(4).5H_(2)O).` Upon heating, they were dehydrated. On cooling, they were again hydrated. The chemical reactions involved are :
`underset(("Blue"))(CuSO_(4)).5H_(2)Ooverset("Heat")to underset(("White"))(CuSO_(4))+5H_(2)O`
`underset(("White"))(CuSO_(4))+5H_(2)to underset(("Blue"))(CuSO_(4)).5H_(2)O`
(b) Hydrated sodium carbonate (Washing soda) `:Na_(2)CO_(3).10H_(2)O`
Hydrated calcium sulphate (Gypsum) `:CaSO_(4).2H_(2)O.`