Question

a substance changed its colour on heating in a closed vessel but regained it after sometime when allowed to cool and exposed in air . name the substance . explain the phenomenon involved .

Answer 1

There could be many such substances.

Crystals with water molecules in them give color. Copper sulphate pentahydrate Cu SO4. (H2O)5 is shining blue crystal. When heated the water evaporates. CuSO4 is a white solid crystal.
Again when cooled in air, the crystals absorb water from the moisture in air and turn blue.

Answer 2

CuSo4 is the answer of your question