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.

(N/A) The substance is Copper sulphate pentahydrate, with the formula $CuSO_{4} \cdot 5H_{2}O$.
When heated, the blue-coloured hydrated copper sulphate crystals lose their water of crystallisation and turn into white anhydrous copper sulphate $(CuSO_{4})$.
This is a reversible physical change. Upon cooling, the anhydrous salt absorbs moisture from the atmosphere or the water vapour present in the test tube to regain its water of crystallisation, thereby returning to its original blue colour.