Question

Blue crystals of copper sulphate on heating in a dry test tube become colourless. Give reasons.

Answer 1

This is because CuSO4 Is chemically  bonded with water to give the crystalline blu colur. On heating, the water evaporated, breaking the bond of CuSO4 and Water, thus giving a white precipitate. The reaction is like this-

                        Heating

CuSO4.H2O-------------> CuSO4 (Water evaporates)

Answer 2

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It is majorly due to evaporation of water of Crystallisation that gives colour to the solution. The reaction is
Hydrated copper sulphate (blue)    →←    anhydrous copper sulphate (white) + water
CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (g)

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