Question

when blue coloured crystals of copper sulphate are heated they become white when few drops of water is sprinkled on them they become blue again what is the reason​

Answer 1

beacuse when u heat them they loose their water molecule

but when u sprinkle some water on it they become blue again beacuse they got their water molecules

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Answer 2

CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (g)

on heating copper sulphate the hydrous / hidrated crystals of copper change into coloueless cause the water molecules assioted with it evaprates and copper sulphate turns anyhydrous  thus changing in to colourless.

and

when we add water into anhydrous copper sulphate solution changes to Blue this shows that coloured hydrated copper sulphate has regained