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Before heating it was blue but after,it turned colourless (white).

This is because, CuSO4 crystals have water as part of their structure.OR, their actual formula is CuSO4.5H2O.When the blue crystals are heated the water of crystallisation is lost and what remains is anhydrous copper sulphate (CuSO4).

As this is a reversible reaction,it's colour can be restored by adding water to it