Question

When blue coloured copper sulphate is heated ,it becomes colorless.Why?

Answer 1

Blue vitriol (CuSO4.5H2O) is the blue substance you are talking about. The colour in the substance is given by this water of crystallisation only. So when we heat it, the water molecules escape into surroundings and a white coloured anhydrous copper sulphate is left behind.

Answer 2

On heating the blue salt of copper sulphate it is seen that it becomes colourless. This is so because the salt strongly looses all the water of crystallization, which leads to the formation of anhydrous copper sulphate which is white in colour.