Question

blue crystals of copper sulphate on heating in air dry test tube become colourless give reason

Answer 1

water of crystalization imparts colour to salt. So when we heat Copper sulphate crystals in a dry tube.Water of crystalization removes and copper sulphate gets converted into anhydrous copper sulphate which is colourless.

Answer 2

Copper sulphate crystals which seems to be dry contains water of crystallization .when we heat the crystals ,this water is removed and the salt turns white.

if you moisten the crystals again with water you will find the blue colour of the crystals reappear.

hope it helped