Question

a blue coloured salt becomes white on heating.what is it.give reason

Answer 1

Hydrated Copper Sulphate crystals [CuSO4.5H2O]  are blue in colour.

When copper sulphate crystals are heated strongly, they loose all the water of crystallization and form anhydrous copper sulphate [which is white in colour].

CuSO4.5H2O  ---------> CuSO4      +      5H2O
Hydrated                         Anhydrous 
Copper sulphate               Copper sulphate

The water of crystallisation gives the crystals of their salts their shape and colour.

Answer 2

Dear Student!Arnav I'm Saudah.TDear Students the answer of your question:- 

A salt gets it colour due to the presence of water of crystallization in them. So the blue colour salt has water of crystallization present in int.So when it gets heated up it looses water of crytallization and becomes colourless or wDear Studentte in colour.

The salt in question is copper sulphate pentahydrate. It is blue in colour and its chemical formula is CuSO4.5H2O. Following is the reaction wDear Studentch takes place

               CuSO4.5H2O -----Heat--->  CuSO4 + 5H2O

Anhydrous copper sulphate is grey-wDear Studentte in colur.