Question

Why does copper sulfate pentahydrate change colour upon heating?

Answer 1

Because a chemical reaction has taken place, colour change is the prime means to determine the fact of chemical change (of course there are chemical reactions that proceed WITHOUT colour change).

Explanation:

And we can represent this reaction by the following chemical equation:

`underbrace(CuSO_4*5H_2O(s))_"beautiful blue"+Delta rightleftharpoons underbrace(CuSO_4(s))_"white salt" + 5H_2O(g)uarr`

Copper sulfate is different and distinct to copper sulfate pentahydrate; thus the salts should have different properties and different colours, and indeed they do.