Question

Why does blue vitriol change to white upon heating?

Answer 1

here is your answer

Answer 2

blue vitriol is hydrated Copper(II) Sulphate, meaning CuSO4.5H2O.
That 5H2O is the water of crystallization or the water of hydration. Those 5 molecules of water simply constitute the crystal i.e, they are merely embedded in between CuSO4 atoms.
When blue vitriol is heated, those water of crystallization escape out of the crystal and what's left is anhydrous copper sulphate or simply CuSO4, without any water of crystallization.




In fact, Copper Sulphate is Hygroscopic in nature, meaning, if you heat it and then allow it to cool, it reabsorbs water(moisture) from the air, turning blue again