Answer: Blue vitriol (copper(II) sulfate pentahydrate, CuSO4·5H2O) is dehydrated by concentrated sulfuric acid and turns into white anhydrous copper(II) sulfate (CuSO4); the acid absorbs the water of crystallization.
Explanation
- Concentrated H2SO4 is a strong dehydrating agent. It removes the water of crystallization from blue vitriol, converting the blue hydrated salt to the white anhydrous salt.
- Observationally you see a colour change from blue to white; if the product is exposed to moisture it will rehydrate and turn blue again.
Chemical equation (summary):
$$\text{CuSO}_4\cdot5\text{H}_2\text{O}\xrightarrow{\text{conc. }H_2SO_4}\text{CuSO}_4+5\text{H}_2\text{O}$$
(The liberated water is taken up by the conc. H2SO4, yielding a more hydrated/dilute sulfuric acid.)
Notes and cautions
- The change is a physical dehydration (loss of water of crystallization), not reduction/oxidation of copper.
- Anhydrous CuSO4 is a white, hygroscopic powder that becomes blue again on adding water.
- Concentrated sulfuric acid is highly corrosive and the process can be exothermic — handle with proper PPE and in a fume hood.