Heating ferrous sulphate crystals

Heating $\text{FeSO}_4\cdot 7\text{H}_2 ext{O}$ decomposes $\text{FeSO}_4$ into $\text{Fe}_2 ext{O}_3$, $\text{SO}_2$, and $\text{SO}_3$. The gases $\text{SO}_2$ and $\text{SO}_3$ have a characteristic choking smell of burning sulphur.