Heating Ferrous Sulphate Crystals

Thermal decomposition of $FeSO_4$ produces ferric oxide ($Fe_2O_3$), sulphur dioxide ($SO_2$), and sulphur trioxide ($SO_3$). $SO_2$ gas has a characteristic smell of burning sulphur: $2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$