On heating, ferrous sulphate crystals lose water of crystallisation (FeSO4.7H2O). So, their colour changes from light green to white due to the formation of anhydrous ferrous sulphate(FeSO4). When FeSO4 is heated, it gives ferric oxide, sulphur dioxide and sulphur trioxide.
FeSO4 → Fe2O3 + SO2 + SO3