Ferrous sulphate crystals contain water of crystallisation (FeSO4.7H2O). On heating, ferrous sulphate crystals lose water. So their colour changes from light green to white due to the formation of anhydrous ferrous sulphate (FeSO4). When ferrous sulphate is heated, it gives ferric oxide, sulphur dioxide and sulphur trioxide.
FeSO4 → Fe 2 O 3 + SO2 + SO3