While SO2 and SO3 evolve out as gases, the residue left is the solid - Fe2O3 (ferric oxide) - which is a reddish brown powder. On heating, ferrous sulphate crystals lose water and anhydrous ferrous sulphate (FeSO4) is formed. So their colour changes from light green to white.
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While SO2 and SO3 evolve out as gases, the residue left is the solid - Fe2O3 (ferric oxide) - which is a reddish brown powder. On heating, ferrous sulphate crystals lose water and anhydrous ferrous sulphate (FeSO4) is formed. So their colour changes from light green to white.