Iron and its oxides

Iron(II) oxide FeO is a black powder that is insoluble in water. On paper it is prepared by reducing a higher oxide, for example Fe₂O₃ + CO → 2FeO + CO₂, or by heating iron(II) oxalate without air: FeC₂O₄ → FeO + CO + CO₂. In hot air it oxidises to the red oxide, 4FeO + O₂ → 2Fe₂O₃. With acids it gives an iron(II) salt and water: FeO + H₂SO₄ → FeSO₄ + H₂O. Iron(III) oxide Fe₂O₃ is reddish-brown. On paper, ammonia solution added to an iron(III) salt gives a reddish-brown precipitate of Fe(OH)₃, and heating that gives the oxide: FeCl₃ + 3NH₄OH → Fe(OH)₃ + 3NH₄Cl, then 2Fe(OH)₃ → Fe₂O₃ + 3H₂O. A second route is heating iron(II) sulphate, 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (the gases are poisonous, so this stays on paper). With hot concentrated acids Fe₂O₃ gives iron(III) salts: Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O. The rule is: oxide + acid → salt + water, and the iron keeps its state, so FeO gives iron(II) salts and Fe₂O₃ gives iron(III) salts.