(1) Oxides of Iron : Iron forms three oxides FeO, Fe₂O₃ (Haematite), Fe₃O₄ (magnetite also called magnetic oxide or load stone).

(i) Ferrous oxide, FeO: It is a black powder, basic in nature and reacts with dilute acids to give ferrous salts.

(ii) Ferric oxide Fe₂O₃: It is a reddish brown powder, not affected by air or water; amphoteric in nature and reacts both with acids and alkalis giving salts. It can be reduced to iron by heating with C or CO.

It is used as red pigment to impart red colour to external walls and as a polishing powder by jewellers.

(iii) Ferroso ferricoxide Fe₃O₄(FeO.Fe₂O₃): It is more stable than FeO and Fe₂O₃ magnetic in nature and dissolves in acids giving a mixture of iron (II) and iron (III) salts.

(2) Ferrous sulphide FeS: It is prepared by heating iron filing with sulfur. With dilute H₂SO₄ it gives H₂S.FeS + H₂SO₄ (dil) → FeSO₄ + H₂S ↑

(3) Ferric chloride FeCl₃: (i) preparation: It is prepared by Fe(OH)₃ treating with HCl

The solution on evaporation give yellow crystals of FeCl₃.6H₂O

(ii) Properties: (a) Anhydrous FeCl₃ forms reddish-black deliquescent crystals.

(c) Due to oxidising nature Fe³⁺ ions FeCl₃ is used in etching metals such as copper

(d) In vapour state FeCl₃ exists as a dimer, Fe₂Cl₆

(4) Ferrous sulphate, FeSO₄ , 7H₂O (Green vitriol): It is prepared as follow,

(i) One pressure to moist air crystals become brownish due to oxidation by air.

(ii) On heating, crystals become anhydrous and on strong heating it decomposes to Fe₂O₃, SO₂ and SO₃.

(iii) It can reduce acidic solution of KMnO₄ and K₂Cr₂O₇

(iv) It is generally used in double salt with ammonium sulphate.

Mohr’s salt is resistant to atmospheric oxidation.

(v) It is used in the ring test for nitrate ions where it gives brown coloured ring of compound FeSO₄.NO.

FeSO₄ + NO → FeSO₄ . NO

(vii) FeSO₄ + H₂O₂ is known as a name of Fenton’s reagent.

(5) Mohr's salt FeSO₄.(NH₄)₂SO₄.6H₂O: It is a double salt and is prepared by crystallising a solution containing equivalent amounts of FeSO₄.7H₂O and (NH₄)₂SO₄ . It may be noted that Mohr’s salt contains only Fe²⁺ ions without any trace of Fe³⁺ ions. In contrast FeSO₄.7H₂O always contains some Fe³⁺ ions due to aerial oxidation of Fe²⁺ ions. Mohr salt is, therefore, used as a primary standard in volumetric analysis since a standard solution of Fe²⁺ ions can be obtained directly by weighing a known amount of the Mohr salt.

It acts as a reducing agent and as such reduces acidified KMnO₄ and K₂Cr₂O₇ solutions.

MnO₄^– + 5Fe²⁺ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O