Iron(III) oxide, also known as ferric oxide, is an inorganic substance that possesses the formula Fe2O3. It is one of the three main oxides of iron, the other two being Iron(II) oxide or FeO, which is rare, and Iron(II,III) oxide Fe3O4, which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is ferromagnetic, dark red in appearance, and is immediately attacked by acids. Iron(III) oxide is recreationally referred to as rust, and is acidic in nature and to some extent this label is useful, because rust shares several properties and has a similar composition. Fe2O3 can be obtained in various polymorphs; within the main ones, α and γ, iron adopts octahedral coordination geometry. That is, each iron center is bound to six oxygen ligands.
The most important reaction by Iron(III) oxide is its [arbothermal reduction, which gives iron used in steel making:
- Fe2O3 + 3 CO → 2 Fe + 3 CO2
- 2 Al + Fe2O3 → 2 Fe + Al2O3
This process is used to bind thick metals such as rails of train tracks by using a ceramic container to funnel the liquid iron in between two sections of railing.
Partial reduction with hydrogen at about 400 °C produces magnetite, a black magnetic substance that contains both Fe(III) and Fe(II):
- 3 Fe2O3 + H2 → 2 Fe3O4 + H2O
Iron(III) oxide is insoluble in water but dissolves readily in strong acid, e.g. hydrochloric and sulfuric acids. It also dissolves well in solutions of chelating agents such as oxalic acid.
Heating iron(III) oxides with other metal oxides or carbonates yields materials known as ferrates:
- ZnO + Fe2O3 → Zn(FeO2)2
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