Rust is an iron oxide, a usually reddish brown oxide formed by the reaction of iron and oxygen in the presence of water or air moisture. Rusting of iron consists of the formation of hydrated oxide, Fe(OH)3, FeO(OH), or even Fe2O3. Rust is hydrated iron(III) oxide. Rust is the common name of the chemical called iron oxide. Answer · Rust is a general term for a series of iron oxides, usually, red oxides, formed by the reaction of iron with oxygen in the presence of water. What is the Chemistry Behind the Rusting of Iron? · Iron(II) oxide or ferrous oxide.