Why does a piece of iron rusts?

• Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen.

• The nature of rust changes with time, reflecting the slow rates of the reactions of solids.

• Rusting is an oxidation reaction. The iron reacts with water (Moisture present in air) and oxygen to form hydrated iron(III) oxide, which we see as rust. 

• Here is the word equation for the reaction:

        Iron + water + oxygen → hydrated iron(III) oxide.

         Fe  + H2O  + O2   → Fe2O3·H2O