Question

Why does iron rust when it comes in contact with water?

Answer 1

Rust is a form of corrosion. Corrosion consists of a transfer of electrons from the anode to the cathode. For rust to occur, there needs to be a donor of electrons (the metal) and an acceptor (oxygen). However, oxygen in the air is in the diatomic form of O2 and will not readily accept electrons. It is actually monoatomic oxygen dissolved in water that will accept electrons by the following equation:

1⁄2 O2 + H2O + 2e- --> 2OH-

Water is actually required for iron to rust, and it also results in the production of OH-. As the iron gives up its electrons to oxygen, the surface becomes positively charged. These iron ions are soluble in aqueous solution and will combine with oxygen to create rust.

So there are two things required for iron to rust: water and oxygen. Deaerated or nitrogen-saturated water will not rust because oxygen will not be able to dissolve into the water and receive electrons

Answer 2

Rust is usually a reddish brown colour compound called iron oxide (Fe2O3).It is formed when iron reacts with oxygen in the presence of water or moisture in the air.

It is very common process because iron readily reacts with oxygen that it actually rare to find pure iron in nature.

Rust is also formed under water due to the reaction of iron and chlorine indicating green colour.
It is a combustion process in which first the iron piece will begin to corrode then will change entirely into rust. Unlike fire, though,the reaction is much slower and don't create a flame.