Chemically rust is?

Hint: We often see some reddish-brown substance which forms over iron objects. But do you know there’s a whole chemical process behind it?

Complete step by step solution:

Rust is the general name for Iron oxide, when we leave iron or it’s alloys in open for a long time, oxygen and hydrogen in the air reacts with the metal to form hydrated oxide.
Rusting occurs in the presence of water or moist air. It’s an example of electrochemical reaction and corrosion.
As oxygen is a very good oxidizing agent and iron a reducing agent, iron atoms give up electrons whenever they are exposed to oxygen.
\[Fe\,\to \,F{{e}_{2}}^{+}\,+\,\,2e\]
The oxidation state of iron further increased by the oxygen atom when water is present
\[4F{{e}_{2}}\,+\,{{O}_{2}}\,\to \,4F{{e}_{3}}^{+}\,+\,\,2{{O}_{2}}^{-}\]
Now, there will be an acid-base reaction occurring between the iron cations and water molecules.
\[F{{e}_{2}}\,+\,2{{H}_{2}}O\,\to \,Fe{{(OH)}_{2}}\,+\,\,2{{H}^{+}}\]
\[F{{e}_{3}}\,+\,3{{H}_{2}}O\,\to \,Fe{{(OH)}_{3}}\,+\,\,3{{H}^{+}}\]
The hydroxides can also be formed when there is a direct contact between iron cation and hydroxides ions.
\[Fe{{(OH)}_{2}}\to \,FeO\,+\,\,{{H}_{2}}O\]
\[4Fe{{(OH)}_{2}}\,+\,{{O}_{2}}\,+\,x{{H}_{2}}O\to \,2F{{e}_{2}}{{O}_{3}}\,.(x+4){{H}_{2}}O\]
\[Fe{{(OH)}_{3}}\,\to \,FeO(OH)\,\,+\,{{H}_{2}}O\]
\[FeO(OH)\,\,\to \,F{{e}_{2}}{{O}_{3}}+\,{{H}_{2}}O\]
Therefore, the chemical formula of rust is \[Fe_{2}O_{3}.nH_{2}O\].

Note: Rusting of iron can be controlled by limiting the amount of oxygen and water surrounding the metal. Always keep this in mind oxygen gas (\[{{O}_{2}}\]) is not the only source of oxygen in air or water. Carbon dioxide (\[C{{O}_{2}}\]) also has oxygen in it. Carbon Dioxide and water react and form weak carbonic acid. As the acid attacks the iron, water breaks into hydrogen and oxygen. Now the free oxygen and dissolved iron form iron oxide and electrons are released which can easily flow to the other parts of the metal. Once rusting starts, it corrodes the whole metal.