Explain how rusting of iron is envisaged as setting up of an electroCHMemical cell.

Explanation

The water layer present on the surface of iron (especially in the rainy season) dissolves acidic oxides of the air like `CO_(2),SO_(2)` etc. tof orm acids which dissociate to give `H^(+)` ions
`H_(2)O+CO_(2)toH_(2)CO_(3)hArr2H^(+)+CO_(3)^(2-)`
In the presence of `H^(+)` ions, iron starts losing electrons at some spot to form ferrous ions, i.e., its oxidation takes place. hence, this spot acts as the anode:
`Fe(s)toFe^(2+)(aq)+2e^(-)`
The electrons thus released move through the metal to reach another spot where `H^(+)` ions and the dissolved oxygen take up these electrons and reduction reaction takes place. hence, this spot acts as the cathode:
`O_(2)(g)+4H^(+)(aq)+4e^(-)to2H_(2)O(l)`
The overall reaction is: `2Fe(s)+O_(2)(g)+4H^(+)(aq)to2Fe^(2+)(aq)+2H_(2)O(l)`
Thus, an electrochemical cell is set up on the surface:
Ferrous ions are further oxidized by the atmospheric oxygen to ferric ions which combine with water molecules to form hydrated ferric oxide, `Fe_(2)O.x H_(2)O`, which is rust.