`CO_(2)` is always present in natural water. Explain its effect as rusting of iron.

Explanation

`CO_(2)` when dissolved in water produces carbonic acid, `H_(2)CO_(3)`.
`CO+H_(2)OhArr H_(2)CO_(3)`
carbonic acid to give `H^(+)` or `H_(3)O^(+)` ions,
`H_(2)CO_(3)+H_(2)OhArr H_(3)O^(+)+HCO_(3)^(-)`
On corrosion, iron first changes to `Fe^(2+)` ions. the electrons lost by iron are taken up by the `H_(3)O^(+)` ions produced by the dissociation of `H_(2)CO_(3)`.
`Fe to Fe^(2+)+2e^(-)`
`2H_(3)O^(+)+1//2O_(2)+2e^(-) to 3H_(2)O`
However, the `H_(3)O^(+)` ions so sued are produced in the last step which involves the oxidation of `Fe^(2+)` to `Fe^(3+)`, but a small amount of `CO_(2)` present in water acts as a catalyst to start the process of rusting.