Correct Answer - D
Rust is an hydrated oxide of iron, mainly `Fe_(2)O_(3).xH_(2)O` formed on the surface of iron when it is exposed to moisture and air.
Consider the rusting that occurs when a drop of water is in contact with iron. The edge of the water drop exposed to the air becomes one pole of a voltaic cell. At this edge, molecular `O_(2)` from air is reduced to hydroxide ion in solution.
`O_(2)(g)+2H_(2)O(l)+4e^(-) rarr 4OH^(-)(aq.)`
The electrons for this reduciton are supplied by the oxidation of metallic iron to the edge of the drop. Thus, the metallic iron funcations, as the ecternal circuit between the cell poles. Ions move within the water drop, completing the electric circuit, Iron `(II)` iron move outward from tghe center of the drop, and hydroxide ions move inward from the edge. The two ions meet in a doughnut-shaped region, where they react to precipitate iron `(II)` hydroxide.
`Fe^(2+)(aq.)+2OH^(-)(aq.) rarr Fe(OH)_(2)(s)`
This precipitate is quickly oxidized by `O_(2)` to rust (approxi-mated by the formula `Fe_(2)O_(3).H_(2)O)`.
`4Fe^(OH)_(2)(s)+O_(2)(g) rarr 2Fe_(2)O_(3).H_(2)O(s)+2H_(2)O(l)`
Thus rust is formed between the anode and cathode. We can regard rust as a mixture of `Fe_(2)O_(3)` and `Fe(OH)_(3)`.
