Question

Rusting of iron takes place in:
A. Ordinary water
B. Distilled water
C. Both ordinary and distilled water
D. None of the above

Answer 1

Hint: In rusting of iron, ferrous ions$\left( \text{F}{{\text{e}}^{2+}} \right)$ formed are further oxidised by oxygen to ferric ions$\left( \text{F}{{\text{e}}^{3+}} \right)$which come out as ‘Rust’ in the form of hydrated ferric oxide ($\text{F}{{\text{e}}_{2}}{{\text{O}}_{3}}\text{.x}{{\text{H}}_{2}}\text{O}$). The rusting of iron is a spontaneous process. Check the factors on which formation of rust depends.

Complete step by step answer:
Let us discuss the factors required for the formation of rust:
(1) Free oxygen: The atmospheric oxygen is required by the iron atoms for the formation of rust. Distilled water readily absorbs oxygen from the atmosphere than ordinary tap water. As, distilled water does not contain impurities and minerals. Thus, rusting takes place easily. The reaction involved is $4\text{Fe}\left( \text{s} \right)+\text{3}{{\text{O}}_{2}}\left( \text{g} \right)+2\text{x}{{\text{H}}_{2}}\text{O}\left( \text{l} \right)\to \text{2F}{{\text{e}}_{2}}{{\text{O}}_{3}}\text{.x}{{\text{H}}_{2}}\text{O}\left( \text{s} \right)$.
It means that rusting of iron can take place in distilled water.
(2) Ions: We know that rusting of iron is a slow process. In ordinary water, rust formation takes place faster than distilled water. As, ordinary water contains only a few more minerals and dissolved ions than distilled water.
It means that rusting of iron can take place in ordinary water also.
Rusting of iron takes place in both ordinary and distilled water, which is option ‘c’.


Additional Information: The other factor on which rusting of iron depends is humidity. As iron requires oxygen in air, water or water vapours for rust. So, areas where humidity is high, the moisture is more, thus, rusting of iron takes place faster.


Note: The rusting of iron occurs slowly in ordinary water than in salty water. This is because salty water is an electrolyte and contains many ions. Generally, the ions that are present are sodium cations $\left( \text{N}{{\text{a}}^{+}} \right)$ and chloride anions $\left( \text{C}{{\text{l}}^{-}} \right)$ and other ions are magnesium ions $\left( \text{M}{{\text{g}}^{+2}} \right)$ , calcium ions $\left( \text{C}{{\text{a}}^{+2}} \right)$ any more. The ions increase the conductance of the solution which is in contact with metal surface and in turn formation of $\text{F}{{\text{e}}^{+2}}$ ions speed up and thus, rust $\left( \text{F}{{\text{e}}_{2}}{{\text{O}}_{3}}\text{.x}{{\text{H}}_{2}}\text{O} \right)$ formed out quickly.