Question

What are the essential conditions for rusting to take place?
(This question has multiple correct options)
(A) Presence of pure water
(B) Presence of oxygen
(C) Presence of impure water
(D) Presence of oxygen alone is enough

Answer 1

Hint: Rusting would take place only under certain conditions. Iron oxide is one of the products formed in rusting. It is formed by the oxidation of iron in presence of water and oxygen.

Complete step by step solution:
-As we are aware, rusting is a part of corrosion. It is a chemical process which results in the formation of red or orange coating on the surface of metals. Thus, rusting of iron can be referred to as the formation of rust, a mixture of iron oxides on the surface of iron objects or structures.
-In the process of rusting, iron or an alloy of iron is exposed to oxygen in the presence of moisture. Since this reaction is not instantaneous, the process will continue for a longer time. In rusting the oxygen atoms bond with the iron atoms to form iron oxides and this weakens the bonds between iron atoms in an object.
-The reactions involved in rusting can be generally written as
 $4Fe+3{{O}_{2}}\to 2F{{e}_{2}}{{O}_{3}}$
 $2F{{e}_{2}}{{O}_{3}}+x{{H}_{2}}O\to F{{e}_{2}}{{O}_{3}}.x{{H}_{2}}O$
 -$F{{e}_{2}}{{O}_{3}}.x{{H}_{2}}O$ is rust. Since oxygen is a very good oxidizing agent and iron is a reducing agent, iron atoms readily give up electrons when exposed to oxygen.
-The water is usually present in the form of water vapour and oxygen is always present in the normal atmosphere. As we have seen, the oxygen needs moisture or water as a catalyst and as a reactant to accelerate the reaction, so in the absence of water, iron won't rust.
- Iron also does not rust in pure water which is free from dissolved salts.
 From the above details, it is clear that the presence of impure water and presence of oxygen is required for rusting.

Therefore, the answers are options (B) and (C).

Note: Some might confuse corrosion with rusting. While these terms express almost similar ideas there are some major differences between them. Corrosion is a type of oxidation and rusting comes under the category of corrosion. Corrosion includes the deterioration of metals or non-metals and rusting occurs only on metals such as iron and steel.