Question

Rusting is a :
a.) chemical change
b.) physical change
c.) can’t be said
d.) both

Answer 1

Hint: The physical change is one in which the identity of atoms in a molecule is the same while in chemical change the atoms lose their initial identity and gain a new identity. In rusting of iron, the iron gets oxidised to form iron oxides.

Complete answer :
First, let us see what are physical and chemical changes.
As we have studied the physical change is the one in which the chemical identity of a substance is constant. It does not change. The chemical change is the one in which the identity of elements is different before the change happened and after the change. The number of atoms of an element may remain the same but their bonding and their identity with respect to molecules get changed. Now, let us see what is rusting. The rust is the common term used for corrosion of iron. The rust is an oxide of iron formed when the iron reacts with oxygen and water. We see that anything made of iron gets corroded when exposed to the outer surface. This is because of oxidation of iron. The process is called rusting. In the outer atmosphere, the oxygen comes from the air and water from the moisture present in the surrounding atmosphere.
The surface of iron once corroded can not get back to its original form. So, it is a chemical change.

Thus, the correct answer is the option a.).

Note: The reactions involved during rusting of iron can be given as -
$Fe \to F{e^{2 + }} + 2{e^ - }$
$4F{e^{2 + }} + {O_2} \to 4F{e^{3 + }} + 2{O^{2 - }}$
$F{e^{2 + }} + 2{H_2}O \rightleftharpoons Fe{(OH)_2} + 2{H^ + }$
Rust results in degradation of iron and iron based structures. To prevent corrosion, the iron is galvanised. In this, a layer of other metal is coated on iron to protect iron from oxidation.