Rusting of iron is a chemical change because it happens due to chemical reaction between iron and air air in the presence of moisture

Rusting of iron is an example of a chemical change.

Explanation:

Chemical changes, also known as chemical reactions, involve the formation of new substances with different chemical properties. In these reactions, the atoms rearrange themselves to form new chemical bonds, resulting in the formation of different substances.

Rusting of iron is a chemical change that occurs when iron comes in contact with moisture and oxygen in the presence of air. It is a slow process that leads to the formation of a new compound called iron oxide or rust.

Let's break down the process of rusting of iron and see why it is considered a chemical change:

1. Formation of Iron Oxide:
When iron comes in contact with moisture and oxygen, a chemical reaction takes place. The iron atoms react with oxygen to form iron oxide. This reaction can be represented by the chemical equation:

4Fe + 3O2 + 6H2O → 4Fe(OH)3

2. Change in Chemical Properties:
The formation of iron oxide results in a change in the chemical properties of the iron. Rust is a reddish-brown compound that is different from the original iron. It is brittle and flaky, unlike the solid and durable nature of iron.

3. Irreversibility:
Once iron has rusted, it is difficult to reverse the process and restore the iron to its original form. This irreversible nature of the reaction is another characteristic of a chemical change.

4. Energy Change:
Chemical changes often involve energy changes. In the case of rusting, the process is exothermic, meaning it releases energy in the form of heat.

In contrast, the other options listed in the question (a) heating of iron rod, (b) melting of wax, and (d) woollen yarn to knitted sweater are examples of physical changes.

(a) Heating of iron rod is a physical change because it does not involve the formation of new substances. The iron rod may change in temperature and expand, but no new chemical bonds are formed.

(b) Melting of wax is also a physical change as it involves a change in state from solid to liquid. The chemical composition of wax remains the same during this process.

(d) Woollen yarn to knitted sweater is a physical change as it involves the rearrangement of yarn without any change in the chemical composition of the wool.

In conclusion, rusting of iron is an example of a chemical change because it involves the formation of a new compound (iron oxide) with different chemical properties.