CHEMISTRY [2 Mark Question Answer]

The original substances undergo changes to form new substances which have composition and properties different from the original substances. These new substances cannot be converted back to the original substances.

Process of boiling and evaporation involves the change of state from liquid to gaseous. Only the physical state of substance changes. Chemical composition of the substance does not change. No new substance is formed and this process can be reversed by condensation.

Rusting of iron is a chemical change as the composition and properties of rust are different from those of iron. Rust cannot be converted back to iron by simple physical methods. Therefore, the change is irreversible and permanent.

Cooking of food is a chemical change because the composition of raw grains and vegetables changes and new substances are formed. It is not possible to get the raw vegetables from cooked food. This shows that the change is permanent and irreversible.

Glowing of an electric bulb is a physical change as it is reversible and no new substance is formed inside the bulb. When an electric bulb is switched on, electric current passes through the filament of the bulb and the filament glows to give light. When switched off, the filament returns to its normal condition and the bulb stops glowing.

Melting of ice is a physical change as it is temporary and reversible. Ice can be formed again by freezing water. No new substance is formed. The chemical composition of water does not change.

Burning of wood is a chemical change as when wood is burnt in air, they undergo changes to form carbon dioxide and water vapour (new substances). The change is permanent and irreversible as ash cannot be converted back to wood.

Burning of a candle involves physical as well as chemical changes –
1. During the burning of a candle, some wax melts and solidifies back to wax on cooling. Therefore, this shows that melting of wax is a physical change.

2. During burning of a candle, some wax undergoes combustion. It bums to produce new substances : carbon dioxide and water vapour. Therefore, this also shows a chemical change.

A physical change is a change in which no new substance is formed. It is temporary and usually reversible. Examples : Drying of wet clothes, stretching a rubber band, glowing of an electric bulb, etc. A chemical change is a permanent change in which the original substance loses its own composition and properties and forms a new substance. Examples : Cooking of food, digestion of food, rusting of iron, burning of paper, etc.