Classify each of the following, as a physical or a chemical change. Give reasons. (a) Drying of a shirt in the sun. (b) Rising of hot air over a radiator. (c) Burning of kerosene in a lantern. (d) Change in the colour of black tea on adding lemon juice to it. (e) Churning of milk cream to get butter.

Answer:

Physical change – A substance is said to undergo a physical change when only the physical properties such as the shape, size, colour, state or appearance of the substance change. Its chemical composition remains intact.

Chemical change – A substance is said to undergo a chemical change when the chemical properties of a substance alter. As a result, there is either formation or breaking of atomic bonds at the molecular level.

(a) Drying of a shirt in the sun.

Drying of the shirt in the sun is a physical phenomenon because there are no chemical reaction or any chemical changed involved in this process.

(b) Rising of hot air over a radiator.

Rising of hot air over radiator is a physical change. Water in a radiator converts to vapours. Hot air becomes lighter and rises.

(c) Burning of kerosene in a lantern.

Burning of kerosene in a lantern is a chemical change because kerosene burns by using atmospheric oxygen and produces carbon dioxide.

(d) Change in the colour of black tea on adding lemon juice to it.

Change in the colour of black tea on adding lemon juice to it is a chemical change. Lemon juice is a source of citric acid, ascorbic acid and malic acid. This acid reacts Flavin antioxidants present in black tea to change the colour of the tea.

(e) Churning of milk cream to get butter.

Churning of milk cream to get butter is a physical change as there is no involvement of chemical reaction. Here the principal is of centrifugation which turn the milk cream to butter.

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