Q 1.

What is the composition of air?

SOLUTION:

Air is a mixture of several gases. It contains 78% nitrogen, 21% oxygen, about 0.9% argon and other noble gases, 0.03% carbon dioxide, traces of other gases and water vapour.

Q 2.

Which gas in the atmosphere is essential for respiration?

SOLUTION:

Oxygen gas is essential for respiration. Human beings, plants and animals take in oxygen from the atmosphere and release carbon dioxide during breathing.

Q 3.

How will you prove that air supports burning?

SOLUTION:

Fix two small candles of the same size in the middle of two shallow containers. Fill the containers with some water and light the candles. Cover each candle with an inverted glass of different size. We will observe that the candle covered with smaller glass goes off first and the water in both the glasses rises up. This is because the candles used the oxygen present inside the glass for burning. Also, some of the space occupied by the oxygen inside the glass becomes empty and as a result, the water rises up to fill or occupy this space.

Q 4.

How will you show that air is dissolved in water?

SOLUTION:

Take some water in a transparent glass bowl or beaker. Heat it slowly and observe the inner surface of it. Tiny bubbles are seen on the inner surface of glass vessel or beaker. These bubbles are air. As the water heats up, the air dissolved in water escapes. On further heating, water itself turns into vapour and finally begins to boil. This activity shows that air is dissolved in water.

Q 5.

Why does a lump of cotton wool shrink in water?

SOLUTION:

The lump of cotton wool remains fluffy because it contains air which is trapped in the vacant spaces in it. When the lump is dipped in water, water displaces the air trapped in it which comes out in the form of bubbles. This results in decreased volume of the cotton wool and the fibres of cotton stick together, hence the lump shrinks.

Q 6.

The layer of air around the Earth is known as ___________.

SOLUTION:

atmosphere

Q 7.

The component of air used by green plants to make their food is ___________.

SOLUTION:

carbon dioxide

Q 8.

List five activities that are possible due to the presence of air.

SOLUTION:

Five activities that are possible due to the presence of air are :
(i) Oxygen present in air helps in breathing.
(ii) Air acts as a medium. It allows sound to travel.
(iii) Compressed air is used in tyres of vehicles which makes their movement easy.
(iv) Moving air i.e. wind helps in dispersal of seeds and in pollination of several flowers.
(v) Birds, insects and bats are able to fly due to the presence of air.

Q 9.

How do plants and animals help each other in the exchange of gases in the atmosphere?

SOLUTION:

Plants and animals help each other in the exchange of gases in the atmosphere. They take in oxygen from atmosphere and release carbon dioxide during respiration.
During photosynthesis, plants use carbon dioxide to synthesise their food and release oxygen. Thus, the balance of oxygen and carbon dioxide in the atmosphere is maintained through respiration in plants and animals and photosynthesis in plants.