CBSE Class 6 Science Air Around Us Exam Notes

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Air Around Us

ATMOSPHERE
Air is all around us. In fact, a thick layer of air surrounds the earth. This blanket of air is called atmosphere and extends to several hundred kilometres above the earth’s surface. It is held in place by the earth’s gravity and not allowed to escape into the space. As we go up in the atmosphere, the availability of air gradually decreases. That is why mountaineers carry oxygen cylinders with them while climbing high mountains.(Figure)

COMPOSITION OF AIR
Air is a mixture of several gases. The gases present in air are mainly nitrogen a (almost four-fifths of air) and oxygen (nearly one-fifth of air). We say ‘nearly’ or ‘almost’ because air also contains small amounts of carbon dioxide, helium, argon, water vapour, dust particles and certain other impurities (Figure).

The composition of air varies from place to place and season to season (Table).

Let us perform the following activities to learn about the various constituents of air.
Activity 1
Air Contains Water

Take a cold drink bottle from the refrigerator. Hold this bottle in your hand. What do you observe after a few minutes? Small drops of water appear on the outer surface of the bottle that make your hand wet. From where did this water come? The water vapour present in the air condenses on the cold surface of the bottle and forms these small drops of water.

Activity 2
Air Contains Carbon Dioxide

Take some lime water in petridish and keep it for a few hours in the open. A milk white crust will be formed on the copper surface of the same water turns milky due to the presence of carbon dioxide in the air.

Activity 2
Air is Necessary for Respiration

Take a live frog and cover it with a bell jar. You may also take any insect in place of the frog. Keep the bell jar undisturbed. Examine the frog after a few days. Do you find any sign of life in the frog? NO, the frog is dead. Why did the frog die? The frog died because of the non-availability of oxygen. The bell jar cut off the supply of air containing oxygen. This experiment proves that air is necessary for respiration.

Terrestrial Animals
Terrestrial animals which live on the surface of land get oxygen directly from the air. They breathe with the help of lungs. The animals which live under the soil breathe air present in the soil. Earthworms respire through their moist skin. Earthworms that live under the soil come out of their burrows during rainy season. Can you give any reason for it?

Aquatic Animals
Aquatic animals are those that live in water. Most aquatic animals like fish, crabs, octopus, etc. respire through special organs called gills. They take in dissolved oxygen from the water. Some aquatic animals like dolphins and whales breathe with the help of lungs by coming to the surface of water in regular intervals.

RESPIRATION IN PLANTS
Like animals, plants are also living organisms. They also need energy for performing vital functions. The energy requirement of plants is less in comparison to animals. Gaseous exchange in plants mainly takes placethrough the stomata that are present i large number on the leaves. Oxygen diffuses in and carbon dioxide diffuses out through these stomata. Respiration is a continuous process, occurring day and night.

Photosynthesis
Green plants use carbon dioxide from the air, water from the soil and light from the Sun to make their own food in the form of sugar or starch. This process is called photosynthesis. During photosynthesis, oxygen, which is vital for all living beings, is given out. The oxygen produced during photosynthesis is more than that required by the plants for respiration.