Class 6 Science Chapter Notes

Chapter 15 Air Around Us

 

Air is one of the natural resources available on Earth.

Properties of Air

(i) Air is invisible, it cannot be seen, can only be felt.

(ii) It occupies space.

(iii) It is colourless, odourless, transparent and tasteless.

(iv) Air takes shape and smell from the object it is present in.

(v) Air is a mixture of different gases and other particles.

A weather cock is used to find the direction of the wind.

Activity to show the presence of air

Let us make a firki of our own by using a paper strip.

Hold the stick of the firki and move it in different directions in an open area. We will observe that the firki starts rotating.

The firki moves/rotates when the air strikes its blades.

This shows that air is present around us.

Presence of Air Around Us

Although, we are not able to see air but we can feel its presence. When fan moves over head, our hair starts flying. It shows the existence of air. Air is present everywhere and it occupies space. The place without air is called vacuum.

Activity to show that air occupies space

Take an empty glass bottle. It is not really empty; something is inside it. Now, we hold the bottle in inverted position (with its open mouth facing downwards).

The inverted glass bottle is put in water, the water does not enter into the inverted glass bottle because the bottle is filled with air.

Now, tilt the bottle in water. Air present in the bottle goes out in the form of air bubbles.

As the air from the bottle escapes, water starts entering the glass bottle and occupies the place of air. We conclude that the glass bottle which appeared to be empty was actually filled with air.

Atmosphere

Our earth is surrounded by a thin layer of air. This layer extends upto many kilometres above the surface of the earth and is called atmosphere.

Atmosphere is the protective blanket/covering that surrounds the earth. It filters harmful radiation's and other particles entering earth's atmosphere.

Mountaineers carry oxygen cylinders with them while climbing on high mountains. This is necessary because the amount of air reduces as we go up. At the top, level of oxygen is very low creating breathing problems.

Components of Air

Air is a mixture of many gases, e.g. nitrogen, oxygen, carbon dioxide, argon, and water vapour.

Water Vapour

Air contains water vapour. When air comes in contact with cool surface, the vapours present in it condenses and drops of water appear on the cooled surface. The presence of water in air is important for the water cycle in nature.

Oxygen

It is vital for life as it is essential for breathing, respiration, etc. Oxygen is non-combustible but it is a supporter of combustion. Water has some dissolved oxygen in it. The aquatic animals (e.g. fish) which live in water, breathe in oxygen dissolved in water of pond, lake, river or sea. The plants that live in water also use the oxygen which is dissolved in water. Thus, there is oxygen in water too. If there were no oxygen in water then no living being could survive in water.

Activity to demonstrate the presence of oxygen in air and it supports burning

Fix two small candles of the same length on a table. Light both the candles.

Cover one of the candles with an inverted glass tumbler.

Observe both the candles carefully.

The candle covered with glass tumbler got extinguished after some time, whereas the other candle continued burning.

The candle got extinguished because the component inside of the glass tumbler, which supports burning, is limited.

Most of the component is used up by the burning candles.

However, the other candle is getting continued supply of air. This component of air, which supports burning, is known as oxygen.

Nitrogen

About 78.9% of total air is nitrogen. It is required by plants for their growth. But plants are not able to absorb the nitrogen directly from the air. Nitrogen is converted into nitrate, then plants can absorb it from soil. We have seen in previous activity that a major part of air is still present in the glass bottle even after the candle blew out, this indicates the presence of some components in the air which do not support burning. That component is the major part of air which is called as nitrogen.

Carbon Dioxide

It is about 0.03-0.04% of total air present in atmosphere is important for green plants as they make their food by using it. It is a very crucial component of air. We intake oxygen of the air while breathing and give out carbon dioxide. During burning also oxygen is used up and carbon dioxide is given out. Without CO₂, plants will not flourish.

Dust and Smoke

Dust particles are always present in air. Dust comes into air through blowing of wind, vehicles driving, dusting at home, etc. Burning of the fossil fuel produces smoke. Smoke contains a few gases and fine dust particles. It is harmful.

We have seen the long chimneys in factories. This takes the harmful smoke and gases away from our nose but bring it closer to the birds flying up in the sky.

The traffic policemen who regulate traffic at a crowded city road crossing often wear masks. The traffic policemen wear masks to protect themselves from the smoke containing harmful gases which are emitted by the vehicles moving around them.

Activity to observe the presence of dust particles in air

Find a room in a school. Close all the doors and windows with the help of black chart paper or black curtains.

A small hole is made in the black chart paper fixed on the window, faces the sun. A thin beam of sunlight enters the room through the small hole made in the chart. We could see the tiny dust particles moving in the beam of light in the dark room.

This shows that air also contains dust particles. The presence of dust particles in air varies from time to time and place to place.

Composition of Air

The gases in air are mainly nitrogen 78.9%, oxygen 20.9%, the remaining 0.2% is made up of gases, e.g. argon, carbon dioxide and small amounts of other gases. Varying quantities of dust, smoke and water vapour are also present.

Presence of Oxygen in Water and Soil

Animals and plants living in water are aquatic organisms. These organisms use the oxygen which is dissolved in water for breathing.

The organisms living in the soil and plant roots use the air present in spaces formed between soil particles.

Activity to show that freshwater contains dissolved air

We take a tumbler/pan filled with water and keep it on a tripod stand. Heat the water slowly by using a burner. Before the water begins to boil, watch carefully at the inner surface of the tumbler/pan, tiny bubbles appear on the inner surface of the tumbler/pan. These bubbles are of the air which was dissolved in water.

If we continue heating, the water itself turns into vapour and finally begins to boil. We conclude that the air (mostly oxygen) dissolves slowly in freshwater.

When the water is warmed, the solubility of air decreases and it comes out of water in the form of bubbles. Hence, water contains dissolved air (oxygen). All the aquatic animals and plants thus survive by using this oxygen.

Activity to show that soil contains trapped air

We take small jar filled with water. Add a handful of soil into the jar and observe air bubbles forming in by water. The small bubbles of air in water shows that the soil contains some air trapped in between the soil particles.

We observe that the organisms that live inside the soil and the roots of plants respire by using this air. A lot of burrows and pores are formed in deep soil by the animals which live in the soil for the purpose of breathing. When it rains heavily, water fills in all these spaces occupied by the air in the soil, thus clogging all the pores cutting of oxygen supply. In this situation, animals living in the soil have to come out for respiration, e.g. earthworms and frogs, etc.

Replacement of Oxygen in Atmosphere

All living organisms consume oxygen present in air. The process of respiration uses oxygen present in air but this is constantly replaced by plants through photosynthesis process. In this process, plants make their own food and oxygen is produced along with it. Plants also consume oxygen for respiration, but they produce more of it than they consume. That is why, we say plants produce oxygen.

It is obvious that animals cannot live without plants. The balance of oxygen and carbon dioxide in the atmosphere is maintained through respiration in plants and animals and by the photosynthesis in plants. This shows the interdependence of plants and animals.

Some Uses of Air

(i) Air exerts pressure on objects that come in its way.

(ii) Air regulates temperature.

(iii) Wind is used to rotate windmills. It is used for various purposes like draw water from tubewells, to run flour mills, generate electricity, etc.

(iv) Air is used by living organisms for breathing.

(v) Air is used to inflate the tyres of vehicles.

(vi) Nitrogen of air is used to manufacture fertilisers.

(vii) Air is also used for winnowing.

(viii) Air helps in travelling of sound and smell. When air moves, it carries sound and smell to distances that's why, we are able to hear.

(ix) Air helps in the movements of sailing yachts, gliders, parachutes and aeroplanes. Birds and insects can fly due to the presence of air.

(x) Air also helps in the dispersal of seeds and pollen of flowers of several plants.

(xi) Air plays an important role in water, nitrogen and oxygen cycle.

 

End of Notes

 

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