Question 1 What is air? State the composition of air?
Question 2 Write few properties of air?
Question 3 Describe an activity to show that a glass bottle which appears to be empty to us is actually filled with air?
Question 4 How will you show that air is a mixture of gases?
Question 5 What is atmosphere?
Question 6 Why do mountaineers carry oxygen cylinders with them while climbing high mountains?
Question 7 Explain why, when an empty glass bottle is inverted into a bucket of water vertically, water does not enter the bottle?
Question 8 What is wind?
Question 9 Why atmosphere is necessary for life?
Air is a mixture of several gases. We cannot see or smell air but air is everywhere around us. Air is necessary for the breathing of all the living things. No living things can survive without air.
We cannot see air but we can feel its presence when it starts moving (or blowing). Moving air is called wind.
(1) Moving air makes the clothes hanging on a clothes line sway.
(2) The air blowing in a garden stirs the leaves and branches of trees.
(3) When a fan is switched on, the air moved by the blades of the fan makes the pages of an open book flutter back and forth.
(4) It is the moving air which makes it possible for us to fly a kite in the sky (by pushing against it).
(5) It is also the moving air which pushes the sails of yacht (sailing boat) and makes it move in water.
(6) In agriculture moving air helps us separate husk from wheat grains during winnowing.
(7) It is the moving air (or blowing air) which makes a phirki rotate.
(8) Weathercock is a device which shows the direction in which the air is moving (or blowing) at a place.
The important properties of air are as follows:
(1) Air is colourless, tasteless and odourless gaseous substance.
(2) Air has mass.
(3) Air occupies space.
(4) Air dissolves in water.
(5) Air can be compressed.
Air is Present Everywhere Around Us
Air is present everywhere around us. Even those containers and vessel which appear to be empty to us are filled with air.An empty bottle is not really empty, it contains air in it. Even when we turn an empty bottle upside down, the air remains in it.
We take an empty glass bottle and hold it in the inverted position (with its open mouth facing downwards).We will find that water does not enter into the inverted glass bottle.The water does not enter into the glass bottle because the bottle is filled with air. All the space in the bottle is occupied by air. The air present in the empty glass bottle prevents the water from entering it.We conclude that air occupies space.
If we tilt the bottle held in water, we will find that the 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. Actually, the space vacated by the air leaving the bottle is now occupied by water. A glass bottle which appears to be empty to us is actually filled with air.
The envelope of air that surrounds the earth is called atmosphere. The atmosphere (or layer of air) extends up to many kilometres above the surface of earth.
Atmosphere is essential for life on earth. This is because the air of atmosphere provides oxygen gas for breathing by all the living organisms including us. As we go higher up in the atmosphere, the amount of air becomes less and less. The air at the top of very high mountains is so thin that it does not have enough oxygen for the people to breathe properly.
So, the people who climb high mountains (called mountaineers) carry cylinders containing oxygen gas with them. They breathe in oxygen from these cylinders (so as to survive on high mountains).
Composition of Air
Air is a mixture of many gases. The major component of air is nitrogen gas.Almost four-fifths of air is nitrogen gas. The second major component of air is oxygen gas. About one-fifth of air is oxygen gas. Air also contains small amount of carbon dioxide gas, water vapours and some other gases.Air also contain some dust particles.
Composition of Air
Carbon dioxide: 1%
Other gases, and Dust particles
The composition of air changes slightly from place to place and season to season.
For example: The air over industrial cities usually has a higher amount of carbon dioxide in it than the air over open spaces.
The air in coastal areas may have more water vapour than inland areas.
The air also contains more water vapour in rainy season.
Similarly, the amount of dust in the air is more in windy places than other areas.
Activity : Air is a mixture of gases
(1) We take a trough (a kind of vessel) and place a gas jar stand in it. Fix a candle on the gas jar stand. Fill half the trough with water.
(2) Dissolve some caustic soda (sodium hydroxide) in this water to form caustic soda solution.
(3) Also add a few drops of ink in water to colour it.
(4) We light the candle with a matchstick. Cover the burning candle by placing an inverted gas jar over it.
(5) After a short time, the candle stops burning and water rises up in the gas jar to a certain level.The candle stops burning because all the oxygen of air present in the gas jar is used up by the burning candle.
(6) Oxygen is a supporter of combustion (or burning). So, when oxygen present in air contained in the gas jar is all used up, the burning candle gets extinguished.
(7) When the candle burns, then oxygen of air in the gas jar is used up and carbon dioxide gas is formed. This carbon dioxide gas is absorbed (or dissolved) by the caustic soda solution in the trough. The absorption of carbon dioxide gas by caustic soda solution creates a vacuum (low pressure) in the gas jar.Water rises up in the gas jar to fill this vacuum.
(8) Water in the gas jar rises to about one-fifth part of the volume of air initially present in the gas jar. The volume of water risen in the gas jar is equal to the volume of oxygen present in the air in the gas jar.
(9) The major part of air which is not used up by a burning candle and remains behind in the gas jar is nitrogen. Since water does not rise in the remaining four-fifth part of this activity, it shows that about four-fifths part of air is nitrogen.