POINTS TO REMEMBER
  1. Wind : When air is in motion it is called wind.
  2. Air : Air is a mixture of gases.
Properties of air
  1. It is colourless i.e., it has no colour and tasteless.
  2. It is transparent i.e., we can see through it.
  3. Air occupies space. It fills all the space in a container which is otherwise empty and seems to be empty.
  4. Air exerts pressure.
  5. All living organisms require air for their survival.
  6. Smoke : These are the dark-coloured gas caused by burning of anything.
  7. Weather cock :It shows the direction in which the air is moving at that place.
Composition of Air
  1. Air is really not one substance but a mixture.
  2. Air is a mixture of some gases, water vapour and dust particles.
  3. The gases in air are mainly nitrogen, oxygen, small amount of carbon dioxide and some other gases.
Water vapours
  • Air contains water vapours
  • When air comes in contact with a cool surface, it gets cooled and fog
  • The presence of water vapour in air is important for water cycle in nature.
Dust particles
  • Air contains dust particles
  • The presence of dust particles in air varies from place to place and time to time.
  1. The component of air that supports burning is called oxygen.
  2. Oxygen is necessary for the survival of all living beings. It is required in respiration.
  3. Percentage of oxygen in air is 20.95%.
  1. The major part of air is nitrogen. It takes up four fifth of the space (78.11%) that air fills.
  2. Nitrogen does not support burning.
Carbon  dioxide
  1. Carbon dioxide makes up a small component (0.03%) of air around us.
  2. It causes feeling of suffocation.
  3. All material, when they burn consume oxygen and produce carbon dioxide.
  4. It is also produced along with water vapours during respiration.
  5. Plants need carbon dioxide to live.
  1. Our earth is surrounded by air in the form of a thin layer. This thin layer is called atmosphere.
  2. The atmosphere extends several kilometers.
  3. Air becomes thinner and thinner as we go high up from the surface of earth.
  4. The atmosphere is quite due to the movement of air, with respect to the earth.
  5. The process like, cloud formation, thundering, rain etc., occur in atmosphere.
Uses of Air
  1. Air exerts force on objects that come in its way this property of air is quite useful.
  2. Fun like firkis, pinwheel are based on the force applied by air.
  3. The air current makes the windmills to rotate.
  4. Air helps in the movements of sailing yachts, sliders, parachutes and aircrafts.
  5. Air also helps in dispersal of seeds and pollen of flowers.
  6. Compressed air is used in tyres of vehicles.
  7. Nitrogen is used on a large scale to manufacture fertilizers.
  8. Winnowing, is possible only because of air.
  9. Air is also useful in several musical instruments.
  10. Birds, bats and many insects fly in air.
  11. We cannot hear the sounds in absence of air.
  12. Various components of air play various important roles.
Q.1. What is the composition of air?

Ans. Various components of air are :
  1. Nitrogen – 78.11%
  2. Oxygen  -  20.95%
  3. Carbon dioxide  -  0.03%
  4. Other gases
  5. Water vapours
  6. Dust particles
Q.2. Which gas in the atmosphere is essential for respiration?

Ans. Oxygen gas helps in the breakdown of the food to provide energy. This process is called respiration. Thus, oxygen helps in respiration.

Q.3. How will you prove that air supports burning?

To show that oxygen supports burning we have to perform an activity as given below. Fix a candle about 6-8 cm long in the centre of a trough or a plate having a raised rim. Now fill the trough or a plate with water to more than half of its height. Light the candle. Next, take a one litre plastic water bottle and with the help of sharp knife remove its base. It will serve the purpose of a vessel. Tighten its cap. Then place it over the candle as shown in figure. Observe carefully what happens to the burning candle and the water level. Light the candle again and perform this experiment with a two-litre plastic water bottle. Next, make several holes all over the surface of smaller plastic bottle with the help of a hot screwdriver or nail. And repeat the experiment with this water bottle.
The candle flickers out after some time in the first two conditions. The candle burns for a shorter period in the smaller bottle. We kept the air limited in the first two conditions. However, in the third experimental condition, air was allowed to come and go through the vessel. Candle burns continuously in the bottle having holes. We also see that, if not covered in any closed space, a candle continues burning till it is completely finished. This reflects that burning cannot take place without a continuous supply of air. We can conclude that air is necessary for burning things.
In the first two conditions, there was a rise in water level, while no rising of water would have been observed in the third set up. This indicates that, during burning, some amount of air was used up. The component of air that supports burning is called oxygen.

Q.4. How will you show that air is dissolved in water?

To show that air is dissolved in water we have to perform following activity.
Take some water in glass vessel or beaker. Heat it slowly on tripod. Look carefully at the inner surface of vessel.
We see that tiny air bubbles are sticking on it.
These air bubbles are mainly of oxygen gas present in air, which is dissolved in water. When we heat up water, air dissolved in it escapes out in form of bubbles.

Q.5. Why does a lump of cotton wool shrink in water?

In a cotton lump a large amount of air is trapped between thin and light cotton fibres making it fluffy. When these fluffy lump come in contact with water they shrink because the fibres get weight after wetting and the air escapes out from it.

Q.6. The layer of air around the earth is known as …………


Q.7. The component of air used by green plants to make their food, is ………

CO2 (Carbon dioxide)

Q.8. List five activities that are possible due to the presence of air.

Air is very important for living beings. Its various activities are listed below:
  1. It helps in burning.
  2. It is essential for the respiration in living being.
  3. It is helpful in movements of sailing yachts, gliders, parachutes and aircrafts.
  4. Air current makes the windmill to rotate. The wind mill is used to draw water from tubewells and to run flour mills. Wind mill also generate electricity.
  5. Birds, bats and insects fly in air.
  6. Air also helps in the dispersal of seeds and pollen of flowers of several plants.
  7. Compressed air is used in tyres in vehicles.
  8. Nitrogen of the air is used on a large scale to manufacture fertilizers.
  9. Helps in winnowing.
  10. Helps in hearing sounds.
  11. CO2 present in air is used by plants in making their food.
  12. Activities like cloud formation, thundering, etc. occur in air.
Q.9. How do plants and animals help each other in the exchange of gases in the atmosphere?

Ans. The carbon dioxide along with water, is used by green plants in the presence of sunlight. Oxygen is produced along with food in photosynthesis. Plants also consume oxygen for respiration, but they produce more of it than they consume. That is why we say plants produce oxygen.
Since plants produce oxygen, while animals consume it. It is obvious that animals cannot live without plants, since plants are their food. Moreover, plants could not survive for long without animals. They would consume all the carbon dioxide in the atmosphere. We can see that both need each other, as the balance of oxygen and carbon dioxide in the atmosphere is thus maintained. This is because of the interdependence of plants and animals.


Q.1. What will happen if the amount of oxygen is increased in the air?

Air will become more combustible and many substances may catch fire undesirably.

Q.2. Name the gases present in air. Which one of them is most abundant?

N2. O2 Ar(Argon), CO2, water vapour, Neon(Ne), Helium(He), Krypton(Kr), Xenon(Xe) are present in air, Nitrogen is most abundant.

Q.3. Name the vital component of air. Specify its importance.

Oxygen is regarded as the most important and vital component of air. The living beings take in oxygen (respiration) and give off carbon dioxide. The oxygen used is replaced by plants in photosynthesis. The concentration of oxygen in the air is thus maintained. Thus, it is very important to sustain life on the earth.

Q.4. Explain why we are able to see upto a long distance just after a rain.

When it rains, the dust in the atmosphere settles down to the ground. This is why you can see upto a long distance just after rain. You might have also observed that the initial drops of first rain bring a lot of dirt along with it.

Q.5. Why air dissolved in water is richer in oxygen than ordinary air?

The air dissolved in water is richer in oxygen because oxygen has appreciable solubility in water whereas nitrogen is insoluble in water.

Q.6. Name the substances which are present in traces in air. What are these together called?

Xenon (Xe), Krypton (Kr), Neon (Ne) and Helium (He) are present in traces in air. These together form 0.02% of air and are called inert gases.

Q.7. Describe the composition of air.

Air contains mostly nitrogen (78.03%) and oxygen (20.99%). The remaining 1% (by volume) is shared by argon (0.34%), carbon dioxide (0.33%), other noble gases, oxides of nitrogen and sulphur etc., in the decreasing order.

Q.8. What constitutes the atmosphere? Name the four different layers of atmosphere.

The atmosphere is constituted of mixture of gases. The four different layers of atmosphere are troposphere, stratosphere, mesosphere and thermosphere.

Q.9. Why is the study of atmosphere important to us?

The study of atmosphere helps us to make weather forecasts. Atmosphere also plays important part for the sustenance of life on earth. Thus it is important to study atmosphere from time to time and take protective steps, if necessary.

Q.10. What is the cause of air pressure?

The air pressure arises due to the constant collision of molecules of the gases present in air with walls of the containing vessel.

Q.11. What is the importance of atmosphere for us?

Atmosphere is important for us because:
  1. it protects us from harmful radiations reaching us.
  2. It plays an important role in the formation of clouds, occurrence of rain and formation of snow and winds.
  3. The survival of mankind on earth has become possible because of a particular composition of atmosphere.
Q.12. What is the important of oxygen in daily life?

Oxygen plays a very important role in our daily life. Some of these are :
  1. As ozone, it provides a blanket in the atmosphere which protects us from harmful ultraviolet radiations emitted by the sun.
  2. It is used for breathing and as a breathing aid in hospitals, aeroplanes, and y deep-sea divers, astronauts.
Q.13. How does oxygen occur in nature?

Oxygen is found in free state in air as diatomic molecule (O2) to the extent of 21% by volume or 23% by weight. In the combined state, it is present in water (89% by weight), earth’s crust (about 50%) and in plants and animal tissues (50-70%).

Q.14. What is the importance of the presence of carbon dioxide in the atmosphere? What will happen if its concentration in the air increases?

Carbon dioxide has the property of absorbing heat rays called infrared rays. Nature has a balance of carbon dioxide in the atmosphere, thus giving an optimum warmth to the atmosphere. The increase in the concentration of carbon dioxide in the air would cause increase in the temperature of the earth and would ultimately lead to melting of glaciers causing floods etc. This is called global warming.

Q.15. Nature maintains a balance of carbon dioxide and oxygen in the atmosphere. Comment.

Nature is its own way has been maintaining a balance of carbon dioxide and oxygen in the atmosphere. Atmosphere and oceans are continuously exchanging these gases with the rocks, plants and living organisms. However, due to rapid industrialization in the past, the concentration of CO2 near the earth’s surface is increasing though in very small proportions till now.

Q.16. Name three processes that will get accelerated if the amount of oxygen in the air increases.

If the amount of oxygen in air increases, then
  1. The rate of metabolism in the bodies of human being and animals will become very fast, thus damaging their health.
  2. All the fuels will burn fast and dangerously with explosion.
  3. Rapid corrosion of metals will take place.
Q.17. There are two jars A and B. One is full of oxygen and the other has nitrogen. Both gases are colourless and odourless. How will you show which jar contains oxygen gas?

Oxygen is a supporter of combustion. So if you introduce a glowing stick into a jar of oxygen, it will burst into a flame. Thus out of jars A and B, the one which makes the glowing stick burst into flame, contains oxygen gas.

Q.18. Can you rotate a firkin in a closed room with its ceiling fan closed?

No, firkin does not rotate in a closed room. We observe that firkin rotates in an open area, or in a closed room with the ceiling fan switched on. This is because open areas allow air to move here and there.
Q.19. State uses of oxygen.

Ans. Uses of Oxygen:

  1. Oxygen mixed with CO2 or helium is used for respiration.
  2. Liquid oxygen filled in cylinders is conveniently used as a breathing aid by deep-sea divers, mountaineers, astronauts and at home or hospitals by heart or asthmatic patients.
  3. Oxygen-hydrogen and oxy-acetylene flames are used for cutting and welding purposes.
  4. Liquid oxygen mixed with finely divided carbon is used as a substitute for dynamite in coal mining.
  5. Oxygen is used as an oxidizing agent in several preparations.
  6. Liquid oxygen is an important constituents of fuels used for rockets.
  7. Molten iron is purified by blowing oxygen into it. Thus, purified iron is then mixed with carbon and other metals to prepare steel alloys.
Q.20. Show that air contains about 20% oxygen.
Ans. Take a trough and fill 2/3rd of it with water. Put an inverted jar in the water and divide the jar into five equal parts starting with zero from the surface of water. Remove the jar and place a burning candle under the jar. When the candle is extinguished, it will be noticed that water from the trough rises to 1/5th of volume of the jar. The candle is extinguished because the supporter of combustion, i.e., oxygen present in air is consumed. The vacant space due to consumption of oxygen is now occupied by water. Since water rises to only 1/5th of the volume occupied by air, it shows that air contains only 1/5th of its volume of supporter of combustion, i.e., oxygen.

Q.21. Why fine hair and mucus are present in our nostrils? Why should we not breath in by mouth?

We inhale air when we breathe through our nostrils. We also know that air contains dust particles. To prevent dust particles from getting into the respiratory system, fine hairs and mucus are present inside the nose.Since, our mouth does not contain, above discussed barriers of dust, if we breathe through it dust particles may enter in our respiratory tract. This is why breathing through mouth is not suggested.

Q.22. Why atmosphere is called the place of exchange of gases in nature?

We know that various gases are found in air. Now some organisms live on land, some in water and some deep in soil. But all need oxygen to respire. The terrestrial organisms take oxygen from the atmosphere. You know that during respiration of living beings and burning of materials, carbon dioxide is produced along with some other gases. Also CO2 used by the plants in the process of photosynthesis in which oxygen is released. All gases go into the atmosphere. Thus, it is a place in Nature for gaseous exchange.

Q.23. How will you prove that air is a mixture of several gases and not a compound?

The following points show that air is a mixture :
  1. The composition of air varies slightly from place to place.
  2. Different components of air are separable by physical processes.
  3. Different components of air exhibit their characteristics, e.g., O2 in air supports combustion, CO2 in air turns time water milky.
Q.24. Why do you feel suffocation if there is any material burning in the room?

In a closed room, if there is any material that is burning, you may have noticed a feeling of suffocation. This comes from excess of carbon dioxide that may be accumulating in the room. Carbon dioxide makes up a small component of Air around us. All materials, when they burn, produce mainly carbon dioxide and a few other gases.

Q.25. Why long chimneys are erected in factories? What is its effect on birds flying in the sky?

The burning of fuel and materials also produces smoke. Smoke contains a few gases and fine particle and is often harmful. That is why we see long chimneys in factories. This takes the harmful smoke and gases away from our noses, but, brings it right under the noses of birds flying up in the sky and adversely affect them.

Q.26. How can we see dust particles present in the air?

To see the dust particles present in air, we have to close all the doors and windows of a room to make it dark. Then, if we open the door/windows facing the sun, just a little, in such a way that it allows sunlight to enter the room only through a slit. We can see some tiny shining dust particles moving rapidly in the slanting beam of sunlight.
The presence of dust particles in air varies from place to place and time to time.


Q.1. Match the following items given in Column A with that in Column B.

Column AColumn B

  1. Air
 2. Wind

3. Atmosphere

4. Oxygen

5. Carbon dioxide

6. Nitrogen

7. Respiration

8. Photosynthesis

9  Wind mill

10. Open areas
i) Thin layer of air surrounding our earth.

ii) Released in respiration.

iii) Makes three fourth part of total air.

iv) Is process of burning of food to get energy.

V) Uses wind power to rotate.

vi) Process of preparation of food by green plants.

vii) Allow air to move here and there.

viii) Is a gaseous mixture.

ix) Supports combustion.

x) Is air in motion.

Q.2. Fill in the blanks with appropriate words:

  1. When air is in ………… it is called wind.
  2. Air has no ………… and one can see through it. It is ………… .
  3. Air is really not one substance but a ………… .
  4. Air contains …………. Particles.
  5. The components of air that supports burning is called ………… .
  6. The oxygen helps in breaking down the digested food in the body to release the ………… .
  7. Our earth is surrounded by …………. .
  8. Initial drops of first rain bring a lot of ………. Along with them.
  9. The aquatic animals use dissolved oxygen in water for respiration. This is possible because oxygen is ………… in water.
  10. Nitrogen is the air is used on a large scale to manufacture ……….. .
  11. Air is present in, atmosphere, water and ………… .
  12. The method of separating two components of a mixture which is based on the wind is …………. .
  13. …………….. air is used in tyres of vehicles.
  14. ……………. is the place in nature for gaseous exchange.
Q.3. State whether the statements given below are True or False:
  1. We can not see air.
  2. Air is a compound and not an element.
  3. A firkin rotates only in an open or in a closed room with the ceiling fan switched on.
  4. Open areas allow air to move here and there.
  5. Air is an opaque material.
  6. All living organism require air for their survival.
  7. Air contains water vapours.
  8. We breath in CO2 and breath out O2.
  9. A policeman, controlling the traffic wears a mask to prevent the dust particles from entering his respiratory system through breathing.
  10. Air is necessary for burning things.
  11. The major part of air is nitrogen.
  12. Air becomes thinner and thinner as we go high up from the surface of earth.
  13. Plants produce oxygen through photosynthesis.
  14. Plants do not respire.
Q.4. Choose the correct option in the following questions :
  1. Major part of the air constituted by
(a) nitrogen                                       (b) oxygen
(c) carbon dioxide                              (d) inert gases

2. Which gas helps in burning?
  • (a)Nitrogen                                        (b) Oxygen
(c) Carbon dioxide                             (d) Carbon monoxide 

3. Which gas we use in breathing?
  • (a)Carbon dioxide                            (b) Nitrogen
             (c) Oxygen                                          (d) None of these

4.What is wind?
  • (a) Air around us                               (b) Rising hot air
            (c) Air in motion                                 (d) None of these

5.Air is present in

              (a)atmosphere                                    (b) soil
              (c) water of ponds, lakes and sea    (d) everywhere

6. Air is
                (a) mixture                                            (b) pure substance
              (c) an element                                     (d) depends on the place where it is found

7. Envelope of air that surrounds the earth is known as

(a) biosphere                                       (b) atmosphere
(c) environment                                   (d) ecosystem

8. Which is not a property of air?

(a) It occupies space                          (b) It is transparent
(c) It is a solution                                 (d) It is a compound

9. The ratio of nitrogen to oxygen in the air is

(a)1 : 4                                                  (b) 4 : 1
(c) 3 : 1                                                  (d) 1 : 2

10.The main constituent of the air is

(a)nitrogen                                          (b) oxygen
(c) carbon dioxide                               (d) hydrogen
Q.1. Match the items in Column A with Column B :

Column AColumn B
1 iii

Q.2. Fill in the blanks :
        (i) motion              (ii) colour, transparent           (iii) mixture                (iv) dust
        (v) oxygen            (vi) energy                             (vii) air                       (viii) dirt
        (ix) soluble           (x) fertilizers                           (xi) soil                      (xii) winnowing
        (xiii) Compressed   (xiv) Atmosphere.
Q.3. True/False :
         (i) True                        (ii) False                       (iii) True                     (iv) True
         (v) False                     (vi) True                        (vii) True                    (viii) False
         (ix) True                      (x) True                         (xi) True                    (xii) True
         (xiii) True                    (xiv) False.
Q.4. Choose the correct option:
  1. (a) 3/4th portion of air is nitrogen.
  2. (b) Oxygen does not burn but supports burning.
  3. (c) We use oxygen and produce carbon dioxide.  
  4. (c) Air in motion is called wind.
  5. (d) Air is present everywhere.
  6. (a) Air is a mixture of various gases, water vapours and dust particles.
  7. (b) Air is all around us, a thick layer of air surrounds the earth, this blanket of air is called atmosphere.
  8. (d) Air is a homogeneous solution, which has occupies space and it is transparent.
  9. (b) 4 : 1 is the ratio of nitrogen to oxygen in the air.
  10. (a) Composition of air (Percentage by volume) nitrogen = 78.03, oxygen 20.99, carbon dioxide 0.03, noble gases = 0.95 and some water vapours, dust particles and other impurities.