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15
Air Around us
W
e have learnt in Chapter 9 that
all living things require air.
But, have you ever seen air?
You might not have seen air, but, surely
you must have felt its presence in so
many ways. You notice it when the
leaves of the trees rustle or the clothes
hanging on a clothes-line sway. Pages
of an open book begin fluttering
when the fan is switched on. The moving
air makes it possible for you to fly your
kite. Do you remember Activity 3 in
Chapter 5 in which you separated the
sand and sawdust by winnowing?
Winnowing is more effective in moving
air. You may have noticed that during
storms the wind blows at a very high
speed. It may even uproot trees and blow
off the rooftops.
Have you ever played with a firki
(Fig. 15.1)?
Fig. 15.2 Making a simple firki
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Move it a little, back and
forth. Observe, what happens.
Does the firki rotate? What makes a firki
rotate — moving air, isn’t it?
Have you seen a weather cock
(Fig. 15.3)? It shows the direction in
which the air is moving at that place.
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Fig. 15.3 A weather cock
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Fig. 15.1 Different types of firki
15.1 IS AIR PRESENT EVERYWHERE
AROUND US?
Close your fist — what do you have in
it? Nothing? Try the following activity
to find out.
Activity 1
Activity 2
Let us make a firki of our own, following
the instructions shown in Fig. 15.2.
Hold the stick of the firki and place it
in different directions in an open area.
Take an empty glass bottle. Is it really
empty or does it have something inside?
Turn it, upside down. Is something
inside it, now?
Fig. 15.4 Experiments with an empty bottle.
Now, dip the open mouth of the bottle
into the bucket filled with water as
shown in Fig. 15.4. Observe the bottle.
Does water enter the bottle? Now tilt the
bottle slightly. Does the water now enter
the bottle? Do you see bubbles coming
out of the bottle or hear any bubbly
sound? Can you now guess what was
in the bottle?
Yes! You are right. It is “air”, that was
present in the bottle. The bottle was not
empty at all. In fact, it was filled
completely with air even when you
turned it upside down. That is why you
notice that water does not enter the
bottle when it is in an inverted position,
as there was no space for air to escape.
When the bottle was tilted, the air was
able to come out in the form of bubbles,
and water filled up the empty space that
the air has occupied.
This activity shows that air occupies
space. It fills all the space in the bottle.
It is present everywhere around us. Air
has no colour and one can see through
it. It is transparent.
Our earth is surrounded by a thin
layer of air. This layer extends up to
many kilometres above the surface of
the earth and is called atmosphere.
Why do you think, mountaineers
carry oxygen cylinders with them, while
climbing high mountains (Fig. 15.5)?
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Fig. 15.5 Mountaineers carry oxygen cylinders
with them
15.2 WHAT
IS
AIR MADE
UP OF?
Until the eighteenth century, people
thought that air was just one substance.
Experiments have proved that it is really
not so. Air is a mixture of many gases.
What kind of a mixture is it? Let us find
out about some of the major
components of this mixture, one by one.
Water vapour
We have learnt earlier that air contains
water vapour. We also saw that, when
air comes in contact with a cool surface,
it condenses and drops of water appear
on the cooled surfaces. The presence of
water vapour in air is important for the
water cycle in nature.
Oxygen
Activity 3
In the presence of your teacher, fix two
small candles of the same size in the
middle of two shallow containers. Now,
fill the containers with some water. Light
the candles and then cover each one of
them with an inverted glass (one much
SCIENCE
component in the air, which does not
support burning. The major part of air
(which does not support burning candle)
is nitrogen. It takes up nearly four-fifth
of the space that air fills.
Carbon dioxide
Fig. 15.6 Air has oxygen
taller than the other) as shown in Fig.
15.6. Observe carefully what happens to
the burning candles and the water level.
Do the candles continue to burn or
go off? Does the level of water inside
glasses remain the same?
The burning of the candle must be
due to presence of some component of
air, isn’t it? Do you find any difference
in your observation with the two glasses
of different heights? What can be the
reason for this?
Burning can occur only in the
presence of oxygen. We see that, one
component of air is oxygen. Now, the
amount of air and hence its oxygen
component inside each glass in our
experiment, is limited. When most of
this oxygen is used up by the burning
candle, it can no longer burn and blows
out. Also, some of the space occupied
by the oxygen inside the glass becomes
empty and the water rises up to fill or
occupy this space.
In a closed room, if there is some
material that is burning, you may
have felt suffocation. This is due to
excess of carbon dioxide that may be
accumulating in the room, as the
burning continues. Carbon dioxide
makes up a small component of the air
around us. Plants and animals consume
oxygen for respiration and produce
carbon dioxide. Plant and animal matter
on burning, also consumes oxygen and
produces mainly carbon dioxide and a
few other gases.
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Nitrogen
In Activity 3 did you observe 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
AIR AROUND US
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Dust and smoke
The burning of fuel also produces
smoke. Smoke contains a few gases and
fine dust particles and is often harmful.
That is why you see long chimneys in
factories. This takes the harmful smoke
and gases away from our noses, but,
brings it closer to the birds flying up in
the sky!
Dust particles are always present
in air.
Activity 4
Find a sunny room in your school/
home. Close all the doors and windows
with curtains pulled down to make the
room dark. Now, open the door or a
window facing the sun, just a little, in
such a way that it allows sunlight to
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Fig. 15.7 Observing presence of dust in air with
sunlight
Fig.15.8 Policemen regulating traffic at a crowded
crossing often wear a mask
enter the room only through a slit.
Look carefully at the incoming beam
of sunlight.
Do you see some tiny shining
particles moving in the beam of sunlight
(Fig. 15.7)? What are these particles?
During cold winters you might have
observed similar beam of sunlight filter
through the trees in which dust particles
appear to dance merrily around!
This shows that air also contains
dust particles. The presence of dust
particles in air varies from time to time,
and from place to place.
We inhale air when we breathe
through our nostrils. Fine hair and
mucus are present inside the nose to
prevent dust particles from getting into
the respiratory system.
Do you recall being scolded by your
parents when you breathe through your
mouth? If you do that, harmful dust
particles may enter your body.
We may conclude, then, that air
contains some gases, water vapour and
dust particles. The gases in air are
mainly nitrogen, oxygen, small amount
of carbon dioxide, and many other
gases. However, there may be some
variations in the composition of air from
place to place. We see that air contains
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Boojho is asking you, why
do you think, the policeman
in Fig. 15.8 is wearing a
mask?
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Paheli wants to know, why
the transparent glass of
windows, if not wiped off
regularly, appears hazy?
Boojho wants to know, why
during an incident of
fire, one is advised to
wrap a woollen blanket
over a burning object.
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carbon dioxide,
water vapour
and other gases
oxygen
Nitrogen
Fig.15.9 Composition of air
mostly nitrogen and oxygen. In fact,
these two gases together make up 99%
of the air. The r emaining 1% is
constituted by carbon dioxide and a few
other gases, water vapour and dust
particles (Fig. 15.9).
Activity 5
Take some water in a glass vessel or
beaker. Heat it slowly on a tripod stand.
Well before the water begins to boil, look
carefully at the inner surface of the
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Fig. 15.10 Water contains air!
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vessel. Do you see tiny bubbles on the
inside (Fig. 15.10)?
These bubbles come from the air
dissolved in water. When you heat the
water, to begin with, the air dissolved in
it escapes. As you continue heating, the
water itself turns into vapour and finally
begins to boil. We learnt in Chapters 8
and 9, that the animals living in water
use the dissolved oxygen in water.
The organisms that live in soil also
need oxygen to respire, isn’t it? How do
they get the air they need, for
respiration?
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15.3 H OW DOES O XYGEN B ECOME
AVAILABLE TO ANIMALS AND PLANTS
LIVING IN WATER AND SOIL?
AIR AROUND US
Here is a question from Paheli, “Will
the tiny air bubbles seen before the
water actually boils, also appear if
we do this activity by reheating
boiled water kept in an air tight
bottle?” If you do not know
the answer you may try doing
it and see for yourself.
Activity 6
Take a lump of dry soil in a beaker or a
glass. Add water to it and note what
happens (Fig. 15.11). Do you see
bubbles coming out from soil? These
bubbles indicate the presence of air in
the soil.
When the water is poured on the
lump of soil, it displaces the air which
is seen in the form of bubbles. The
organisms that live inside the soil and
the plant roots respire in this air. A lot
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Fig. 15.11 Soil has air in it
of burrows and holes are formed in deep
soil by the animals living in the soil.
These burrows also make spaces
available for air to move in and out of
the soil. However, when it rains heavily,
water fills up all the spaces occupied by
the air in the soil. In this situation,
animals living in the soil have to come
out for respiration. Could this be the
reason why earthworms come out of the
soil, only during heavy rains?
Have you ever wondered why all the
oxygen of atmosphere does not get used
up though a large number of organisms
are consuming it? Who is refilling the
oxygen in the atmosphere?
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
shows the interdependence of plants
and animals.
We can now appreciate, how important
air is for life on earth. Are there any other
uses of air? Have you heard about a
windmill? Look at Fig. 15.12.
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15.4 HOW IS THE OXYGEN IN THE
ATMOSPHERE REPLACED?
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In Chapter 7, we read about
photosynthesis. 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. Similarly, plants can not
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Fig. 15.12 A windmill
The wind makes the windmill
rotate. The windmill is used to draw
water from tubewells and to run flour
mills. Windmills are also used to
generate electricity. Air helps in the
movements of sailing yachts, gliders,
parachutes and aeroplanes. Birds,
bats and insects can fly due to the
presence of air. Air also helps in the
dispersal of seeds and pollen of
flowers of several plants. Air plays an
important role in water cycle.
SCIENCE
Atmosphere
Carbon dioxide
Composition of air
Oxygen
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Nitrogen
Smoke
Windmill
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„
Air is found everywhere. We cannot see air, but we can feel it.
„
Air in motion is called wind.
„
Air occupies space.
„
Air is present in water and soil.
„
Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and a few
other gases. Some dust particles may also be present in it.
„
Oxygen supports burning and is necessary for living organisms.
„
The envelope of air that surrounds the earth is known as atmosphere.
„
Atmosphere is essential for life on earth.
„
Aquatic animals use dissolved air in water for respiration.
„
Plants and animals depend on each other for exchange of oxygen and carbon
dioxide from air.
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1.
What is the composition of air?
2.
Which gas in the atmosphere is essential for respiration?
3.
How will you prove that air supports burning?
4.
How will you show that air is dissolved in water?
5.
Why does a lump of cotton wool shrink in water?
AIR AROUND US
153
6.
The layer of air around the earth is known as ___________.
7.
The component of air used by green plants to make their food, is ___________.
8.
List five activities that are possible due to the presence of air.
9.
How do plants and animals help each other in the exchange of gases in the
atmosphere?
SUGGESTED PROJECTS AND ACTIVITIES
1.
On a clear glass window facing towards an open area, fix a small rectangular
strip of paper. Remove the strip after a few days. Do you notice a difference
between the rectangular section that was left covered with paper and the
rest of the glass window? By repeating this exercise every month, you can
have an idea about the amount of dust present in air around you at different
times of the year.
2.
Observe the leaves of trees, shrubs or bushes planted by the roadside. Note
whether their leaves have some dust or soot deposited over them. Take similar
observations with the leaves of trees in the school compound or in a garden.
Is there any difference in deposition of soot on leaves of trees near the roadside?
What could be the possible reasons for this difference? Take a map of your
city or town and try to identify regions in the map where you have noticed
very thick layer of soot on the plants by the roadside. Compare with results
obtained by other classmates and mark these areas on the map. Perhaps
the results from all the students could be summarised and reported in
newspapers.
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SCIENCE
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