136 SCIENCE

14
Water
S
uppose for some reason your
family gets only one bucket of
water everyday for a week.
Imagine what would happen? Would you
be able to cook, clean utensils, wash
clothes or bathe? What are the other
activities you would not be able to do?
What would happen if we do not have
easy access to water for a long period
of time?
Apart from drinking, there are
so many activities for which we use
water (Fig. 14.1). Do you have an idea
about the quantity of water we use in a
single day?
each activity by you and other family
members. You may use a mug, a glass,
a bucket or any other container to
measure the amount of water used.
Fig. 14.1 Uses of water
14.1 HOW MUCH WATER DO WE USE?
Activity 1
List all the activities for which you use
water in a day. Some activities are listed
in Table 14.1. Make a similar table in
your notebook. Throughout the day,
measure the amount of water used for
Table 14.1 Estimation of the
amount of water used by a
family in a day
y t i v i t c A
f o t n u o m A
d e s u r e t a w
g n i k n i r D
g n i h s u r B
g n i h t a B
s l i s n e t u g n i h s a W
s e h t o l c g n i h s a W
s t e l i o T
r o o l f g n i n a e l C
r e h t o y n A
n i d e s u r e t a w l a t o T
y l i m a f a y b y a d a
You now have a rough idea as to how
much water your family uses in a day.
Using this information, calculate the
amount of water needed by your family
in a year. Now, divide this amount by
the number of members of your family.
This will give an idea of the amount of
water needed by one member of your
family in a year. Find the number of
people that live in your village or town.
137 WATER
You have listed a number of activities
for which you use water. Do you think,
our water requirement is limited to
activities like these? We use wheat, rice,
pulses, vegetables and many other food
items everyday. We know that some of
the fibres that we use for making fabric
come from plants. Is water not needed
to grow these? Can you think of some
more uses of water? Water is used in
industries for producing almost all the
things that we use. So, we need water
not only for our daily activities but also
for producing many things.
water from a river, spring, pond, well or
a hand pump. Some others might say,
We get water from taps. Have you ever
wondered where water in the taps comes
from? Water that we get from taps is also
drawn from a lake or a river or a well
(Fig. 14.2). It is then supplied through
a network of pipes.
Fig. 14.2 Water in taps comes from rivers, lakes
or wells
Boojho wonders whether people
living in different regions of our
country get the same amount of
water. Are there regions where
people do not get adequate amount
of water? How do they manage?
You may now get an idea of the amount
of water needed by your village or town
in a year.
Paheli wants to tell you that
about two glasses of water
are required to produce
each page of a book.
Each of us may be getting water into
our homes in different ways. But, finally,
all of us get water from the same sources
such as ponds, lakes, rivers and wells.
We have discussed some of the
sources of water. Where does the water
come from, to fill these ponds, lakes,
rivers and wells?
Boojho wants you to imagine a
day in your life when water supply
through taps is not available. So,
you have to fetch it yourself from
a far away place. Would you
use the same amount of water
as on any other day?
14.2 WHERE DO WE GET WATER
FROM?
Where do you get the water that you
use? Some of you may say, We draw
138 SCIENCE
Do you know that about two thirds
of the Earth is covered with water? Most
of this water is in oceans and seas
(Fig. 14.3).
The water in the oceans and seas has
many salts dissolved in it the water
is saline. So, it is not fit for drinking
and other domestic, agricultural and
industrial needs. You might have heard
the famous lines of the poem Rime of
the Ancient Mariner written by S.T.
Coleridge in 1798:
Water water every where
Nor any drop to drink
Here the poet has described the
plight of sailors on a ship lost in the
ocean.
Yet, oceans play an important role
in supplying the water that we use. Do
you find this surprising? After all, the
water that we use is not salty. Many of
us live in places far away from the
oceans. Does the water supply in these
places also depend on the oceans? How
does the ocean water reach ponds, lakes,
rivers and wells, which supply us water?
How come the water from these sources
is not saline anymore?
Do you remember Activity 6 in
Chapter 5 in which water with salt
dissolved in it was heated? What did we
find? The water evaporated and the salt
was left behind. This activity gives us
an idea that, on heating, water changes
into its vapour. We also realise from this
activity, that water vapour does not carry
away the salt with it. Water vapours so
formed become a part of the air and
cannot usually be seen. We also found
that heating is essential to convert water
into its vapour. However, we have seen
that water changes into its vapour also
Fig. 14.3 Oceans cover a major part of the earth
Fig. 14.4 Clothes drying on a clothes-line
That is where the water cycle
comes in!
14.3 WATER CYCLE
Disappearing Trick of Water
How many times have you noticed that
water spilled on a floor dries up after
some time? The water seems to disappear.
Similarly, water disappears from wet
clothes as they dry up (Fig. 14.4). Water
from wet roads, rooftops and a few other
places also disappears after the rains.
Where does this water go?
139 WATER
from the fields, roads, rooftops and
other land areas. We also discussed in
Chapter 5 that to obtain salt, water from
the sea is left in shallow pits to let the
water evaporate. From where does this
water get the heat it needs to evaporate?
Let us find out.
Activity 2
Take two similar plates. Place one of the
plates in sunlight and keep the other
under shade. Now, pour equal amount
of water in each of the plates (Fig. 14.5).
You can use a cap of a bottle to measure
water. Make sure that water does not
spill over. Observe the two plates after
every 15 minutes. Does the water seem
to disappear? From which plate does it
disappear first? What is the source of
heat for this evaporation?
During the daytime, sunlight falls on
the water in oceans, rivers, lakes and
ponds. The fields and other land areas
also receive sunlight. As a result, water
from all these places continuously
changes into vapour. However, the salts
dissolved in the water are left behind.
In Activity 2, did you find that water
also disappeared from the plate kept in
the shade, though it could have taken
more time? Does the heat from the
sunlight reach here? Yes, during the
daytime all the air surrounding us gets
heated. This warm air provides heat for
evaporation of water in the shade. Thus,
evaporation takes place from all open
surfaces of water. As a result, water
vapour gets continuously added to air.
However, evaporation of water is a slow
process. That is why we rarely notice its
loss from a bucket full of water. In
sunlight, evaporation takes place faster.
On heating water on a burner, its
evaporation takes place even faster. Is
there any other process through which
water vapour gets transferred into air?
Loss of Water by Plants
You have learnt in Chapter 7 that plants
need water to grow. Plants use a part of
this water to prepare their food and
Fig.14.5 Evaporation of water in sunlight and in
shade
Boojho has been reading about
transpiration. He asked himself -
how much water is lost through
transpiration by wheat plants
that give us one kilogram of
wheat? He found out that this is
nearly 500 litres, that is, roughly
25 large sized buckets full of
water. Can you now imagine the
amount of water lost by plants
of all the forests, crops and
grasslands together?
140 SCIENCE
retain some of it in their different parts.
Remaining part of this water is released
by the plants into air, as water vapour
through the process of transpiration. Do
you remember observing transpiration
of water by plants in Activity 4 in
Chapter 7?
Water vapour enters the air through
the processes of evaporation and
transpiration. Is it lost for ever? No, we
get it back again, as we will see.
How are clouds formed?
Activity 3
Take a glass half filled with water. Wipe
the glass from the outside with a clean
piece of cloth. Add some ice into the
water. Wait for one or two minutes.
Observe the changes that take place on
the outer surface of the glass (Fig.14.6).
From where do water drops appear
on the outer side of the glass? The cold
surface of the glass containing iced water,
cools the air around it, and the water
vapour of the air condenses on the
surface of the glass. We noticed this
process of condensation in Activity 7 in
Chapter 5.
Fig. 14.6 Drops of water appear on outer
surface of the glass containing water with ice
The process of condensation plays an
important role in bringing water back
to the surface of earth. How does it
happen? As we go higher from the
surface of the earth, it gets cooler. When
the air moves up, it gets cooler and
cooler. At sufficient heights, the air
becomes so cool that the water vapour
present in it condenses to form tiny
drops of water called droplets. It is these
tiny droplets that remain floating in air
and appear to us as clouds (Fig. 14.7).
It so happens that many droplets of
water come together to form larger sized
Paheli has noticed dew on leaves of
grass on winter mornings. Did you
notice something similar on leaves
or metal surfaces like iron grills and
gates on a cold morning? Is this
also due to condensation? Do you
see this happening on hot
summer mornings?
Fig. 14.7 Clouds
141 WATER
drops of water. Some drops of water
become so heavy that they begin to fall.
These falling water-drops are, what we
call rain. In special conditions, it may
also fall as hail or snow.
Thus, water in the form of vapour
goes into air by evaporation and
transpiration, forms clouds, and then
comes back to the ground as rain, hail
or snow.
14.4 BACK TO THE OCEANS
What happens to the water that rain and
snow bring to different regions of earth?
Almost all land surfaces are above the
level of oceans. Most of the water that
falls on the land as rain and snow
sooner or later goes back to the oceans.
This happens in many ways.
Snow in the mountains melts into
water. This water flows down the
mountains in the form of streams and
rivers (Fig. 14.8). Some of the water that
falls on land as rain, also flows in the
form of rivers and streams. Most of the
rivers cover long distances on land and
ultimately fall into a sea or an ocean.
However, water of some rivers flows
into lakes.
The rainwater also fills up the lakes
and ponds. A part of the rainwater gets
absorbed by the ground and seems to
disappear in the soil. Some of this water
is brought back to the air by the process
of evaporation and transpiration. The
rest seeps into the ground. Most of this
water becomes available to us as ground
water. Open wells are fed by ground
water. Ground water is the source for
many lakes as well. It is also this ground
water which is drawn by a handpump
or a tubewell. The more handpumps or
tubewells that are used in an area, the
deeper we need to dig to find this ground
water. The loss in the level of ground
water due to over use, is worrisome.
Paheli wants to share a concern with
you. In those areas where the land has
little or no vegetation, the rainwater flows
away quickly. Flowing rainwater also
takes the top layer of the soil away with
it. There are few areas where most of the
land is covered with concrete. This
reduces the seepage of rainwater into the
ground which ultimately affects the
availability of ground water.
Boojho has noticed fog near the
ground in winter mornings. He
wonders if this is also condensation
of water vapour near the
ground. What do you
think?
Fig. 14.8 Rain water flows down in the form of
streams and rivers
142 SCIENCE
Fig. 14.9 Water cycle
We now know that water brought
back to the surface of the earth by rain,
hail or snow, goes back to oceans. Thus,
water from the ocean and surface of the
earth goes into air as vapour; returns
as rain, hail or snow and finally goes
back to the oceans. The circulation of
water in this manner is known as the
water cycle (Fig.14.9). This circulation
of water between ocean and land is a
continuous process. This maintains the
supply of water on land.
14.5 WHAT IF IT RAINS HEAVILY?
The time, duration and the amount of
rainfall varies from place to place. In
some parts of the world it rains
throughout the year while there are
places where it rains only for a few days.
In our country, most of the rainfall
occurs during the monsoon season.
Rains bring relief especially after hot
summer days. The sowing of many crops
depends on the arrival of monsoon.
However, excess of rainfall may lead
to many problems (Fig. 14.10). Heavy
Fig. 14.10 A scene after heavy rains
143 WATER
rains may lead to rise in the level of water
in rivers, lakes and ponds. The water
may then spread over large areas causing
floods. The crop fields, forests, villages,
and cities may get submerged by water
(Fig. 14.11). In our country, floods cause
extensive damage to crops, domestic
animals, property and human life.
During floods, the animals living in
the water also get carried away with the
waters. They often get trapped on land
areas and die when floodwater recedes.
Rains also affect the animals living in
the soil.
14.6 WHAT HAPPENS IF IT DOES NOT
RAIN FOR A LONG PERIOD?
Can you imagine what would happen if
it does not rain in a region for a year or
more? The soil continues to lose water
by evaporation and transpiration. Since
it is not being brought back by rain,
the soil becomes dry. The level of water
in ponds and wells of the region goes
down and some of them may even dry
up. The ground water may also become
scarce. This may lead to drought.
In drought conditions, it is difficult
to get food and fodder. You might have
heard about droughts occurring in some
parts of our country or the world. Are
you aware of the difficulties faced by
the people living in these areas? What
happens to the animals and the
vegetation in these conditions? Try and
find out about this by talking to your
parents and neighbours and by
reading about it from newspapers
and magazines.
14.7 HOW CAN WE CONSERVE WATER?
Only a small fraction of water available
on the Earth is fit for use of plants,
animals and humans. Most of the water
is in the oceans and it cannot be used
directly. When the level of the ground
water decreases drastically, this can not
be used any more. The total amount of
water on Earth remains the same, but,
the water available for use is very limited
and is decreasing with over usage.
The demand for water is increasing
day-by-day. The number of people using
water is increasing with rising
population. In many cities, long queues
for collection of water are a common site
(Fig. 14.12). Also, more and more water
is being used for producing food and by
the industries. These factors are leading
Fig. 14.12 A queue for collecting water
Fig. 14.11 A scene of a flooded area
144 SCIENCE
Fig. 14.13 Rooftop rainwater harvesting
to shortage of water in many parts of
the world. Hence, it is very important
that water is used carefully. We should
take care not to waste water.
14.8 RAINWATER HARVESTING
One way of increasing the availability
of water is to collect rainwater and store
it for later use. Collecting rainwater in
this way is called rainwater harvesting.
The basic idea behind rainwater
harvesting is Catch water where it falls.
What happens to the rainwater that
falls in places that are mostly covered
with concrete roads and buildings? It
flows into the drains, isn't it? From there
water goes to rivers or lakes, which could
be far away. A lot of effort will then be
required to get this water back into our
homes as the water did not seep into
the ground.
Discussed here are two techniques
of rainwater harvesting:
1. Rooftop rainwater harvesting: In this
system the rainwater is collected from
the rooftop to a storage tank, through
pipes. This water may contain soil
from the roof and need filtering
before it is used. Instead of collecting
rainwater in the tank, the pipes
can go directly into a pit in the
ground. This then seeps into the
soil to recharge or refill the ground
water (Fig. 14.13).
2. Another option is to allow water
to go into the ground directly
from the roadside drains that
collect rainwater.
s d u o l C
n o i t a s n e d n o C
t h g u o r D
n o i t a r o p a v E
d o o l F
r e t a w d n u o r G
l i a H
n a e c O
g n i t s e v r a h r e t a w n i a R
w o n S
r u o p a v r e t a W
e l c y c r e t a W
145 WATER
1. Fill up the blanks in the following:
(a) The process of changing of water into its vapour is called _________________.
(b) The process of changing water vapour into water is called _________________.
(c) No rainfall for a year or more may lead to _________________ in that region.
(d) Excessive rains may cause _________________.
2. State for each of the following whether it is due to evaporation or condensation:
(a) Water drops appear on the outer surface of a glass containing cold water.
(b) Steam rising from wet clothes while they are ironed.
(c) Fog appearing on a cold winter morning.
(d) Blackboard dries up after wiping it.
(e) Steam rising from a hot girdle when water is sprinkled on it.
3. Which of the following statements are true ?
(a) Water vapour is present in air only during the monsoon. ( )
(b) Water evaporates into air from oceans, rivers and lakes but not from the
soil.( )
(c) The process of water changing into its vapour, is called evaporation.( )
(d) The evaporation of water takes place only in sunlight.( )
(e) Water vapour condenses to form tiny droplets of water in the upper layers of
air where it is cooler.( )
Water is essential for life.
Water vapour gets added to air by evaporation and transpiration.
The water vapour in the air condenses to form tiny droplets of water, which
appear as clouds. Many tiny water droplets come together and fall down as
rain, snow or hail.
Rain, hail and snow replenish water in rivers, lakes, ponds, wells and soil.
The circulation of water between ocean and land is known as the
water cycle.
Excessive rains may cause floods while lack of it for long periods may
cause droughts.
The amount of usable water on earth is limited so it needs to be used
carefully.
146 SCIENCE
SUGGESTED PROJECTS AND ACTIVITIES
1. List three activities in which you can save water. For each activity describe
how you would do it.
2. Collect pictures relating to floods or droughts from old magazines or
newspapers. Paste them in your notebook and write about the problems that
people would have faced.
3. Prepare a poster on ways of saving water and display it on your school notice
board.
4. Write a few slogans of your own on the topic Save Water.
4. Suppose you want to dry your school uniform quickly. Would spreading it near
an anghiti or heater help? If yes, how?
5. Take out a cooled bottle of water from refrigerator and keep it on a table. After
some time you notice a puddle of water around it. Why?
6. To clean their spectacles, people often breathe out on glasses to make them
wet. Explain why the glasses become wet.
7. How are clouds formed?
8. When does a drought occur?