Page 172 - ICSE Chemistry 8

Potable Water

We already know that the water which is fi t for consump on is known as potable water. Potable water is
clear, transparent, colourless, odourless and free from all impuri es. It contains minerals that are essen al
for our body.

Water obtained from natural sources contains impuri es such as dirt, germs, dissolved and suspended
impuri es like clay, sand, dead and decaying organic ma er, sewage, and other pollutants. These impuri es
are known as water pollutants.

Human wastes, animal dung and industrial wastes are the major factors responsible for contamina on of
water. In order to make water fi t for consump on, it should be treated to remove all the impuri es. Once
the water is treated in water treatment plants, it becomes fi t for consump on.

Purification of Water

The process of removal of undesirable chemicals, impuri es and biological contaminants from water is
called purifi ca on of water.

The main processes involved in purifi ca on of water obtained from lakes or rivers are sedimenta on,
fi ltra on and chlorina on.

• Sedimenta on: Water collected from sources like rivers or lakes consists of dirt. It is fi rst taken
to sedimenta on tanks and allowed to stand for a few hours. Here, heavy impuri es se le at
the bo om as sediments. Use of potash alum (potassium aluminium sulphate) and slaked lime
(calcium hydroxide) speeds up the process of sedimenta on.

Following sedimenta on, water is run-off at the top of the sedimenta on tank without disturbing

the sediments at the bo om. Now the water is transferred to fi lter beds.
• Filtra on: Water containing suspended impuri es is
passed through a fi lter. The impuri es are retained on
the fi lter, while clear water is collected as fi ltrate in

another container.
When water has to be treated on a large scale, it is

fi ltered through layers of sand, gravel, ac vated
charcoal, etc. This process removes suspended
impuri es.

• Chlorina on: The fi ltered water may s ll contain some Fig. 8.5: Purifi cation of water
harmful bacteria which can cause diseases. So, fi ltered
Sterilisa on
water is sterilised to remove such harmful bacteria.
The process of removal of
Sterilisa on is done by adding chlorine. The addi on
germs is called sterilisa on.
of chlorine kills bacteria and other harmful germs
present in water.

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