Page 91 - Chemistry - XI

2. Storage: The water from rivers may be stored in large reservoirs during which natural biological
purifi cation takes place. Solid impurities will settle down. Some bacteria cannot survive in water and
decrease in a storage reservoir.

3. Coagulation and Sedimentation: Suspended impurities can be coagulated by using potash alum
(potassium aluminium sulphate). It is the most commonly used coagulant. A layer of sludge is formed
at the bottom which is removed from the bottom of a tank from time to time.
4. Filtration: The remaining suspended impurities are removed through fi ltration by using rapid sand
fi lter in which water moves vertically through sand which often has a layer of activated charcoal above
the sand. The activated charcoal removes compounds which impart taste, colour and odour to water.
The space between sand particles is larger than the size of suspended particles. Most of the particles

pass through surface layer, but trapped in pore spaces or adhere to sand particles. The fi lter is cleaned
by back fl ushing, i.e. by passing water through fi lter in the opposite direction to the normal direction.
5. Disinfection: Bleaching powder and sodium hypochlorite are used as disinfectant. They destroy
bacteria, but not protozoans and viruses completely. These chemicals react with organic compounds
and lead to the formation of harmful compounds. Chlorine liberated from bleaching powder changes
taste and odour of water. Ozone is a very strong and powerful disinfectant and is used in developed

countries. It is most eff ective for almost all type of microorganism. It does not change taste or odour
of water. UV radiations can also be used as a disinfectant. It is very eff ective and can destroy viruses,
also. It is used in aquaguard these days. R.O. purifi er makes use of reverse osmosis and semipermeable
membrane that needs to be replaced from time to time.
6. Boiling: Water is boiled to inactivate or kill microorganisms which cannot survive at high temperatures.
Boiling is done for 2–3 minutes. If water is hard then calcium carbonate and magnesium carbonate will
be formed which gets deposited on the heating element.

Result
The various methods of purifi cation of water are used depending upon the source, impurity present and
purpose for which it is to be used.

COMPETENCY BASED QUESTIONS

Multiple Choice Questions

Q1. Which of the following is safe for drinking?
(a) Mineral water (b) Distilled water (c) Heavy water (d) All of these

Q2. Which of the following is/are disinfectant(s)?
(a) U.V. light (b) O (c) Cl (d) All of these
2
3
Q3. Water needs to be R.O. purifi ed when TDS is
(a) <100 (b) 500 or more (c) < 50 (d) < 20

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