of Air and Water
Q21. What is water pollution? Name any four major water
Ans. Water pollution is the contamination of water by
substances harmful to life. Sewage, agricultural chemicals and industrial waste
are some of the major contaminants of water.
Q22. What is global warming?
Ans. Accumulation of CO2 in the atmosphere
traps heat and does not allow it to escape into space. As a result, the average
temperature of the earth’s atmosphere is gradually increasing. This is called
Q23. How do pesticides cause water pollution?
Ans. Pesticides are sprayed on crop plants. These
chemicals dissolve in water and are washed into water bodies from the fields.
They also seep into the ground to pollute ground water.
Q24. Write one source and one harmful effect of oxides of
Ans. Smoke may contain oxides of nitrogen which
combine with other air pollutants and fog to form smog. The smog causes
breathing difficulties such as asthma, cough and wheezing in children.
Q25. Why hot water released by power plants and industries is
considered a pollutant?
Ans. Hot water released by power plants and industries
is considered a pollutant because it raises the temperature of the water body
and adversely affect the animals and plants living in it.
Q26. Clear, transparent water is always fit for drinking.
No, clear and transparent water is not always fit for drinking. Water which looks
clean may still have disease carrying microorganisms and dissolved impurities.
So, it is essential to purify water before drinking, for example, by boiling.
Q27. How do industries cause water pollution?
Many industries discharge harmful chemicals into rivers and streams, causing
the pollution of water. Examples are oil refineries, paper factories, textile
and sugar mills and chemical factories. These industries cause
chemical contamination of water. The chemicals released include arsenic, lead
and fluorides which lead to toxicity in plants and animals.
Q28. Explain the differences between pure air and polluted air.
Ans. Pure air consists of a mixture of gases. By
volume, about 78% of this mixture is nitrogen and about 21% is oxygen. Carbon
dioxide, argon, methane, ozone and water vapour are also present in very small
air is contaminated by unwanted substances which have a harmful effect on both
the living and the non-living, it is referred to as polluted air.
Q29. What is smog? How is it formed? What are its harmful
Ans. A thick fog-like layer in the atmosphere, especially
during winters made up of smoke and fog is called smog. Smoke may contain
oxides of nitrogen which combine with other air pollutants and fog to form
smog. The smog causes breathing difficulties such as asthma, cough and wheezing
Q30. Write any three ways of controlling water pollution.
Ways of controlling water pollution
Sewage water should be treated to make it
harmless before discharging it into water bodies such as lakes, rivers, and
All the industries should treat the toxic
wastes before discharging them into water bodies.
Garbage should not be thrown into open
drains, rivers, lakes or ponds.
Q31. Why does the increased level of nutrients in the water
affect the survival of aquatic organisms?
Ans. Excessive quantities of chemicals (fertilizers, pesticides
and weedicides) which get washed from the fields act as nutrients for algae to
flourish. Once these algae die, they serve as food for decomposers like
bacteria. A lot of oxygen in the water body gets used up. This results in a
decrease in the oxygen level which may kill aquatic organisms.
Q32. Describe the ‘Green House Effect’ in your own words.
Ans. The greenhouse effect is a warming of Earth's
surface and the air above it. The sun’s rays warm the earth’s surface. A part
of the radiation that falls on the earth is absorbed by it and a part is
reflected back into space. A part of the reflected radiation is trapped by the
atmosphere. The trapped radiations further warm the earth. This process is
called the greenhouse effect. Without this process, life would not have been
possible on the earth.