SUBJECT – SCIENCE
CHAPTER – 18
POLLUTION
NCERT SOLUTION
PINKI SINGH
(1) What are the different ways in which water gets contaminated?
Answer: Water gets contaminated in the following ways:
(a) Various industrial units dispose – off wastage including harmful chemicals
in rivers. These chemicals are poisonous and contaminate water.
(b) Water gets contaminated when sewage is disposed – off (without proper
treatment) in rivers.
(c) Ground water gets contaminated by harmful insecticides and pesticides
sprayed by farmers and it get mixed with drinking water through rain water.
(2) At an individual level, how can you help reduce air pollution?
Answer: Air pollution should be checked at individual level by the following
ways:
• At an individual level, we can plant trees as trees reduce air pollution.
• We can use public transportation and encourage carpooling so that air
pollution can be reduced.
• Instead of going by cars and scooters for short distances, we can go by foot
or by bicycles.
• By saying no to crackers we can minimize air pollution as well as noise
pollution.
• We should not burn garbage and dry leaves. The burning of dry leaves and
garbage release greenhouse gases which are responsible for global
warming.
(3) Clear, transparent water is always fit for drinking. Comment.
Answer: Transparent water is not always fit for drinking. It may contain harmful
microorganisms, soluble salts and other pollutants. Sometime water looks like
pure but it contains dissolved salts or bacteria which are not good for us. So we
should purify water before drinking.
(4) You are a member of the municipal body of your town. Make a list of
measures that would help your town to ensure the supply of clean water to all
its residents.
Answer: We can take following measures:
(i) Water should be treated properly to make it free from harmful germs,
chemical and physical impurities.
(ii) Chemical methods such as chlorination must be used for purifying water.
(iii) The area around water pipes must also be clean.
(iii) Proper maintenance of pipes supplying tap water to avoid any corrosion,
breakage and leakage.
(iv)Organize awareness programmes to keep water resources clean and make a
habit of conserving water.
(5) Explain the differences between pure air and polluted air.
Answer:
Pure Air Polluted Air
Pure air is free from germs and Polluted air contains harmful
harmful gases. It contains 78% gases like carbon monoxide, sulphur
nitrogen, 21% oxygen and small dioxide etc. and unwanted
amount of other gases and dust substances.
particles.
Smog is not present. Smog can be Present.
Not harmful for organisms. Harmful for organisms as it causes
diseases like Asthma.
( 6) Explain circumstances leading to acid rain. How does acid rain affect us?
Answer: When harmful gases like sulphur dioxide and nitrogen dioxide, react
with the water vapour present in the atmosphere to form sulphuric acid and
nitric acid. The acids drop down with rain, making the rain acidic. This is called
acid rain.
Following are the affects of acid rain:
• It increases corrosion of metals.
• It damages buildings and sculptural materials.
• It removes the basic nutrients such as calcium from the soil.
• It causes direct damage to plants and crops.
• It causes skin eruptions in human being.
(7) Which of the following is not a greenhouse gas?
(a)Carbon dioxide
(b) Sulphur dioxide
(c) Methane
(d) Nitrogen
Answer: Nitrogen
(8)Describe the ‘Green House Effect’ in your own words.
Answer:
➢ Green House effect is a process by which solar radiation is absorbed by
greenhouse gases and the temperature of Earth atmosphere is increased.
➢ This increase in the temperature of the surroundings is responsible for
global warming.
➢ Gases like CO2, Methane, nitrous oxide, water vapour, etc. trap heat
radiation and are called greenhouse gases.
➢ Green houses are often used for growing flowers, vegetables and plants
(Mainly in cold region).
➢ Now because of increase in concentration of greenhouse gases in our
environment is adversely affecting the climate.
(9) Prepare a brief speech on global warming. You have to deliver the speech
in your class.
Answer: Global warming is an increase in the average temperature of the Earth’s
surface. It occurs as a result of an increased concentration of greenhouse gases
in the atmosphere. The greenhouse gases include carbon dioxide, methane, and
water vapour. These gases trap solar radiations released back by the Earth. This
helps in keeping our planet warm and thus, helps in human survival. However,
an increase in the amount of greenhouse gases can lead to an increase in the
Earth’s temperature leading to global warming.
(10) Describe the treat to the beauty of the Taj Mahal.
Answer: (i) Acid rain is a major threat to the beauty of the Taj Mahal.
(ii) When acid rains fall on the monument (that is completely made of marble),
they react with marble to form a powder-like substance that is then washed
away by the rain.
(iii) Acid rain corrodes the marbles of this monument. This phenomenon is
known as marble cancer.
(iv)Also, the soot particles emitted from the Mathura oil refinery located near
Agra is leading to the yellowing of the marble.
(11) Why does the increased level of nutrients in the water affect the survival
of aquatic organisms?
Answer: (i) An increase in the level of nutrients in a water body leads to an
excessive increase in the algae population in the water body.
(ii) When these algae die, they serve as food for decomposers, like bacteria.
(iii)A lot of oxygen is utilized in this process.
(iv)This decreases the level of oxygen dissolved in the water body, which may
kill aquatic organisms.
