NCERT Solutions for Class 8 Science Chapter 18 - Pollution of Air and Water

Question 1:

What are the different ways in which water gets contaminated?


Water gets contaminated by various ways, such as by sewage, by silt, by washing, bathing or other household activities. Additionally, toxic chemicals from industries, garbage and dead bodies of animals and human beings, run-off fertilisers, insecticides, pesticides, etc., also contaminate water.

Question 2:

At an individual level, how can you help reduce air pollution?


At an individual level, the following steps can be taken to reduce air pollution:
(i) Reduce the use of automobiles; use unleaded petrol or diesel in the automobiles
(ii) Plant trees or vegetation around residential areas
(iii) Use public transport or car pools
(iv) Enforce laws to make sure that industrial and vehicular emissions are treated before being released into the air

Question 3:

Clear, transparent water is always fit for drinking. Comment.


This statement is incorrect. Clear, transparent water may appear clean but it may not necessarily be safe or fit for drinking. There may be dissolved impurities and disease-causing microorganisms present in the clear water.

Question 4:

You are a member of the municipal body of your town. Make a list of the measures that would help your town to ensure the supply of clean water to all its residents.


To obtain clean water:
(i) Industrial waste must be properly treated before being released into water resources.
(ii) Sewage must be treated by physical and chemical methods before it is dumped into water bodies.
(iii) Use of fertilisers should be minimised to reduce its run-off into water bodies.
(iv) Dumping of wastes into water bodies, washing clothes or giving bath to animals on the banks of rivers and disposal of household sewage into water bodies without treating it first should be stopped.

Question 5:

Explain the difference between pure air and polluted air.


Question 6:

Explain the circumstances leading to acid rain. How does acid rain affect us?


Some pollutants like sulphur dioxide, nitrogen dioxide, etc. react with the water vapour present in the atmosphere to form sulphuric and nitric acids respectively. The acid mixes in the rain water and thus makes it acidic. Acid rain destroys crops, wild plants, steel rail tracks, monuments, and electrical equipment. It causes irritation in the eyes, nose and throat and adversely affects water bodies.

Question 7:

Which of the following is not a greenhouse gas?

  1. Carbon dioxide
  2. Sulphur dioxide
  3. Methane
  4. Nitrogen


Question 8:

Describe ‘Greenhouse Effect’ in your own words.


The heating of the Earth’s atmosphere due to trapped infrared radiations of the Sun is called the greenhouse effect. There are four gases which can trap infrared radiations: carbon dioxide (CO2), ozone (O3) and methane (CH4).

Question 9:

Prepare a brief speech on global warming. You have to deliver the speech in your class.


When the Sun’s rays reach the Earth’s surface, some of them are reflected back and the rest are absorbed by greenhouse gases, which include water vapour, carbon dioxide, methane and ozone. The absorbed energy maintains the temperature of the Earth. Increased concentration of greenhouse gases in the atmosphere is leading to global warming, which is the increase in the average temperature of Earth. The increase in temperature is leading to melting of polar ice caps and increase in the water level in the sea. This poses a danger for the people living in coastal areas.

Question 10:

Describe the threat to the beauty of the Taj Mahal.


Taj Mahal is made up of white marble. The whiteness of the marble used in the monument is at threat owing to air pollution. Industrial establishments around the Taj Mahal emit many harmful gases such as sulphur dioxide, nitrogen oxides, etc. These gases react with the water vapour present in the atmosphere to form sulphuric and nitric acids. These acids come down as acid rain and corrode the marble of the monument.

Question 11:

Why does the increased level of nutrients in water affect the survival of aquatic organisms?


Excessive quantities of chemicals like nitrates and phosphates present in the fertilisers act as nutrients for algae to flourish. When these algae die, decomposers such as bacteria use the oxygen present in the water to decompose the algae. Thus, there is a reduction in the oxygen level of the water, which adversely affects the survival of aquatic organisms.