NCERT Class VIII Science Solutions: Chapter 18-Pollution of Air and Water Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 25:

What do stand for? Name some devices where are used. Why are considered as pollutants?



stands for chlorofluorocarbons. These are the chemical compounds made of chlorine, fluorine and carbon elements. are used in refrigerators, air conditioners and aerosol sprays.

are considered as pollutants because these are depleting the useful ozone layer of the upper atmosphere. This happens as follows:

released into the air, go up and ultimately reach high into the atmosphere where the protective ozone layer exists. The chlorofluorocarbons react with ozone gas of ozone layer and destroy it gradually.

This allows the harmful ultraviolet rays of the sun to reach on the earth and can cause skin cancer, cataract and destruction of plants, including crops.

Question 26:

Why is it advised that industries should switch over to cleaner fuels such as CNG and LPG in the Taj Mahal Zone in Agra?


Pollutants in air are discoloring white marble of Taj Mahal. The pollutants like sulphur dioxide and nitrogen dioxide react with the water vapour present in the atmosphere to form sulphuric acid and nitric acid respectively

Acid rain react with the carbonates present in marble (calcium carbonate) and limestone of a monument and corrode it slowly (dissolve it slowly) .

Question 27:

It is said, “ contributes to global warming.” Explain.


Plants utilise from the atmosphere for photosynthesis, thereby decreasing the amount of in the air Deforestation leads to an increase in the amount of in the air because the number of trees which consume , is reduced.

Human activities, thus contribute to the accumulation of 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 global warming. Thus, increased contributes to global warming.

Question 28:

We should plant trees and nurture the ones already present in the neighborhood. Why?


We should plant more trees and also nurture the existing trees in our neighborhood because trees reduce air pollution by absorbing excess carbon dioxide from the air Thus, planting more trees will mitigate global warming.

All of us should participate in programmes such as Van Mahotsav in which lakhs of trees are planted in the month of July every year

Question 29:

Explain the traditional way of purifying water to make it fit for drinking.


The various traditional ways of purifying water to make it fit for drinking are as follows:

  1. By filtering this is a physical method of removing impurities. A popular household water filter is a candle type water filter
  2. By boiling when water is heated, it boils at a temperature of . At this high temperature, all the harmful microorganisms (or germs) present in water are killed and it becomes absolutely safe for drinking. Many household uses boiling as a method for obtaining safe drinking water
  3. Chlorination it is a commonly used chemical method for purifying water it is done by adding chlorine tablets or bleaching powder to the water Chlorine kills all the harmful microorganism (or germs) present in water and makes it absolutely safe for drinking.
Way to Purifying Water