CBSE Class 8 Science • Chapter 19 • Detailed Master Notes
Chapter Overview
Clean air is essential for life on Earth. In this chapter, we explore how human activities introduce harmful substances into the atmosphere, causing air pollution, and what we can do to reduce its effects.
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 quantities.
Air Pollution: When air is contaminated by unwanted substances which have a harmful effect on both the living and the non-living, it is referred to as air pollution.
Air Pollutants: The substances which contaminate the air are called air pollutants.
Vehicles produce high levels of pollutants like carbon monoxide, carbon dioxide, nitrogen oxides and smoke. Carbon monoxide is produced from incomplete burning of fuels such as petrol and diesel. It is a poisonous gas that reduces the oxygen-carrying capacity of the blood.
Smog: This is a thick fog-like layer in the atmosphere, especially during winters, made up of smoke and fog. 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.
Chlorofluorocarbons (CFCs): These are chemicals used in refrigerators, air conditioners, and aerosol sprays. CFCs damage the ozone layer of the atmosphere, which protects us from harmful ultraviolet rays of the sun.
The Taj Mahal in Agra is a famous monument made of white marble. Over the years, its marble has become discoloured due to air pollution from industries located in and around Agra.
Acid Rain: Industries produce pollutants like sulphur dioxide and nitrogen dioxide. These gases react with the water vapour present in the atmosphere to form sulphuric acid and nitric acid. These acids drop down with rain, making the rain acidic. This is called acid rain.
Acid rain corrodes the marble of the monument. The phenomenon is also called "Marble cancer". Moreover, suspended particulate matter emitted by factories has contributed towards the yellowing of the marble.
AI Image Prompt: A diagram showing the formation of acid rain. Factories on the ground emit plumes of sulphur dioxide and nitrogen dioxide gases into the sky. The gases mix with clouds to form sulphuric acid and nitric acid, which fall as rain onto trees and a marble monument below. Clear educational style.
We know that 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.
The Greenhouse Effect: 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 earth because it would be too cold.
However, carbon dioxide ($\text{CO}_2$) is one of the gases responsible for this effect. When $\text{CO}_2$ increases excessively in the atmosphere due to human activities (burning fossil fuels, deforestation), it traps more heat than normal.
Global Warming: The trapping of excess heat leads to an average increase in the temperature of the earth's atmosphere. This is called global warming. It causes glaciers to melt and sea levels to rise.
Q1. Apart from $\text{CO}_2$, what are other major greenhouse gases?
Ans: Other significant greenhouse gases include methane, nitrous oxide, and water vapour. Methane, for instance, is released from agriculture and landfills and is highly effective at trapping heat in the atmosphere.
There are many steps we can take to reduce air pollution and help save the environment: