Class 8 Science Pollution of Air and Water Exam Notes

Class 8 Science Pollution of Air and Water Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Covers the following topics:

Air and water are the most important natural resources, and their pollution affects every living thing. This Class 8 Science exam notes page covers air pollution and its gaseous and particulate pollutants, natural and man-made sources, effects such as acid rain and global warming, control measures, water pollution and its sources and effects, prevention of water pollution, potable water, and the purification of water through sedimentation, filtration and chlorination.

Topics Covered

1. Pollution
2. Air Pollution
3. Air Pollutants
4. Prevention and Control of Air Pollution
5. Water Pollution
6. Source of Water Pollution
7. Effect of Water Pollution
8. Prevention of Water Pollution
9. Potable Water
10. Purification of Water

POLLUTION

Change in the chemical, physical or biological properties of a substance is known as pollution.

AIR POLLUTION

The contamination of air by the addition of unwanted and harmful substances is known as air pollution.

AIR POLLUTANTS

The substances which contaminate the air are called air pollutants. For example: fly ash, carbon monoxide and sulphur dioxide.

Types of air pollutants
(1) Gaseous: pollutants which are in the gaseous state at normal temperature and pressure are called gaseous pollutants. For example: SO₂, CO₂, NO₂ and CO.
(2) Particulate: pollutants that are in the form of tiny particles are called particulate pollutants. They may be solid or liquid. Particulate matter usually remains suspended in the atmosphere and is therefore also termed suspended particulate matter (SPM).

Sources of air pollution
(1) Natural sources of air pollution
(2) Man-made sources of air pollution

Natural sources of air pollution
(1) Strong winds during dust storms lift dust particles into the air and pollute it.
(2) Forest fires release soot (carbon particles) and ash into the air.

[Figure: Photograph captioned Forest fire leads to air pollution]

(3) Volcanic eruptions release various gases and ash into the atmosphere.

[Figure: Photograph captioned Volcanic eruption releases gases and ash]

(4) Decay of organic matter releases ammonia gas into the air.
(5) Decay of organic matter lying under water releases methane gas as an air pollutant.

Man-made sources of air pollution
(i) Burning fuels: burning of wood, cow dung cakes, coal and kerosene in homes pollutes the air by producing carbon monoxide (CO), sulphur dioxide (SO₂), carbon dioxide (CO₂), smoke, soot and ash.
(ii) Vehicles: exhaust gases from motor vehicles produce harmful pollutants such as sulphur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), unburnt hydrocarbons, lead compounds and soot.

[Figure: Photograph captioned Vehicular pollution is a major source of air pollution]

(iii) Industries: various industries pollute the air by releasing pollutants such as SO₂, NO, chlorine, cement dust and asbestos dust.

[Figure: Photograph captioned Industries pollute air]

(iv) Fertilisers and pesticides: use of fertilisers and pesticides in agriculture pollutes the air.
(v) Deforestation: it pollutes the air by increasing the amount of carbon dioxide in it.
(vi) Chlorofluorocarbons (CFCs): their use pollutes the air by depleting the ozone layer.
(vii) Smoking: it pollutes the air by emitting carbon monoxide (CO) and nicotine.

Effects of air pollution
(1) Respiratory diseases: air pollution causes several respiratory diseases such as bronchitis, asthma, lung cancer, tuberculosis, pneumonia, chest pain and allergies.
(2) Carbon monoxide poisoning: carbon monoxide is a poisonous gas. It combines with the haemoglobin of our blood and forms a stable compound called carboxy-haemoglobin. This reduces the oxygen carried in the blood and leads to respiratory problems, suffocation and even death. This is called carbon monoxide poisoning.
(3) Depletion of ozone: chlorofluorocarbons are harmful chemicals widely used in industries. When they escape into the atmosphere, they destroy ozone. Ozone in the upper atmosphere absorbs harmful ultraviolet radiation from the sun and protects plants and animals on the earth. Ultraviolet radiation causes skin cancer, cataract and damage to the immune system.
(4) Greenhouse effect: burning of fossil fuels such as coal and petroleum products in homes, transport and industry releases a lot of carbon dioxide into the air. The trapping of radiated heat due to increased concentration of carbon dioxide in the air is called the greenhouse effect. It leads to heating of the earth. The rise in temperature of the earth's surface is called global warming.
(5) Acid rain: high concentrations of gases such as carbon dioxide, sulphur dioxide and oxides of nitrogen in the air dissolve in rainwater and produce what is called acid rain.

Harmful effects of acid rain
(1) Acid rain is very corrosive and harmful to both living and non-living things.
(2) It gradually destroys living tissues, eats up cement, limestone and marble of buildings, and corrodes steel and other metals.
(3) It damages old statues and old monuments.
(4) It causes premature leaf fall in plants and makes the soil acidic.

PREVENTION AND CONTROL OF AIR POLLUTION

Some of the methods for controlling air pollution are given below.
(1) Tall chimneys should be installed in factories to reduce air pollution at ground level.
(2) Better designed fuel burning equipment should be used in homes and industries so that fuel is burnt completely.
(3) Reduce vehicular emissions by using non-polluting fuels such as CNG.
(4) Use LPG for domestic purposes.
(5) Improve the quality of fuel in automobiles and use catalytic converters in them.
(6) Make use of renewable alternative sources of energy such as solar energy, wind energy and hydro energy.
(7) Use unleaded petrol.
(8) Plant and grow more and more trees.

WATER POLLUTION

The contamination of water with unwanted and harmful substances such as sewage, toxic chemicals and industrial waste is called water pollution.
Water pollutants are the unwanted, harmful substances which pollute water. For example: sewage, industrial waste and chemical waste.

SOURCES OF WATER POLLUTION

(1) Industrial wastes: the discharge of untreated, toxic industrial wastes into rivers and lakes is one of the main causes of water pollution.

[Figure: Photograph captioned Industrial discharge pollutes water]

(2) Sewage: dumping of sewage into rivers and lakes is the second major cause of water pollution in big cities. Excreta and cattle dung contain harmful microbes that cause diseases.
(3) Domestic waste: this includes human excreta, food waste, soaps and detergents, garbage, wrappers of eatables, plastics and other materials.
(4) Chemical fertilisers and pesticides: excessive use of fertilisers, pesticides and weedicides pollutes water, because some of these chemicals are washed into rivers and lakes with rainwater.
(5) Petroleum oil: leakage of petroleum oil into the sea during drilling and shipping operations pollutes sea water.

[Figure: Photograph captioned Oil spill]

EFFECTS OF WATER POLLUTION

(1) Polluted water contains disease-causing pathogens such as bacteria, protozoa and viruses, which cause water-borne diseases.
(2) The acids and alkalis present in industrial wastes kill the microorganisms that are the natural cleaning agents of water.
(3) Sewage and fertilisers in polluted water are rich in nutrients. This leads to excessive growth of algae (algal bloom), which covers the whole surface of the water body. This process is called eutrophication.
(4) Industrial wastes from industries such as paper, paint, pesticide and metal contain toxic chemicals like mercury, lead, copper and cadmium. When polluted water is used, these toxic substances can enter the human body through food chains and affect health.

PREVENTION OF WATER POLLUTION

Water pollution can be prevented or minimised by adopting the following measures.
(1) Toxic industrial wastes should be treated chemically to neutralise the harmful substances before they are discharged into rivers and lakes.
(2) Sewage should not be dumped into rivers. It should first be treated at a sewage treatment plant to remove the organic matter, which is obtained as manure.
(3) The use of excessive fertilisers and pesticides should be avoided.
(4) The use of synthetic detergents should be minimised, or biodegradable detergents should be used.
(5) Dead bodies of human beings and animals should not be thrown into rivers.
(6) Excreta and other garbage should be treated in a biogas plant to get fuel as well as manure.
(7) The water of rivers, streams, ponds and lakes should be purified or cleaned. This can be done by industries and by the government. One such example is the Ganga Action Plan launched by the Indian government.
(8) Trees and shrubs should be planted along the banks of rivers.
(9) There should be general awareness among the masses about the harmful effects of water pollution and the ways of preventing it.
(10) Waste paper, plastics, waste food materials and rotten food and vegetables should not be thrown into open drains.

POTABLE WATER

Water which is suitable for drinking by human beings is called potable water.

PURIFICATION OF WATER

People in big cities get purified river or lake water through a network of water pipelines. The three main processes that water undergoes during purification are sedimentation, filtration and chlorination.

Sedimentation
Water is pumped from a river into a large tank and allowed to stand for some time. The heavy suspended particles settle down at the bottom of the tank as sediments. This process is called sedimentation. The upper layer of water is then sent for filtration.

Filtration
After sedimentation, the water is filtered through thick layers of sand and gravel. In this process the remaining suspended impurities are removed.

Chlorination
The filtered water is chlorinated by adding chlorine or bleaching powder. Chlorine kills all the harmful germs. This process of adding chlorine to filtered water to kill germs is known as chlorination.

The purified and chlorinated water is then supplied for human use through a network of pipes. The stages in the purification of city water supply are shown in the figure below.

[Figure: Flow diagram of purification of water for city supply. River water is pumped into a Sedimentation tank, then to an Overhead tank, then to a Filtration tank, then to a second Overhead tank with a dome-shaped top, then to a Domestic tap in a house, and the supply continues as City supply through pipes]

5.Pollution

6.Water Pollution

Pollutions 

Change in Chemical, Physical & Biological property of substance, known as pollutions.