Class -8, Science, Chapter - 18, Pollution of Air and Water Notes, Ncert Solutions & Frequently Asked Questions

 Science

Class - 8, Chapter - 18

Pollution of Air and Water

Notes, Ncert Solutions & Frequently Asked Questions 

-- Notes --

⭐Air:- Air is a combination of different gases such as nitrogen (78%), oxygen (21%), Carbon dioxide (0.04%) & left 0.96% comprises water vapour and other gases (helium, ozone, argon etc) along with dust particles, pollens and spores. 

Figure 1.- Constituents of air

⭐ Air Pollution:- Air pollution is defined as the presence of different substances in a certain concentration in the air that causes harm to human beings and the environment.

Figure 2.- Pollution of Air due to industries

⭐ How does Air get polluted? :- 

 ➧ Air Pollutants:- Any unwanted or harmful substance present in the air that decreases its quality and contaminates it is called air pollutants. 

• For example, smoke is an air pollutant. 

•Air pollutants can be classified into different categories based on their sources.

a) Indoor and Outdoor Air Pollutants:- 

i) Indoor pollutants:- These pollutants are generated indoors in houses, institutions, buildings or commercial facilities. 

• Indoor air pollution may not severely impact people's health but can certainly affect them in the long run. 

• Different indoor pollutants are:-

           -- tobacco smoke

           -- biological pollutants like pet hair, fungi, bacteria, pollens etc

          -- building materials like lead and asbestos (asbestos can cause cancer)

          -- gases such as carbon monoxide

Figure 3.- Indoor Air Pollutants

⭐ Indoor Air quality (IAQ):- Indoor air quality is defined as the quality of the air within houses, buildings and commercial structures. 

• Low indoor air quality can lead to immediate effects such as irritation in the eyes, redness of eyes, headaches or dizziness. 

• It may also lead to adverse effects in the long run such as the problem of asthma. 

• Air purifiers are used in many houses and offices to ensure clean air in the surroundings.

ii) Outdoor Pollutants:- Outdoor air pollution refers to the contamination of the air in the open environment. 

• Outdoor air pollutants generally arise from the burning of fossil fuels, smoke from industries and vehicles. 

• They adversely affect the quality of the air and hence the whole environment of the earth. 

• Different outdoor pollutants are:-

               -- carbon monoxide

               -- nitrogen oxide

               -- Sulphur Dioxide

               -- hydrocarbons

Figure 4.- Outdoor Air Pollution Sources

b) Primary and Secondary Air Pollutants:- Depending upon the emission into the air, the air pollutants can be categorized as primary or secondary. 

• Primary pollutants are the ones that are emitted directly in the environment. 

       -- For eg.- harmful gases (carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen oxide etc) are emitted directly from industries and vehicles into the air.

• Secondary Pollutants are the ones that get generated due to the reactions between various constituents of the year. 

        -- For eg.- acid rain is a secondary pollutant. Nitrogen oxides react with the water vapour present in the air and form nitric acid.

Figure 5.- Primary and Secondary Air Pollutants

⭐ Causes of Air Pollution:- 

• Air pollution from Natural Sources:- Sometimes forest fires or volcanic eruptions can lead to the release of an excess of smoke and dust in the atmosphere.

• Burning of fossil fuels:- coal and petroleum used in industries and vehicles when burnt release harmful substances in the air such as carbon monoxide, nitrogen oxides and sulphur dioxide.

• Exhaust from factories:- many industries release harmful substances like carbon dioxide, hydrocarbons and other harmful chemicals in the air which decrease its quality.

• Agricultural activities:- usage of insecticides, fertilizers and pesticides lead to release of chemicals in the air. Agricultural activities also lead to the release of ammonia in the atmosphere which is extremely hazardous for us.

• Pollution due to households:- paints, cleaning products, air conditioners, refrigerators and other appliances used in houses also contribute to air pollution. The air conditioners and refrigerators release chlorofluorocarbons that damage the ozone layer of the earth. Burning of wood and cow dung cakes in rural areas also leads to air pollution.

• Mining activities:- mining results in the release of a large amount of dust and chemicals in the air.

⭐ Effects of Pollutants on the Environment:- 

⭐ Air pollution and the case study of the Taj Mahal:- Air pollution can severely affect the environment. One great example of the harmful effects of air pollution can be viewed on the Taj Mahal, the most famous and beautiful tourist attraction of India.

• The air pollution in that region has led to decolourization of the white marbles of the Taj Mahal.

Figure 6.- Effect of Air Pollution on Taj Mahal

⭐ Causes of air pollution in Agra and effects on the Taj Mahal:- 

• The main sources of pollutants in the air around Taj Mahal are the industries around Agra.

• The Mathura oil refinery and other industries including automobiles, rubber processing and chemicals are releasing harmful substances in the air.

• The major pollutants are Nitrogen dioxide and Sulphur Dioxide. These pollutants lead to acid rain in the Agra region.

• When Nitrogen dioxide combines with water it forms nitric acid and when Sulphur Dioxide combines with water it forms sulphuric acid.

• The rainwater which falls on the Taj Mahal leads to decay of the marble. This is also called ‘marble cancer’.

• The Mathura oil refinery is a major source of suspended particulate matter (SPM) in the air around the Taj Mahal which is making the white marble look yellow in colour.

⭐ Steps taken to reduce air pollution in the area near Taj Mahal:- 

• The industries are switching to clean fuels such as CNG and LPG to prevent air pollution and protect the monument.

• Unleaded petrol should be used in automobiles to prevent harmful smoke from the vehicles near the Taj Mahal area.

⭐ The Greenhouse Effect:- As the sun rays enter the earth's atmosphere, some of the radiation gets reflected back into space while some of it is trapped inside the earth's atmosphere.

• This is possible because of certain greenhouse gases in the earth's atmosphere which can trap the heat of the sun. 

• For eg.- carbon dioxide, methane, nitrous oxide and water vapour can trap the sun’s radiation inside the earth. This phenomenon is called the greenhouse effect.

• Due to this greenhouse effect, the earth has a stable temperature that can support the existence of life on it. The greenhouse effect provides the required warmth to the earth.

• However, due to excess of carbon dioxide in the atmosphere, the greenhouse effect is becoming a serious trouble for the earth.

• The excess of greenhouse gases leads to absorption of more heat inside the Earth which further leads to a gradual rise in the temperature of the Earth. This phenomenon of increased temperature of the earth is called global warming.

Figure 7.- Greenhouse Effect

⭐ Sources of carbon dioxide on earth:-

• Human activities such as the burning of fossil fuels and industrial activities etc are causing a release of carbon dioxide in the atmosphere.

• Cutting of forests imbalances the carbon dioxide and oxygen amounts in the air as plants take up the carbon dioxide from the environment and in return release oxygen. Therefore deforestation leads to increased presence of carbon dioxide in the atmosphere.

⭐ Effects of global warming:- 

• Global warming leads to an increased temperature of the Earth which causes the melting of the glaciers and therefore increased sea levels. Hence, it results in flooding the areas around the sea.

• It also leads to extreme weather conditions in different places of the earth.

• Increased rainfall in different places has occurred due to the higher temperatures of the earth.

• Extinction of species or loss of habitats is occurring due to global warming as the animals are not able to adapt to changing climatic conditions of the earth.

Figure 8.- Effects of Global Warming

⭐The Ozone Layer:- The earth’s atmosphere is covered with a layer of ozone gas all around.

• This ozone gas prevents the dangerous ultraviolet rays of the sun from reaching the surface of the earth.

• The ultraviolet rays of the sun if reach the earth’s surface can cause severe problems.

• They can affect the plants, cause cancer in human beings, skin and eye diseases and can harm all other biotic and abiotic components of the earth.

• Hence, the ozone layer has a significant role in the environment.

⭐ Ozone layer depletion:- Ozone layer can be termed as deterioration of the ozone layer due to the presence of harmful substances in the environment. 

• The main cause for the depletion of the ozone layer is CFCs or Chlorofluorocarbons. They react with the ozone gas and form molecular oxygen. 

Figure 9.- Ozone Layer Depletion

⭐ Prevention of Air Pollution:- 

• Switching to cleaner fuels like CNG and LPG can lead to decreased air pollution.

• Generating awareness among people about air pollution and its harmful effects will encourage them to participate in preventing it.

• Increasing the usage of solar energy, wind energy and hydropower.

• Planting more and more trees around the cities in rural areas. India celebrates ‘Van Mahotsav’ in July every year where people come together and plant trees on a large scale.

• Instead of burning dry leaves and other organic materials they should be dumped into compost.

• Using gas stoves instead of burning coal or wood to cook food.

• Sharing vehicles or using public transport to reduce the number of vehicles on the road and hence the air pollution.

• Using eco-friendly paints and cleaning products for household and other buildings.

⭐ Water Pollution:- Water is not only necessary for the existence of life on the earth but it is required to perform several days to day activities like cleaning, washing clothes, bathing, cooking etc.

• Due to these activities, several substances get mixed up in water which makes it unsafe for further consumption.

• This addition of biological, chemical and physical substances in water is called water pollution.

• These substances contaminate water and make it unfit for consumption of human beings and animals are called water pollutants.

Figure 10.- Sources of Water pollution

⭐ How water gets polluted? :- 

➧ A case study of Ganga River:- Ganga is one of the most significant rivers of India. It flows through many towns, cities and villages of the northern, eastern and central India.

• People living in these regions are dependent upon Ganga water for their livelihood as well as for their day-to-day needs.

• But it has been found that Ganga is one of the 10 endangered rivers of the world according to the (WWF) World Wide Fund for Nature. This is because of its extreme pollution. The river has been called in dead at several places as it has no aquatic life there.

⭐ Causes of water pollution of Ganga:- 

• Various towns, villages and cities through which the Ganga River passes release untreated sewage water, garbage, dead bodies and other harmful substances into it.

• The Ganga River flowing through Kanpur region in Uttar Pradesh is the most polluted part of this river. This is so because of the large population of Kanpur.

• People use river water for bathing, cleaning, washing clothes and they even defecate in the river. From polythene bags to idols of Gods, flowers and garbage, people through several things in the river. All this leads to its pollution.

• The flow of river Ganga near Kanpur region is slow which leads to more pollution of it in still water.

• There are more than 5000 industries in Kanpur including leather, detergent, paint and Fertilizer Industries. All of these released toxic wastes in the river which leads to its pollution.

• In order to prevent the river Ganga from getting polluted the Government of India had initiated the Ganga Action Plan in 1985. However, increased industries and growing population of India have damaged the river to use extend. Hence the Government of India has now started another initiative called the National Mission for Clean Ganga in 2016.

⭐ Water pollution and its Effects:- 

⭐ Eutrophication:- Eutrophication is the presence of an excess of nutrients in the water, especially stagnant water.

• Eutrophication mainly occurs due to the addition of chemicals in the water bodies.

• As a result, a large number of algae and other plants start growing in water.

• As they die, decomposers begin to rise in the water. These decomposers take up most of the oxygen of the water.

• Hence, the lack of oxygen leads to the killing of aquatic animals in the water body.

Figure 11.- Eutrophication

⭐ Potable Water:- Drinking water or the water that is fit for consumption and food preparation is also called Potable water.

⭐ How is water purified? :- In order to prevent water pollution, water from households, industries and other sources is treated in a sewage treatment plant and then it is released in the water bodies.

• Also, water from the water bodies is treated before passing it on to the households and other industries to ensure that it is fit for the usage.

⭐ How water is made safe for drinking? :- 

• Filtration:- it is a physical method of removing impurities from water. Many households use water purifiers that can filter water and make it safe for drinking.

• Boiling:- it kills the germs present in the water. Many people boil water before drinking it.​

• Chlorination:- Chlorine is a natural cleaning agent for water which makes it fit for the consumption. Often chlorine tablets are added in water to remove impurities from it.

⭐ How to prevent water pollution? :- 

• Industries should strictly adhere to the laws and do not dispose untreated sewage water into the rivers or any other water body directly.

• More water treatment plants should be established near the industrial areas.

• We should save water as much as we can, for example, we can make sure that there are no leaking taps in the houses.

• We should reuse water if we can, for example, the water that we use for washing clothes can also be used for cleaning the house floors.

Figure 12.- Steps to Prevent Water Pollution

-- NCERT Solutions -- 

Question 1.- What are the different ways in which water gets contaminated?

Answer:- Following are the ways by which water gets contaminated:-

• People dispose of garbage in the water.

• Many industries discharge their harmful chemicals directly into rivers and streams.

• Chemical fertilisers, pesticides and weedicides used in agricultural practices get dissolve in water and are washed away into water bodies from the fields.

• Untreated sewage is directly discharged into water from household activities.

Question 2.- At an individual level, how can you help to reduce air pollution?

Answer:- I can do the following things to reduce air pollution:-

• I will use public transport as far as possible.

• I will generate awareness among people to reduce air pollution.

• I will plant more and more trees because trees help to keep the environment clean and reduce air 1 pollution.

• I will not use vehicles for short distances.

Question 3.- Clear, transparent water is always fit for drinking. Comment.

Answer:- No, this is not true that clean, transparent water is always fit for drinking. It is possible that they may carry disease-causing microorganisms. Thus, it is good to purify water before drinking.

Question 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:- These are the measures that would help our town to ensure the supply of clean water to all its residents:-

• Chlorine tablets should be made available.

• Proper cleaning of the water tank from time to time.

• The water supply pipes should not come in contact with the sewage pipes.

Question 5.- Explain the differences between pure air and polluted air.

Answer:- Pure air contains 78% nitrogen, 21% oxygen and 0.03% CO₂. Other gases such as argon, methane, neon, water vapour, etc. are also found in small quantities. When this air is contaminated by unwanted substances which have harmful effect on both living and non-living things like nitrogen oxide, sulphur oxide, carbon monoxide, etc., is said to be polluted.

Question 6.- Explain circumstances leading to acid rain. How does acid rain affect us?

Answer:- Nitrogen dioxide, sulphur dioxides are the pollutants which react with the water vapour present in atmosphere to form sulphuric acid and nitric acid. These acids come down with the rain, making the rain acidic. This is called acid rain. Following are the impacts 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.

Question 7.- Which of the following is not a greenhouse gas?

a) Carbon dioxide

b) Sulphur dioxide

c) Methane

d) Nitrogen

Answer:- d) Nitrogen

Question 8.- Describe the ‘Greenhouse Effect’ in your own words.

Answer:- Basically, the greenhouse effect means warming of the earth’s environment. When the sun’s ray falls on earth, a part of the radiation that falls on the earth is absorbed by it and a part is reflected back into space. Greenhouse gases present in the atmosphere trap these radiations and do not allow heat to leave. This helps in keeping our planet warm and thus helps in survival. Carbon dioxide (CO₂), methane (CH₄) and Carbon Monoxide (CO) are the gases responsible for this effect.

Question 9.- Prepare a brief speech on global warming. You have to deliver the speech in your class.

Answer:-

Good Morning, respected Principal Sir/Ma'am, teachers and all my dear friends. Today, I am  _____________ of class 8th, have got an opportunity to share my views on global warming & I hope that you all will listen me very carefully. 

Global Warming is caused by the increase of carbon dioxide levels in the earth’s atmosphere and a result of human activities that have been causing harm to our environment for the past few centuries now. Global Warming is something that can’t be ignored and steps have to be taken to tackle the situation globally. The average temperature is constantly rising by 1.5 degrees Celsius for the last few years. The best method to prevent future damage to the earth, cutting down of more forests should be banned and Afforestation should be encouraged. Start by planting trees near your homes and offices, participate in events, teach the importance of planting trees. It is impossible to undo the damage but it is possible to stop further harm.

Thank you for the attention. 

Have a nice day.

Question 10.- Describe the threat to the beauty of the Taj Mahal.

Answer:- Taj Mahal has become a matter of concern due to pollutants in air that are discolouring its white marble. Acid rains due to increasing air pollution has started corroding the marble. Suspended particulate matter, such as the soot particles emitted by Mathura oil refinery, has contributed towards the yellowing of the marble. Thus making Taj Mahal losing its beauty.

Question 11.- Why does the increased level of nutrients in the water affect the survival of aquatic organisms?

Answer:- Increased level of nutrients in the water affect the survival of aquatic organisms as these act as nutrients for algae to flourish and once these algae die, they serve as food for decomposers like bacteria and a huge amount of oxygen in water body gets used up. This results in decrease in the oxygen level, which may kill aquatic organisms.

-- Frequently Asked Questions --

Question 1.- What should be our main strategy to deal with the environmental problems faced by us ? 

Answer:- To develop technology that uses:- 

• Renewable sources of energy like solar energy, wind energy, etc. and avail maximum benefit from them.

• Less amount of non-renewable sources of energy, so that they last longer.

Question 2.- Mention two human activities which affect the quality of the soil on a large scale.

Answer:- 

• Use of pesticides and fertilisers affect the quality of soil.

• Overgrazing done by animals damages the soil by eating the vegetation.

Question 3.- What are the two ways to conserve soil ?

Answer:- 

• By planting more trees and grass.

• By constructing dams and river embankments.

Question 4.- Mention three causes for erosion of soil. 

Answer:- Soil erosion is caused by water and wind due to three major factors:-

• Deforestation — due to increasing population and increasing industrialisation.

• Overgrazing — by large animal populations has destroyed vegetation and result in barren land.

• Poor Farming Method — ploughing, loosens the soil and destroys its natural resources.

Question 5.- What is acid rain ? Why is it harmful to us ?

Or

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

Answer:- The industries release 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 come down with the rain, making the rain acidic. Acid rain is harmful to us because it destroys buildings and damages crops and trees.

Examples:-

• affects the monuments like Taj Mahal.

• spoils the growth of plants.

• causes irritation of eyes in human beings.

Question 6.- Why is greenhouse effect called so ? How does it cause global warming ? 

Answer:- It is called so because earth’s atmosphere acts like a green house made of glass in which sunlight enters through transparent glass but the heat released by plants in the form of infrared radiations is not allowed to escape. Carbon dioxide does not allow heat to escape causing global warming.

-- By Durgesh Pandey

(Eklavya Coaching Institute)