Pollution of Air and Water
Pollution of Air and Water
Pollution of Air and Water-Air and water are the two things no living thing can go without. This chapter is about what we are putting into them, what it is doing to us and to the Taj Mahal, and what can actually be done about it.
1. What Is Pollution?
Two definitions to learn word for word
Pollutant: any substance that contaminates air, water or soil and harms living things.
Pollution: the contamination of the environment by undesirable substances or changes, making it unfit for living organisms.
Pollutants come from two kinds of sources. Natural sources include volcanic eruptions, forest fires, dust storms and pollen grains. Human-made sources include vehicles, factories, power stations, thermal plants, brick kilns, refineries and the burning of crop residue — and these are the ones we can control.
2. Air Pollution
What clean air is made of
| Component | Approximate share |
|---|---|
| Nitrogen | 78% |
| Oxygen | 21% |
| Argon and other inert gases | about 0.9% |
| Carbon dioxide | about 0.03–0.04% |
| Water vapour and dust | variable |
Air is polluted when unwanted substances enter it, or when the proportions of these normal components are altered. Notice how small the carbon dioxide share is — and yet nudging that tiny number upward is enough to warm an entire planet.
The major air pollutants
| Pollutant | Main source | Harmful effect |
|---|---|---|
| Carbon monoxide (CO) | Incomplete burning of fuels, especially in motor vehicles | Reduces the oxygen-carrying capacity of the blood; can be fatal in closed rooms |
| Carbon dioxide (CO2) | Burning of fossil fuels; deforestation reduces its removal | Main greenhouse gas — drives global warming |
| Sulphur dioxide (SO2) | Power plants and petroleum refineries burning coal and oil | Respiratory problems, including permanent lung damage; causes acid rain |
| Nitrogen dioxide (NO2) | Vehicles and power stations | Causes acid rain; a component of smog |
| CFCs (chlorofluorocarbons) | Refrigerators, air conditioners, aerosol sprays, fire extinguishers | Damage the ozone layer of the atmosphere |
| SPM (suspended particulate matter) | Smoke, soot, dust, fly ash from vehicles, industries and burning of waste | Respiratory ailments, asthma, allergies, even cancer |
| Methane | Paddy fields, cattle, landfills, coal mines | A powerful greenhouse gas |
CO and CO2 are not interchangeable
Examiners love this pair. Carbon monoxide comes from incomplete combustion and poisons the blood directly. Carbon dioxide comes from complete combustion, is a normal part of clean air, and harms us indirectly by trapping heat. Different gas, different source, different damage.
Acid rain
Sulphur dioxide and nitrogen dioxide released into the air react with water vapour present in the atmosphere to form sulphuric acid and nitric acid. These acids come down with the rain, and that is acid rain.
How acid rain forms
Damage: corrodes buildings and monuments, makes lakes and soil acidic, kills fish and aquatic plants, harms crops and forests.
Case study: The Taj Mahal
The Taj Mahal at Agra, built in the seventeenth century of gleaming white marble, is one of the seven wonders of the world. Over the past few decades its marble has been turning yellow and pitted.
Marble cancer — the mechanism
Industries around Agra — including the Mathura oil refinery, iron foundries, a rubber-processing plant and chemical units — release sulphur dioxide and nitrogen dioxide. These form acid rain.
Marble is calcium carbonate. Acid attacks it, corroding the stone away. This corrosion of marble by acid rain is called marble cancer.
Separately, SPM such as soot from the refinery settles on the surface and stains the white marble yellow.
What was done about it
The Supreme Court of India issued directives to protect the monument: industries in the Taj zone were ordered to switch to cleaner fuels such as CNG and LPG, and vehicles in the area were required to use unleaded petrol. It is a useful reminder that pollution control is as much a matter of law and policy as of science.
Smog
The word smog is made from smoke + fog. In cold weather, water condenses out of the air and mixes with smoke and unburnt hydrocarbons. Nitrogen dioxide in this mixture reacts in sunlight to produce ground-level ozone, which is a pollutant near the ground even though it protects us high above.
Smog causes breathing difficulties, asthma, coughing, wheezing and irritation of the eyes. Winter smog over north Indian cities is now an annual event.
The greenhouse effect and global warming
The greenhouse effect is natural — and necessary
Sunlight passes through the atmosphere and warms the Earth’s surface. The Earth radiates some of this heat back. A part of that heat is trapped by gases in the atmosphere and cannot escape into space. This trapping is the greenhouse effect.
Without it the Earth would be far too cold for life. The problem is not the greenhouse effect — it is the excess of it.
The gases responsible are called greenhouse gases: carbon dioxide, methane, nitrous oxide and water vapour. Carbon dioxide is the chief culprit, because we keep adding it in two ways at once — burning fossil fuels puts more in, while deforestation removes the trees that would take it out.
Consequences of global warming
Rising average temperature worldwide · Melting of glaciers and polar ice · Rising sea levels, threatening to submerge low-lying coastal cities such as Mumbai · Changes in rainfall patterns and crop yields · More frequent floods and droughts · Loss of habitats and species.
Depletion of the ozone layer
High in the atmosphere sits a layer of ozone that absorbs harmful ultraviolet (UV) radiation from the Sun. It is a shield: UV rays cause skin cancer, cataracts and damage to crops.
CFCs released from refrigerators, air conditioners and aerosol sprays rise up and destroy ozone molecules, thinning the layer. A severe thinning — the ozone hole — was discovered over Antarctica. Many countries have since phased out CFCs, and the layer has begun a slow recovery.
Do not confuse these two problems
Global warming is caused by greenhouse gases such as CO2 trapping heat. Ozone depletion is caused by CFCs destroying the ozone shield and letting UV through. They are separate problems with separate causes. Writing that CFCs cause global warming, or that CO2 makes the ozone hole, is a standard mistake.
Controlling air pollution
Measures that work
Use public transport, car pools, cycles and walking · Switch to clean fuels: CNG, LPG, unleaded petrol · Use solar and wind energy instead of coal · Fit filters and scrubbers to factory chimneys · Do not burn dry leaves, crop stubble or garbage · Plant trees — they absorb CO2 and trap dust · Maintain vehicles and get pollution checks done.
A real example: when Delhi converted its buses, taxis and auto-rickshaws to CNG, measured levels of several pollutants fell. CNG burns more completely than petrol or diesel, so it produces far less unburnt carbon.
3. Water Pollution
About 71% of the Earth’s surface is covered with water, but only a tiny fraction is fresh water available for use. Water pollution is the contamination of water by substances that make it harmful for drinking, bathing, farming or aquatic life.
Sources of water pollution
| Source | What it adds | Effect |
|---|---|---|
| Sewage | Untreated human waste from towns and cities | Spreads cholera, typhoid, dysentery, jaundice; consumes dissolved oxygen |
| Industrial waste | Arsenic, lead, mercury, fluorides, chromium, oils, chemicals | Poisonous; accumulate in the body and in fish; damage nerves, kidneys, bones |
| Agricultural chemicals | Excess fertilisers and pesticides washed off fields by rain | Cause excessive growth of algae, which uses up dissolved oxygen and kills fish |
| Hot water (thermal pollution) | Coolant water discharged by power plants and factories | Raises water temperature; harms aquatic plants and animals and lowers dissolved oxygen |
| Social and religious practices | Immersion of idols, throwing of ashes, offerings, polythene bags | Add paints, dyes, plastics and organic load to rivers |
| Oil spills | Leakage from ships and tankers | Form a film on the surface, cutting off oxygen and sunlight; kill sea birds and marine life |
Why fertiliser in a river kills fish
This chain confuses many students, so trace it slowly. Fertiliser is food for plants → washed into a lake it feeds algae, which multiply enormously → the algae later die and decompose → the bacteria that decompose them use up the dissolved oxygen in the water → fish and other aquatic animals suffocate. The fertiliser never poisons the fish directly. It starves them of oxygen.
Case study: The river Ganga
The Ganga flows about 2,500 km from Gangotri in the Himalayas to Ganga Sagar in the Bay of Bengal. It is among the most polluted rivers in the world.
The load comes from the untreated sewage of more than 25 towns along its banks, industrial effluents, agricultural run-off, the dumping of unburnt corpses and ashes, and the throwing of rubbish and polythene during rituals. The Ganga Action Plan was launched in 1985 to clean the river; work continues under later programmes.
Potable water and how to purify it
Potable water
Water that is fit for drinking — free from germs, harmful chemicals and unpleasant taste, colour or smell.
A word of caution: water can look perfectly clean and still be dangerous. Disease-causing microorganisms are invisible to the eye. Clear is not the same as safe.
| Method | How it works | Removes |
|---|---|---|
| Filtration (candle filter, cloth) | Water passes through fine pores that hold back solids | Suspended dirt and some germs |
| Boiling | Heat kills microorganisms | Germs (not dissolved chemicals) |
| Chlorination | Chlorine tablets or bleaching powder are added — one tablet per 20 litres | Germs |
| UV treatment | Ultraviolet light destroys microorganisms | Germs |
The limit of chlorine tablets
Chlorination kills germs but does not remove dissolved chemicals such as arsenic or fluoride, and an excess of the tablets is itself harmful. Follow the stated dose — one tablet for 20 litres.
Conserving water
Reduce, Reuse, Recycle
Close the tap while brushing · Repair leaking taps and pipes · Use a bucket instead of a shower or hose · Reuse the water from washing vegetables to water plants · Harvest rainwater · Treat sewage before releasing it into rivers · Use fertilisers and pesticides in moderation · Do not throw waste into water bodies.
4. Quick Revision
The twenty facts that carry the chapter
1. Clean air: nitrogen 78%, oxygen 21%, carbon dioxide about 0.03–0.04%.
2. A pollutant is any substance that contaminates air, water or soil.
3. Carbon monoxide comes from incomplete combustion and reduces the blood’s oxygen-carrying capacity.
4. SO2 and NO2 cause acid rain; SO2 comes mainly from power plants and refineries.
5. Acid rain forms when SO2 and NO2 react with water vapour, making sulphuric and nitric acid.
6. Marble is calcium carbonate; acid rain corrodes it — this is marble cancer.
7. The Taj Mahal also yellows because of SPM, mainly soot from the Mathura oil refinery.
8. The Supreme Court ordered CNG/LPG for Taj-zone industries and unleaded petrol for vehicles.
9. SPM causes respiratory ailments, asthma and cancer.
10. Smog = smoke + fog; it causes breathing difficulty and eye irritation.
11. Greenhouse gases: carbon dioxide, methane, nitrous oxide, water vapour.
12. The greenhouse effect is natural and necessary; excess of it causes global warming.
13. Global warming melts glaciers, raises sea levels and shifts rainfall patterns.
14. CFCs from refrigerators, ACs and sprays damage the ozone layer; the ozone hole is over Antarctica.
15. The ozone layer shields us from ultraviolet radiation.
16. Delhi cut pollution by converting public transport to CNG.
17. Excess fertiliser causes algal growth, which uses up dissolved oxygen and kills fish.
18. Hot water discharged from factories causes thermal pollution.
19. The Ganga flows 2,500 km from Gangotri to Ganga Sagar; the Ganga Action Plan began in 1985.
20. Potable water is water fit for drinking; purify by filtration, boiling, chlorination (1 tablet per 20 litres) or UV.
Worksheet
Eight question types, ten questions each, with answers given right below every question.
A. Fill in the Blanks
1. Any substance that contaminates air, water or soil is called a ____________.
Answer: pollutant
2. The gases ____________ and ____________ react with water vapour in the atmosphere to cause acid rain.
Answer: sulphur dioxide; nitrogen dioxide
3. The corrosion of the Taj Mahal’s marble by acid rain is known as ____________.
Answer: marble cancer
4. ____________ released from refrigerators and air conditioners damage the ozone layer.
Answer: CFCs (chlorofluorocarbons)
5. Water that is fit for drinking is called ____________ water.
Answer: potable
6. Smog is a mixture of ____________ and ____________.
Answer: smoke; fog
7. The ozone layer protects us from harmful ____________ radiation of the Sun.
Answer: ultraviolet (UV)
8. The Ganga Action Plan was launched in the year ____________.
Answer: 1985
9. One chlorine tablet is sufficient to disinfect about ____________ litres of water.
Answer: 20
10. The discharge of hot water from factories into rivers is called ____________ pollution.
Answer: thermal
B. True or False
1. Carbon dioxide is not present in clean air.
False. It is a normal component, about 0.03–0.04%. Only the excess is a problem.
2. The greenhouse effect is entirely harmful and should be eliminated.
False. It is natural and necessary — without it the Earth would be too cold for life. The excess is the problem.
3. CFCs are responsible for the depletion of the ozone layer.
True.
4. Water that is clear and colourless is always safe to drink.
False. Disease-causing microorganisms and dissolved chemicals are invisible.
5. Carbon monoxide is produced by the complete combustion of fuels.
False. It is produced by incomplete combustion.
6. Excessive use of fertilisers can kill fish in a lake.
True. They cause algal growth whose decay uses up the dissolved oxygen.
7. CNG is a cleaner fuel than petrol and diesel.
True. It burns more completely and leaves far less unburnt carbon.
8. The ozone hole was first noticed over the Arctic.
False. It was detected over Antarctica.
9. Boiling removes dissolved chemical impurities from water.
False. Boiling kills germs only; dissolved chemicals remain.
10. Planting trees helps reduce air pollution.
True. Trees absorb carbon dioxide, release oxygen and trap dust.
C. Multiple Choice Questions
1. Which of the following is not a greenhouse gas?
(a) Carbon dioxide (b) Methane (c) Oxygen (d) Nitrous oxide
Answer: (c) Oxygen.
2. Which gas reduces the oxygen-carrying capacity of blood?
(a) Carbon dioxide (b) Carbon monoxide (c) Sulphur dioxide (d) Nitrogen
Answer: (b) Carbon monoxide.
3. Marble cancer is caused by
(a) smog (b) acid rain (c) CFCs (d) global warming
Answer: (b) Acid rain.
4. Which of these is a natural source of air pollution?
(a) Thermal power plant (b) Volcanic eruption (c) Motor car (d) Brick kiln
Answer: (b) Volcanic eruption.
5. The Taj Mahal is turning yellow chiefly because of
(a) suspended particulate matter (b) CFCs (c) methane (d) ozone
Answer: (a) Suspended particulate matter — soot settling on the marble.
6. The Ganga Action Plan was launched to
(a) increase the flow of the Ganga (b) reduce pollution of the Ganga
(c) build dams on the Ganga (d) divert the Ganga
Answer: (b).
7. Which fuel was adopted for public transport in Delhi to reduce air pollution?
(a) Diesel (b) Kerosene (c) CNG (d) Coal
Answer: (c) CNG (compressed natural gas).
8. Excessive growth of algae in a water body leads to
(a) an increase in dissolved oxygen (b) a decrease in dissolved oxygen
(c) purification of the water (d) a rise in fish population
Answer: (b). Decomposition of dead algae consumes the oxygen.
9. The ozone layer is damaged mainly by
(a) CO2 (b) SO2 (c) CFCs (d) SPM
Answer: (c) CFCs.
10. Which disease is not spread through polluted water?
(a) Cholera (b) Typhoid (c) Asthma (d) Jaundice
Answer: (c) Asthma — it is linked to air pollution, not water.
D. Match the Following
| Column A | Column B |
|---|---|
| 1. Carbon monoxide | (a) Smoke + fog |
| 2. CFCs | (b) Reduces oxygen-carrying capacity of blood |
| 3. Smog | (c) Hot water from factories |
| 4. Sulphur dioxide | (d) Ozone layer depletion |
| 5. Thermal pollution | (e) Water fit for drinking |
| 6. Potable water | (f) Acid rain |
| 7. Ganga Action Plan | (g) Marble cancer |
| 8. Taj Mahal | (h) Chief greenhouse gas |
| 9. Carbon dioxide | (i) 1985 |
| 10. Chlorine tablet | (j) Disinfects 20 litres of water |
Answers:
1 → (b) 2 → (d) 3 → (a) 4 → (f) 5 → (c)
6 → (e) 7 → (i) 8 → (g) 9 → (h) 10 → (j)
E. Assertion and Reason
Choose the correct option for each:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
1. A: The marble of the Taj Mahal is being corroded. R: Marble is calcium carbonate, which reacts with the acids present in acid rain.
Answer: (a).
2. A: The greenhouse effect is harmful and must be stopped completely. R: Greenhouse gases trap heat in the atmosphere.
Answer: (d). The reason is true, but the assertion is false — the effect is natural and necessary; only the excess is harmful.
3. A: Fish die in lakes that receive heavy run-off of fertilisers. R: Fertilisers are directly poisonous to fish.
Answer: (c). The assertion is true, but the reason is false — the fish die of oxygen depletion caused by decaying algae.
4. A: CNG is preferred over diesel for public transport. R: CNG burns more completely and produces less unburnt carbon.
Answer: (a).
5. A: Boiling makes any water safe to drink. R: Heat kills microorganisms present in water.
Answer: (d). The reason is true, but the assertion is false — boiling does not remove dissolved chemicals such as arsenic.
6. A: The ozone layer is essential for life on Earth. R: It absorbs harmful ultraviolet radiation coming from the Sun.
Answer: (a).
7. A: Deforestation contributes to global warming. R: Trees absorb carbon dioxide during photosynthesis.
Answer: (a). Fewer trees means less CO2 is removed from the air.
8. A: Ozone is a pollutant when it is present near the ground. R: Ozone is harmful wherever it occurs in the atmosphere.
Answer: (c). The assertion is true, but the reason is false — high in the atmosphere ozone is a protective shield.
9. A: Hot water should be cooled before being released into a river. R: A rise in water temperature lowers the dissolved oxygen and harms aquatic life.
Answer: (a).
10. A: Water that looks clean may still be unfit for drinking. R: Disease-causing microorganisms and dissolved chemicals cannot be seen with the naked eye.
Answer: (a).
F. Very Short Answer Questions (1 mark)
1. Define air pollution.
Answer: The contamination of air by unwanted substances that harm living organisms.
2. Name two natural sources of air pollution.
Answer: Volcanic eruptions and forest fires. (Also dust storms and pollen.)
3. What does SPM stand for?
Answer: Suspended Particulate Matter.
4. Name any two greenhouse gases.
Answer: Carbon dioxide and methane. (Also nitrous oxide and water vapour.)
5. Which acid rain gases attack the Taj Mahal?
Answer: Sulphur dioxide and nitrogen dioxide.
6. What is potable water?
Answer: Water that is fit for drinking.
7. Where is the ozone hole located?
Answer: Over Antarctica.
8. Name two diseases caused by drinking contaminated water.
Answer: Cholera and typhoid. (Also dysentery, jaundice, hepatitis.)
9. Expand CNG and CFC.
Answer: CNG — Compressed Natural Gas. CFC — Chlorofluorocarbon.
10. What are the three R’s of conservation?
Answer: Reduce, Reuse, Recycle.
G. Short Answer Questions (2–3 marks)
1. What is acid rain? How is it formed?
Answer: Acid rain is rain that contains acids. Burning of fossil fuels in power plants, refineries and vehicles releases sulphur dioxide and nitrogen dioxide into the air. These gases react with water vapour present in the atmosphere to form sulphuric acid and nitric acid, which come down along with the rain.
2. Why is carbon monoxide dangerous?
Answer: Carbon monoxide is produced by the incomplete combustion of fuels, mainly in motor vehicles. It reduces the oxygen-carrying capacity of the blood, so the body’s tissues are starved of oxygen. Because it is colourless and odourless it gives no warning, and in a closed room it can be fatal.
3. Explain the greenhouse effect in your own words.
Answer: Sunlight passes through the atmosphere and warms the Earth’s surface. The Earth radiates part of this heat back towards space, but gases such as carbon dioxide, methane and water vapour absorb and trap some of it in the atmosphere. This trapping of heat is the greenhouse effect. It is natural and keeps the Earth warm enough for life; an excess of greenhouse gases traps too much heat and causes global warming.
4. How is the Taj Mahal being damaged, and what has been done to protect it?
Answer: Industries around Agra release sulphur dioxide and nitrogen dioxide, which cause acid rain. Acid rain corrodes the Taj Mahal’s marble — calcium carbonate — a process called marble cancer. Soot and other suspended particulate matter also settle on the surface and turn the white marble yellow. To protect it, the Supreme Court directed industries in the Taj zone to switch to CNG and LPG, and vehicles in the area to use unleaded petrol.
5. What is smog? Why is it harmful?
Answer: Smog is a mixture of smoke and fog that forms in cold weather when water condenses around smoke and unburnt hydrocarbons. It contains nitrogen dioxide, which produces ground-level ozone in sunlight. Smog causes breathing difficulties, asthma, coughing, wheezing and irritation of the eyes.
6. Why is excessive use of fertilisers in fields harmful to a nearby lake?
Answer: Rain washes the excess fertiliser into the lake, where it acts as food for algae. The algae multiply rapidly, and when they die their decomposition by bacteria uses up the dissolved oxygen in the water. Fish and other aquatic animals then die of oxygen shortage.
7. Distinguish between global warming and ozone layer depletion.
Answer: Global warming is the rise in the Earth’s average temperature caused by greenhouse gases such as carbon dioxide and methane trapping excess heat. Ozone depletion is the thinning of the ozone layer caused by CFCs destroying ozone molecules, which allows more harmful ultraviolet radiation to reach the Earth. They have different causes and different effects.
8. Clear water is not necessarily safe water. Explain.
Answer: Water may be perfectly clear and colourless and yet contain disease-causing microorganisms and dissolved chemicals such as arsenic or fluoride, none of which can be seen with the naked eye. Such water can cause cholera, typhoid or long-term poisoning. Water must therefore be purified by boiling, filtration, chlorination or UV treatment before drinking, however clean it appears.
9. What is thermal pollution of water? How does it harm aquatic life?
Answer: Thermal pollution is the release of hot water — used as a coolant in power plants and factories — into rivers and lakes. The rise in water temperature lowers the amount of dissolved oxygen the water can hold and disturbs the breeding and growth of aquatic plants and animals, many of which can survive only within a narrow temperature range.
10. Suggest four ways in which a school student can help reduce air pollution.
Answer: (i) Walk, cycle, use a school bus or share a car instead of travelling alone. (ii) Plant and care for trees. (iii) Never burn dry leaves, crackers or garbage. (iv) Switch off lights, fans and appliances when not needed, since the electricity is largely produced by burning coal.
H. Long Answer Questions (5 marks)
1. Describe the major air pollutants, their sources and their harmful effects.
Answer: (i) Carbon monoxide — from incomplete combustion of fuels in vehicles; reduces the oxygen-carrying capacity of blood.
(ii) Carbon dioxide — from burning fossil fuels; the chief greenhouse gas, causing global warming.
(iii) Sulphur dioxide — from power plants and petroleum refineries; causes respiratory illness and acid rain.
(iv) Nitrogen dioxide — from vehicles and power stations; causes acid rain and forms part of smog.
(v) CFCs — from refrigerators, air conditioners and sprays; deplete the ozone layer.
(vi) SPM — smoke, soot and dust from vehicles, industries and burning of waste; causes asthma, allergies, respiratory ailments and cancer.
Together these pollutants damage human health, corrode monuments, harm crops and forests, and alter the climate.
2. Explain global warming: its cause, mechanism and consequences.
Answer: Mechanism: Sunlight warms the Earth’s surface, which radiates heat back. Greenhouse gases — carbon dioxide, methane, nitrous oxide and water vapour — absorb part of this heat and prevent it from escaping into space. This natural greenhouse effect keeps the Earth habitable.
Cause of the problem: Human activity has raised the concentration of these gases sharply. Burning of fossil fuels in vehicles, factories and power plants adds carbon dioxide, while deforestation removes the trees that would absorb it. More heat is trapped than before, and the Earth’s average temperature rises. This is global warming.
Consequences: Melting of glaciers and polar ice; a rise in sea level that threatens low-lying coastal cities such as Mumbai; changed rainfall patterns leading to floods in some regions and droughts in others; falling crop yields; loss of habitats and extinction of species.
Remedies: Use clean fuels and renewable energy, use public transport, plant trees, and reduce wasteful consumption of electricity.
3. What is acid rain? Explain its formation and its effects on monuments, soil and aquatic life.
Answer: Formation: The burning of coal and oil in power plants, refineries and vehicles releases sulphur dioxide and nitrogen dioxide. These gases react with water vapour in the atmosphere to form sulphuric acid and nitric acid, which dissolve in rain droplets and fall to the ground as acid rain.
Effect on monuments: Marble is calcium carbonate and reacts with acids. Acid rain therefore corrodes marble structures — the damage to the Taj Mahal is called marble cancer. Buildings, statues and metal structures are similarly eaten away.
Effect on soil and plants: Acid rain makes the soil acidic, damages the leaves of plants and reduces crop yields; whole forests have been damaged in parts of Europe and North America.
Effect on aquatic life: It makes lakes and rivers acidic, killing fish, eggs and aquatic plants; some lakes have become entirely lifeless.
Control: Use low-sulphur or clean fuels, fit scrubbers to chimneys, and switch to CNG, LPG and renewable energy.
4. Discuss the main sources of water pollution and their effects.
Answer: (i) Sewage: untreated waste from towns carries germs that cause cholera, typhoid, dysentery and jaundice, and its decomposition consumes dissolved oxygen.
(ii) Industrial effluents: factories discharge arsenic, lead, mercury, chromium, fluorides and oils, which are poisonous and accumulate in the bodies of fish and of the people who eat them.
(iii) Agricultural chemicals: excess fertilisers and pesticides are washed into water bodies, causing excessive growth of algae whose decay uses up dissolved oxygen and suffocates fish.
(iv) Hot water: coolant water from power plants raises the temperature of rivers, lowering dissolved oxygen and harming aquatic organisms.
(v) Social and religious practices: immersion of idols, ashes, offerings and polythene adds paints, dyes and organic waste.
(vi) Oil spills: leaked oil forms a film that blocks oxygen and sunlight and kills marine life and sea birds.
The overall result is disease in humans, destruction of aquatic ecosystems and a shrinking supply of usable fresh water.
5. Describe the pollution of the river Ganga and the steps taken to clean it.
Answer: The Ganga flows about 2,500 km from Gangotri in the Himalayas to Ganga Sagar in the Bay of Bengal, and is revered across India. It is nevertheless one of the most polluted rivers in the world.
Sources of its pollution: the untreated sewage of more than 25 towns along its banks; effluents from tanneries, paper mills and other industries; fertilisers and pesticides washed in from farmland; the dumping of unburnt corpses and ashes; and the throwing of flowers, polythene bags and other offerings during religious practices, along with washing of clothes and bathing of cattle.
Effects: the water has become unfit for drinking and even for bathing in many stretches; dissolved oxygen has fallen sharply; fish and other aquatic life have declined; and waterborne diseases have spread among the people who depend on the river.
Steps taken: the Ganga Action Plan was launched in 1985 to build sewage treatment plants, prevent industrial discharge and create public awareness; the work has continued under later national programmes. Real success also needs cooperation from citizens — not dumping waste, using treated sewage, and choosing eco-friendly materials for rituals.
6. Explain the methods of purifying water for drinking, with the merits and limits of each.
Answer: (i) Filtration: water is passed through a candle filter or fine cloth, whose pores hold back suspended dirt and some germs. Simple and cheap, but it cannot remove dissolved impurities or all microorganisms.
(ii) Boiling: heating kills the microorganisms present. Very effective against germs, but it does not remove dissolved chemicals such as arsenic or fluoride, and it uses fuel.
(iii) Chlorination: chlorine tablets or bleaching powder are added — one tablet is enough for 20 litres. Cheap and convenient, but the dose must be exact, since excess chlorine is itself harmful, and it kills germs only.
(iv) Ultraviolet treatment: UV light destroys microorganisms without adding any chemical. Effective, but it needs electricity and does not remove dissolved impurities.
In practice a combination is used: filter first to remove solids, then disinfect by boiling, chlorination or UV.
7. What is the ozone layer? Why is it important, how is it being destroyed, and what has been done?
Answer: The ozone layer is a region high in the atmosphere containing a concentration of ozone gas.
Importance: it absorbs the harmful ultraviolet radiation coming from the Sun. Without it, UV rays would reach the surface freely and cause skin cancer, cataracts, weakened immunity in humans, and damage to crops and to the tiny organisms at the base of ocean food chains.
Destruction: chlorofluorocarbons (CFCs) used in refrigerators, air conditioners, aerosol sprays and fire extinguishers escape into the air and drift upward, where they break apart ozone molecules. A severe thinning known as the ozone hole was discovered over Antarctica.
Action taken: countries across the world agreed to phase out CFCs and replace them with safer substitutes, and the layer has begun a slow recovery. It is one of the few environmental problems on which international action has visibly worked.
8. Suggest a detailed plan to reduce air pollution in a large Indian city.
Answer: Transport: expand and improve buses and the metro so that people choose them; convert public transport to CNG, as Delhi did; enforce unleaded petrol and regular pollution checks; encourage car pooling, cycling and walking.
Industry: shift polluting factories away from residential areas; require filters, scrubbers and electrostatic precipitators on chimneys; make industries switch to cleaner fuels such as CNG and LPG.
Energy: generate electricity from solar and wind rather than coal; discourage the wasteful use of power.
Waste: ban the burning of garbage, dry leaves and crop stubble; collect and process waste properly.
Green cover: plant trees along roads and in parks, since they absorb carbon dioxide and trap dust.
Awareness and law: monitor and publish air quality data, enforce penalties for violations, and educate citizens.
No single measure is enough; the sources are many, so the response must be on many fronts at once.
9. Water is a limited resource. Explain why it must be conserved and how.
Answer: Although about 71% of the Earth’s surface is covered with water, almost all of it is salty. Only a very small fraction is fresh water, and much of even that is locked in glaciers and ice caps or is now polluted. Meanwhile the population, farming and industry all demand more water each year, and groundwater levels are falling. Fresh water is therefore genuinely scarce.
How to conserve it: apply the three R’s — reduce, reuse, recycle. Close the tap while brushing or soaping; repair leaking taps and pipes promptly; use a bucket instead of a shower or hosepipe; reuse the water from washing vegetables and rice to water plants; harvest rainwater from rooftops; use drip irrigation in fields; treat sewage and industrial effluent before releasing it, and reuse treated water for gardens and construction; and above all keep water bodies free of waste, because polluting water is the fastest way to lose it.
10. A pond near a village suddenly has a thick green covering, and dead fish start floating on it. Explain what has happened and suggest what should be done.
Answer: What happened: the thick green covering is an excessive growth of algae. It is almost certainly caused by fertiliser washed into the pond by rain from the surrounding fields, possibly along with sewage. Fertilisers and sewage are rich in nutrients, which act as food for algae, and the algae multiply enormously.
Why the fish died: the algal mat blocks sunlight from reaching the plants below. When the algae die, bacteria decompose them, and this decomposition consumes large quantities of the oxygen dissolved in the water. Fish and other aquatic animals then suffocate for want of oxygen — they are not poisoned directly.
What should be done: stop the run-off at its source by using fertilisers in moderation and only as needed; prevent sewage from entering the pond; leave a strip of grass or trees between the fields and the water to trap run-off; remove the algal mat and the dead fish; and aerate the water if possible. Long term, the villagers should treat any waste water before it reaches the pond.
