Atmospheric Pollution

Exercise 8A

Question 1

Define the following terms :

(i) pollution

(ii) pollutant

(iii) air pollution.

Answer

(i) Pollution is defined as the effect of undesirable changes in our surroundings that have a harmful effect on plants, animals and human beings.

(ii) Pollutants are toxic and otherwise harmful substances that have an undesirable impact on the different components of the environment and life forms.

(iii) Air pollution means degradation of air quality due to the increase in the concentration of harmful contaminants that affects human, plant and animal lives.

Question 2

Name any four gaseous pollutants.

Answer

Four gaseous pollutants are :

1. Sulphur dioxide
2. Hydrogen sulphide
3. Nitrogen oxides
4. Carbon monoxide

Question 3

Name the compounds of sulphur that cause air pollution. Also, state their harmful effects.

Answer

Compounds of sulphur like sulphur dioxide (SO₂), sulphur trioxide (SO₃) and hydrogen sulphide ((H₂S) cause air pollution.

Harmful effects of hydrogen sulphide (H₂S) :

• It causes nausea and irritates the eye and the throat. Being acidic in nature H₂S destroys vegetable matter.

Harmful effects of oxides of sulphur:

• It causes headache, vomiting and even death due to respiratory failure.
• It destroys vegetation and weakens building materials/constructions.
• It mixes with smoke and fog to form smog, which is very harmful.
• It causes acid rain.

Question 4

State:

(i) natural sources of air pollution

(ii) man-made sources of air pollution

Answer

(i) Natural sources of air pollution are :

1. Volcanic eruptions.
2. Lightning discharge.
3. Decaying vegetation.
4. Forest fires.
5. Winds and dust storms.

(ii) Man-made sources of air pollution are :

1. Automobiles.
2. Factories.
3. Industrial processes.
4. Burning of fossil fuels.
5. Decay of crop residue.

Question 5

(a) How do oxides of nitrogen enter the atmosphere?

(b) What are their harmful effects?

Answer

(a) Oxides of Nitrogen enter the atmosphere in the following ways :

• On the burning of fuels in furnaces, temperature increases. At high temperature, nitrogen and oxygen present in the air combine to form oxides of nitrogen.
• When fuel burns in internal combustion engine, oxides of nitrogen are produced and they enter the atmosphere as exhaust gases from automobile engines.
• Nitric oxide is formed by the reaction between atmospheric nitrogen and oxygen in the presence of electric discharge, which happens during thunderstorms, when there is lightening.
• Nitric oxide further reacts with atmospheric oxygen and ozone to form nitrogen dioxide.

b) Harmful effects of oxides of nitrogen are :

• Nitrogen dioxide causes irritation in the mucous membrane.
• Large concentrations of nitrogen dioxide may cause serious lung diseases.
• Nitrogen dioxide causes injuries to vegetation by damaging plant leaves.
• In sunlight, nitrogen dioxide oxidises hydrocarbons to form photochemical smog. It causes eye irritation, asthma attacks, nasal and throat infections.

Question 6

State the origin and health impact of smog.

Answer

A pollutant which is a combination of oxides of nitrogen and sulphur and partially-oxidized hydrocarbons and their derivatives produced by industries and automobiles forms a dark, thick, dust and soot laden fog and is known as smog.

Smog is noxious and irritating. It reduces visibility, induces respiratory troubles, and can cause death by suffocation.

Question 7

What are the harmful effects of oxides of Sulphur?

Answer

Harmful effects of oxides of sulphur:

1. It causes headache, vomiting and even death due to respiratory failure.
2. It destroys vegetation and weakens building materials/constructions.
3. It mixes with smoke and fog to form smog, which is very harmful.
4. It is oxidised by atmospheric oxygen into sulphur trioxide which combines with water to form sulphuric acid. Sulphuric acid is the cause of acid rain.
   2SO₂ (g) + O₂ (g) ⟶ 2SO₃ (g)
   SO₃ (g) + H₂O (l) ⟶ H₂SO₄ (aq)

Question 8

State the main sources and effects of carbon monoxide.

Answer

Carbon monoxide is formed by incomplete combustion of fuels in homes, factories and automobiles.

Harmful effects of carbon monoxide :
Carbon monoxide is a highly poisonous gas. If inhaled, it binds with haemoglobin 200 times more strongly than oxygen reducing the blood's ability to carry oxygen by converting haemoglobin into carboxyhaemoglobin. So, even in small quantities, it is fatal and can lead to death.

Question 9

Give the mechanism of the action of carbon monoxide.

Answer

When inhaled, carbon monoxide enters the bloodstream through the lungs. It binds to haemoglobin, which carries oxygen to body tissues. Carbon monoxide binds with haemoglobin 200 times more strongly than oxygen, so even small amounts in the air can greatly affect the body. It reduces the blood's ability to carry oxygen by converting haemoglobin into carboxyhaemoglobin. The heart and brain, being highly sensitive to oxygen deprivation, are most severely impacted by carbon monoxide exposure. In high concentrations, carbon monoxide can lead to death by paralyzing normal brain function.

Question 10

How can we control carbon monoxide poisoning?

Answer

Carbon monoxide poisoning can be controlled in the following ways :

1. By switching over from internal combustion engines to electrically powered cars.
2. Many pollution control devices are now installed in cars. Most of these devices help reduce pollution by burning gasoline completely. Complete combustion of gasoline produces only carbon dioxide and water vapour.
   2C₈H₁₈ +25O₂ ⟶ 16CO₂ + 18H₂O
3. By using substitute fuels for gasoline: Natural gas in both compressed (CNG) and liquefied forms (LNG) is now increasingly being used as fuel. Alcohols are other feasible substitutes.
4. By using catalytic converters:
   Carbon monoxide changes to carbon dioxide in the presence of finely divided platinum as a catalyst.
   $2\text{CO} \xrightarrow[\text{[O]}]{\text{Pt}} \text{CO}_2 + \text{H}_2\text{O}$

Exercise 8B

Question 1

Why does rainwater have pH less than 7?

Answer

pH of rain water is about 5.6, because carbon dioxide reacts with it to form weak carbonic acid.

CO₂ + H₂O ⟶ H₂CO₃

Question 2

pH of acid rain is sometimes as low as 2. Explain.

Answer

Acid rain results from acids like nitric acid and sulphuric acid present in polluted air. These acids are formed when oxides of nitrogen and sulphur come into contact with rain water. Due to high pollution, when amount of these acids dissolved in rain water is high, the pH of acid rain can be as low as 2.

Question 3

Explain the formation of acid rain due to:

(i) oxides of sulphur

(ii) oxides of nitrogen

Answer

(i) Sulphur is a non-metallic element found in coal and other fossil fuels. When these fuels are burned, sulphur combines with oxygen in air to form its gaseous oxides, sulphur dioxide (SO₂) and sulphur trioxide (SO₃)

S + O₂ ⟶ SO₂

2SO₂ + O₂ ⟶ 2SO₃

Sulphur dioxide and sulphur trioxide react with water to form H₂SO₄, which is the main cause of acid rain.

2SO₂ + O₂ + 2H₂O ⟶ 2H₂SO₄

SO₃ + H₂O ⟶ H₂SO₄

(ii) Oxides of nitrogen are produced naturally during thunder and lightning. They are also produced by internal combustion engines.

N₂ + O₂ $\xrightarrow[(3000°\text{C})]{\text{high temperature}}$ 2NO

Nitrogen oxide then gets oxidized in the atmosphere of nitrogen dioxide.

2NO + O₂ ⟶ 2NO₂

Nitrogen dioxide combines with water to form a mixture of nitrous acid and nitric acid which causes acid rain.

2NO₂ + H₂O ⟶ HNO₂ + HNO₃

Question 4

What are the causes of acid rain?

Answer

The main cause of acid rain is the formation of mineral acids like carbonic acid, nitric acid and sulphuric acid during rains.
Combustion of fossil fuels releases large amounts of sulphur dioxide, oxides of nitrogen and carbon dioxide into the atmosphere. These oxides dissolve in rain water forming mineral acids and causing acid rain.

Question 5

Give the impact of acid rain:

(i) on plants

(ii) on soil

(iii) on water bodies

Answer

Impact of acid rain on:

(i) On plants — Acid rain causes nutrient loss from plants thus damaging their leaves. It also suppresses growth of plants.

(ii) On soil — Acid rain affects soil chemistry. It removes calcium and potassium, both the basic ingredients of soil, thus making it lose its fertility.

(iii) On water bodies — The water of lakes and rivers is gradually becoming acidic due to acid rain, which is affecting aquatic life.

Question 6

How does a scrubber help in reducing the formation of acid rain?

Answer

A scrubber is a device that absorbs gaseous pollutants. It consists of a fine spray of water and gas rising from the stack, which is passed through the scrubber where water absorbs sulphur dioxide. Thus the formation of this constituent of acid rain is reduced. Below diagram shows the working of a scrubber :

Exercise 8C

Question 1

What do you understand by the 'greenhouse effect' ?

Answer

Heating of the earth and its environment due to solar radiations trapped by greenhouse gases such as carbon dioxide and water vapour in the atmosphere is called greenhouse effect.

Question 2

What are greenhouse gases? How are they responsible for global warming?

Answer

Carbon dioxide, water vapour, oxides of nitrogen, methane, ozone, chloro-fluoro carbons, etc. are called greenhouse gases.

The greenhouse gases in the atmosphere allow the infrared rays from the Sun to penetrate the Earth but do not allow the heat radiated by the heated Earth to penetrate back into space. This trapped radiation warms the earth's surface and lower parts of the atmosphere leading to increase in temperature which is known as global warming.

Question 3

State the sources and effects of the following gases:

(i) Carbon dioxide

(ii) Methane

(iii) Water vapour

Answer

(i) Carbon dioxide

Sources :

1. Burning of fossil fuels like coal, natural gas and petroleum.
2. Industrial processes like the manufacture of lime and those seen in fermentation units.
3. Biological decay of plants.
4. Respiration by animals, humans beings and plants.

Effects :

1. Carbon dioxide is a greenhouse gas.
2. It has the largest contribution of about 50% towards greenhouse effect.

(ii) Methane

Sources :

1. Anaerobic decomposition of organic matter in soil, water and sediments.
2. Incomplete combustion of fossil fuels.

Effects :

1. Methane is a greenhouse gas.
2. It has the second largest contribution of about 19% towards greenhouse effect.

(iii) Water vapour

Sources :

1. Burning of hydrocarbons.
2. Evaporation from water bodies.
3. Transpiration from plants.

Effects :

1. Water vapour is a greenhouse gas.
2. It contributes 2% towards greenhouse effect.

Question 4

State the ways of reducing the presence of greenhouse gases.

Answer

The ways of reducing the presence of greenhouse gases are :

1. Minimize use of automobiles : depending on the situation, one can use a bicycle or the public transport system and car pools.
2. Plant more trees to increase the green cover.
3. Avoid burning of dry leaves, wood etc.
4. Avoid smoking
5. Spread awareness about global warming.

Question 5

State the effects of greenhouse gases on the atmosphere.

Answer

Effects of the greenhouse gases on the atmosphere are :

1. Greenhouse gases are crucial for the evolution of life on earth. Without them, solar heat would escape, leaving the planet cold and barren like the moon.
2. They are necessary for evaporation of water and formation of clouds, leading to rain.
3. The increase of greenhouse gases above an optimum level is causing global warming by increasing the earth's temperature.
4. Global warming due to greenhouse gases will lead to rise in sea level, change in rainfall pattern and destruction of forests and wildlife.

Question 6

State the role of a greenhouse in growing plants.

Answer

A greenhouse is a glass structure that is used to grow green plants in colder regions of the earth. The glass structure allows sunlight to reach inside it but does not allow reflected radiations to escape, thus heating up the greenhouse and providing optimum temperature for the plants to grow.

Question 7

Our atmosphere acts as a greenhouse. Explain.

Answer

The atmosphere acts as a greenhouse by trapping and retaining heat energy from the Sun, similar to how a greenhouse retains heat to create a warmer environment for plants.

The greenhouse gases in the atmosphere allow the infrared rays from the Sun to penetrate the Earth but do not allow the heat radiated by the heated Earth to penetrate back into space. Thus, they act like a blanket around the Earth.

By trapping heat, the atmosphere helps to regulate the Earth's temperature and maintain suitable conditions for life.

Question 8

How can we reduce global warming?

Answer

We can reduce global warming in the following ways :

1. Minimize use of automobiles : depending on the situation, one can use a bicycle or the public transport system and car pools.
2. Plant more trees to increase the green cover.
3. Avoid burning of dry leaves, wood etc.
4. Avoid smoking
5. Spread awareness about global warming.

Exercise 8D — Multiple Choice Type

Question 1

Excessive release of carbon dioxide in the atmosphere is the cause of :

1. depletion of ozone
2. formation of polar vortex
3. global warming
4. formation of smog

Answer

global warming

Reason — Carbon dioxide is a greenhouse gas and it traps heat radiated from the sun. An excessive release of carbon dioxide in the atmosphere will lead to rise in temperature, thus leading to global warming.

Question 2

Inhalation of air polluted with carbon monoxide is dangerous because :

1. CO combines with O₂ dissolved in the blood.
2. CO combines with haemoglobin of blood.
3. CO removes water from the body and causes dehydration.
4. CO causes coagulation of proteins in the body.

Answer

CO combines with haemoglobin of blood

Reason — Carbon monoxide is dangerous because it combines with haemoglobin of blood and prevents it from carrying oxygen to different parts of the body.

Question 3

A decrease in the amount of ozone in the stratosphere is called depletion of ozone and it is caused by :

1. UV radiations of the sun
2. excessive use of detergents
3. burning coal and wood
4. Use of CFC compounds

Answer

Use of CFC compounds

Reason — Chlorofluorocarbons (CFCs) entering the atmosphere cause ozone layer depletion.

Question 4

Which is not a greenhouse gas ?

1. Carbon monoxide
2. Methane
3. Nitrous oxide
4. Carbon dioxide

Answer

Carbon monoxide

Reason — Carbon monoxide is not a greenhouse gas. Carbon dioxide, water vapour, oxides of nitrogen, methane, ozone, chloro-fluoro carbons are examples of greenhouse gases.

Question 5

The acid which is not responsible for acid rain is :

1. Formic acid
2. Carbonic acid
3. Sulphuric acid
4. Nitric acid

Answer

Formic acid

Reason — The main cause of acid rain is the formation of mineral acids like carbonic acid, sulphuric acid and nitric acid during rains. Formic acid being an organic acid does not cause acid rain.

Question 6

The branch of chemistry that aims to design product/process which minimize the use and/or generation of pollution is :

1. Industrial chemistry
2. Green chemistry
3. Analytical chemistry
4. Biochemistry

Answer

Green chemistry

Reason — Green chemistry is the study of science dealing with the development of environment friendly technology in order to reduce the use and production of hazardous and poisonous chemicals.

Exercise 8D — Very Short Answer Type

Question 1

Fill in the blanks:

(a) The pollutants such as NO₂, SO₂ and SO₃ dissolved in the moisture of air are the cause of ...............

(b) Excessive release of carbon dioxide in the atmosphere is the cause of ............... effect which produces global warming.

(c) Ozone layer prevents the harmful ............... radiation of the sun to reach the earth.

(d) Decrease in the concentration of ozone in the stratosphere is the cause of the formation of ............... holes.

(e) Ozone depletion is mainly caused by the active ............... atoms generated from CFC in the presence of UV radiation.

Answer

(a) The pollutants such as NO₂, SO₂ and SO₃ dissolved in the moisture of air are the cause of acid rain

(b) Excessive release of carbon dioxide in the atmosphere is the cause of greenhouse effect which produces global warming.

(c) Ozone layer prevents the harmful ultraviolet radiation of the sun to reach the earth.

(d) Decrease in the concentration of ozone in the stratosphere is the cause of the formation of ozone holes.

(e) Ozone depletion is mainly caused by the active chlorine atoms generated from CFC in the presence of UV radiation.

Question 2

Name any two :

(a) natural sources of atmospheric pollution.

(b) gases which are responsible for the formation of acid rain.

(c) particulate pollutants.

Answer

(a) Volcanic eruptions, Decaying vegetation.

(b) Sulphur dioxide, Nitrogen dioxide.

(c) Dust, Smoke.

Question 3

State two effects of ozone depletion.

Answer

Depletion of ozone layer allows the ultraviolet rays of the Sun to reach the Earth. The harmful effects of these ultraviolet rays are :

1. They cause skin cancer.
2. They destroy many organisms necessary for life in general.

Exercise 8D — Short Answer Type

Question 1

What is a pollutant ?

Answer

Pollutants are toxic and otherwise harmful substances that have an undesirable impact on the different components of the environment and life forms.

Question 2

What do you understand by ppm ?

Answer

PPM stands for "parts per million." It is a unit of measurement used to express the concentration of one substance in relation to a million units of another substance. It is commonly used to quantify the presence of pollutants in air.

Question 3

State an advantage of CNG (Compressed Natural Gas).

Answer

CNG (Compressed Natural Gas) is a cleaner and cheaper fuel than petrol or diesel that helps to control carbon monoxide pollution.

Question 4

State how CFCs break ozone layer.

Answer

CFCs are decomposed by ultra-violet rays to highly reactive chlorine, which is produced in its atomic form.

CF₂Cl₂ (g) $\xrightarrow {\text{Ultraviolet rays}}$ CF₂Cl (g) + Cl (g) [free radical]

This free radical [Cl] reacts with ozone forming chlorine monoxide.

Cl(g) + O₃(g) ⟶ ClO (g) + O₂ (g)

This causes depletion of ozone, and chlorine monoxide further reacts with atomic oxygen to produce more free radicals of chlorine.

ClO (g) + O (g) ⟶ Cl (g) + O₂ (g)

Again this free radical [Cl] destroys ozone, and the process continues, giving rise to large scale ozone depletion.

Question 5

State the chemicals responsible for ozone layer depletion.

Answer

Chemicals responsible for the depletion of the ozone layer are :

1. Excessive use of chlorofluoro carbons: CFCs (freons) enter the atmosphere because of their excessive use in solvents, aerosol sprays, propellants, refrigerants and blowing agents for plastic foams.
2. Fuel of planes: When the fuel of planes burns, a large quantity of nitric oxide and other gases is emitted in the atmosphere.
3. The chemicals used in industries like methyl chloride (CH₃Cl), methyl bromide (CH₃Br), carbon tetrachloride (CCl₄) also deplete ozone layer.

Question 6

What is the function of ozone in the atmosphere ?

Answer

The ozone layer acts as a blanket in the atmosphere above 16 km from the earth’s surface. Ozone absorbs the harmful ultra-violet rays coming from the sun and prevents them from reaching the earth’s surface. Thus, it protects the life on earth from harmful effects of ultra-violet rays that can cause skin cancer and destroy many organisms necessary for life in general.

Question 7

What is the effect of the following pollutants on living beings (one in each case)?

(a) Fluorides

(b) Smoke particles

(c) Lead

(d) Nitrogen oxide

(e) Smog

Answer

(a) Fluorides — causes destruction of vegetation.

(b) Smoke particles — get lodged in lungs damaging its functioning.

(c) Lead — damages the nervous and digetsive systems and can cause cancer.

(d) Nitrogen oxide — causes irritation in mucous membranes.

(e) Smog — it induces respiratory troubles.

Question 8

What is smog ? State its damaging effects.

Answer

A pollutant that is a combination of oxides of nitrogen and sulphur and of partially oxidized hydrocarbons and their derivatives produced by industries and automobiles forms a dark, thick, dust and soot laden fog and is known as smog.

Smog is noxious and irritating. It reduces visibility, induces respiratory troubles, and can cause death by suffocation.

Exercise 8D — Descriptive type

Question 1

Give the composition, causes and effects of acid rain.

Answer

Composition
Acid rain results from acids like nitric acid and sulphuric acid present in polluted air. These acids are formed when oxides of nitrogen and sulphur come into contact with rain water.

Causes
Fossil fuels contain compounds of nitrogen and sulphur in addition to carbon. Due to combustion of these fuels, large amounts of sulphur dioxide, oxides of nitrogen and carbon dioxide get discharged into the atmosphere. These oxides dissolve in rainwater to form mineral acids which is the main cause of acid rain.

Effects

1. Acid rain affects soil chemistry. It removes calcium and potassium, both the basic ingredients of soil, thus making it lose its fertility.
2. Acid rain causes nutrient loss from plants thus damaging their leaves.
3. The water of lake and rivers is gradually becoming acidic due to acid rain, which is affecting aquatic life.
4. Acid rain causes extensive damage to building and sculptural materials like marble, limestone, slate, mortar, etc. These materials become pitted and thus weaken mechanically.
5. Acid rain increases corrosion of metals.
6. In high concentrations, it affects a person's breathing. In lower concentrations, it injures lung tissues.

Question 2

How do you propose to control:

(a) carbon monoxide emission

(b) SO_x emission

Answer

(a) Carbon monoxide pollution can be controlled in the following ways:

1. By switching over from internal combustion engines to electrically powered cars.
2. Many pollution control devices are now installed in cars. Most of these devices help reduce pollution by burning gasoline completely. Complete combustion of gasoline produces only carbon dioxide and water vapour.
   2C₈H₁₈ + 25O₂ ⟶ 16CO₂ + 18H₂O
3. By using substitute fuels for gasoline: Natural gas in both compressed (CNG) and liquefied forms (LNG) is now increasingly being used as fuel. Alcohols are other feasible substitutes.
4. By using catalytic convertors :
   Carbon monoxide changes to carbon dioxide in the presence of finely divided platinum as a catalyst.
   $2\text{CO} \xrightarrow[\text{[O]}]{\text{Pt}} \text{CO}_2 + \text{H}_2\text{O}$

(b) The main source of SO_x emissions are industrial and metallurgical processes. Such emissions can be reduced by using a scrubber, a device which absorbs gaseous pollutants. It consists of a fine spray of water. The gas rising from the stack is passed through the scrubber where water absorbs sulphur dioxide.

Question 3

What are the components of clean, dry air?

Answer

Question 4

What is air pollution? How is it caused?

Answer

Air pollution means degradation of air quality due to the increase in the concentration of harmful contaminants that affects human, plant and animal lives.

Causes of air pollution are :

Natural Sources

1. Volcanoes — Volcanoes release large amounts of air pollutants such as carbon monoxide, sulphur dioxide, hydrogen sulphide, chlorine, hydrogen chloride, hydrocarbons and particulates.
2. Decaying vegetation — Microbial action on organic matter in soil releases a pollutant - nitrous oxide.
3. Forest fires release the poisonous gas - carbon monoxide.
4. Winds and dust storms carry particulate matter like sand and dust.

Man-made sources

1. Automobiles use diesel or petrol as fuel. Incomplete combustion of these fuels releases carbon monoxide, sulphur dioxide, hydrocarbons, nitrogen oxide and particulates like lead.
2. Factories release carbon dioxide, sulphur dioxide, nitrogen monoxide and particulates.
3. Industrial processes release different types of air pollutants, depending upon the type of process involved.
4. Decay of crop residue in rural areas is the main source of carbon monoxide and methane.
5. Burning of plastics releases carbon monoxide and other harmful gases.

Question 5

Why is cigarette-smoking harmful?

Answer

Cigarette smoking is harmful to both the smoker and those exposed to secondhand smoke. Below are a few key points highlighting the harmful effects of cigarette smoking :

1. It increases the risk of cancer due to the many toxic carcinogens present in the cigarette smoke.
2. It damages the respiratory system causing respiratory diseases like pneumonia, asthma, chronic bronchitis, etc.
3. It significantly increases the risk of heart diseases and stroke.
4. It affects almost every organ in the body. It weakens the immune system, damages the skin, teeth, and gums, and increases the risk of vision problems.

Question 6

Describe the major air pollutants. How does carbon monoxide pollute our environment?

Answer

Major air pollutants are :

1. Nitrogen oxides (N₂O, NO and NO₂)
2. Hydrocarbons (mainly methane, CH₄)
3. Sulphur oxides (SO₂ and SO₃)
4. Hydrogen sulphide (H₂S)
5. Carbon monoxide (CO)
6. Particulates (small solid particles and liquid droplets)

Carbon monoxide is a highly poisonous gas. If inhaled, it binds with haemoglobin 200 times more strongly than oxygen reducing the blood's ability to carry oxygen by converting haemoglobin into carboxyhaemoglobin. So, even in small quantities, it is fatal and can lead to death.
In atmosphere, Carbon monoxide combines with oxygen to increase carbon dioxide concentration leading to global warming.

Question 7

Explain the effect of sulphur dioxide on the atmosphere.

Answer

Effects of Sulphur dioxide on the atmosphere are :

1. It mixes with smoke and fog to form smog, which is very harmful.
2. It is oxidised by atmospheric oxygen into sulphur trioxide (SO₃) which combines with water to form sulphuric acid. Sulphuric acid (H₂SO₄) is the cause of acid rain.
   2SO₂ (g) + O₂ (g) ⟶ 2SO₃ (g)
   SO₃ (g) + H₂O (l) ⟶ H₂SO₄ (aq)
3. It destroys vegetation and weakens building materials/constructions.
4. It causes headache, vomiting and even death due to respiratory failure.

Question 8

Explain the formation of ozone in the atmosphere.

Answer

Ozone is formed by the action of ultraviolet rays of the Sun on oxygen.

3O₂ (g) ⟶ 2O₃ (g)

High energy ultra violet radiation, help in breaking oxygen molecules into oxygen atoms.

O₂ + UV ⟶ O + O

Oxygen atoms then react with oxygen molecule to form ozone.

O + O₂ ⟶ O₃

Net reaction : 3O₂ + UV ⟶ 2O₃

Question 9

Explain the term 'global warming'. State two ways by which global warming can be reduced.

Answer

The rise in temperature of earth surface due to the heat trapped by greenhouse gases i.e., carbon dioxide, methane, nitrous oxide, ozone, chloro-fluoro-carbon compounds (CFCs) and water vapour present in the atmosphere is called global warming.

Two ways by which global warming can be reduced are :

1. Minimize use of automobiles : depending on the situation, one can use a bicycle or the public transport system and car pools.
2. Plant more trees to increase the green cover.

Question 10

What is the cause of acid rain? Give any two impacts of acid rain.

Answer

Fossil fuels contain compounds of nitrogen and sulphur in addition to carbon. Due to combustion of these fuels, large amounts of sulphur dioxide, oxides of nitrogen and carbon dioxide get discharged into the atmosphere. These oxides dissolve in rainwater to form mineral acids causing acid rain.

Two impacts of acid rain are :

1. Acid rain affects soil chemistry. It removes calcium and potassium, both the basic ingredients of soil, thus making it lose its fertility, which ultimately damages forests. Acid rain causes nutrient loss from plants thus damaging their leaves.
2. The water of lakes and rivers is gradually becoming acidic due to acid rain, which is affecting aquatic life.

Question 11

Describe the methods of saving the ozone layer.

Answer

Methods of saving the ozone layer are :

1. Use of products that contain CFC's to be replaced by alternative products.
2. Substitutes for CFC's have been initiated by scientists and use of HCFC [hydrochlorofluorocarbons] is being considered. Labelling products as CFC free is initiated.
3. International Treaty [Montreal protocol] was also initiated to prevent ozone depletion.

Exercise 8D — Structured/Application/Skill Type

Question 1

Study the following figure and answer the questions that follow :

(a) What phenomenon is depicted in this figure ?

(b) Mention the cause of it and give the reactions involved.

(c) State any two consequences of the above phenomenon.

(d) List the ways by which you can reduce and/or prevent it from happening.

Answer

(a) Acid rain

(b) Fossil fuels contain compounds of nitrogen and sulphur in addition to carbon. Due to combustion of these fuels, large amounts of sulphur dioxide, oxides of nitrogen and carbon dioxide get discharged into the atmosphere. These oxides dissolve in rainwater to form mineral acids causing acid rain.

Formation of sulphuric acid : Sulphur dioxide is oxidised by atmospheric oxygen into sulphur trioxide which combines with water to form sulphuric acid.

2SO₂ (g) + O₂ (g) ⟶ 2SO₃ (g)
SO₃ (g) + H₂O (l) ⟶ H₂SO₄ (aq)

Formation of nitric acid : Oxides of nitrogen are produced naturally during thunder and lightning. They are also produced by internal combustion engines.

N₂ + O₂ $\xrightarrow[(3000°\text{C})]{\text{high temperature}}$ 2NO

Nitrogen oxide then gets oxidized in the atmosphere to nitrogen dioxide.

2NO + O₂ ⟶ 2NO₂

Nitrogen dioxide combines with water to form a mixture of nitrous acid and nitric acid which causes acid rain.

2NO₂ + H₂O ⟶ HNO₂ + HNO₃

(c) Two consequences of acid rain are :

1. Acid rain affects soil chemistry. It removes calcium and potassium, both the basic ingredients of soil, thus making it lose its fertility, which ultimately damages forests. Acid rain causes nutrient loss from plants thus damaging their leaves.
2. The water of lakes and rivers is gradually becoming acidic due to acid rain, which is affecting aquatic life.

(d) The ways to reduce and/or prevent acid rain are :

1. Use coal or oil with low sulphur content to reduce the emission of oxides of sulphur and nitrogen.
2. Use of scrubber (a device which absorbs gaseous pollutants) for removing sulphur dioxide from a smoke stack.