NCERT Textbook: Environmental Chemistry | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


CHEMISTRY 398
UNIT 14
ENVIRONMENTAL CHEMISTRY
After studying this unit, you will be
able to
• • • • • understand the  meaning of
environmental chemistry;
• • • • • define atmospheric pollution, list
reasons for global warming. green
house effect and acid rain;
• • • • • identify causes for ozone layer
depletion and its effects;
• • • • • give reasons for water pollution
and know about international
standards for drinking water;
• • • • • describe causes of soil pollution;
• • • • • suggest and adopt strategies
for control of environmental
pollution;
• • • • • appreciate the importance of green
chemistry in day to day life.
You have already studied about environment in your earlier
classes. Environmental studies deal with the sum of all
social, economical, biological, physical and chemical
interrelations with our surroundings. In this unit the focus
will be on environmental chemistry. Environmental
chemistry deals with the study of the origin, transport,
reactions, effects and fates of chemical species in the
environment. Let us discuss some important aspects of
environmental chemistry.
14.1 ENVIRONMENTAL POLLUTION
Environmental pollution is the effect of undesirable changes
in our surroundings that have harmful effects on plants,
animals and human beings. A substance, which causes
pollution, is known as pollutant. Pollutants can be solid,
liquid or gaseous substances present in greater
concentration than in natural  abundance and are
produced due to human activities or due to natural
happenings. Do you know, an average human being
requires nearly 12-15 times more air than the food. So,
even small amounts of pollutants in the air become
significant compared to similar levels present in the food.
Pollutants can be degradable, like discarded vegetables
which rapidly break down by natural processes. On the
other hand, pollutants which are slowly degradable, remain
in the environment in an unchanged form for many
decades. For example, substances such as dichlorodi-
phenyltrichloroethane (DDT), plastic materials, heavy
metals, many chemicals, nuclear wastes etc., once released
into the environment are difficult to remove. These
The world has achieved brilliance without wisdom, power
without conscience. Ours is a world of nuclear giants and
ethical infants.
© NCERT
not to be republished
Page 2


CHEMISTRY 398
UNIT 14
ENVIRONMENTAL CHEMISTRY
After studying this unit, you will be
able to
• • • • • understand the  meaning of
environmental chemistry;
• • • • • define atmospheric pollution, list
reasons for global warming. green
house effect and acid rain;
• • • • • identify causes for ozone layer
depletion and its effects;
• • • • • give reasons for water pollution
and know about international
standards for drinking water;
• • • • • describe causes of soil pollution;
• • • • • suggest and adopt strategies
for control of environmental
pollution;
• • • • • appreciate the importance of green
chemistry in day to day life.
You have already studied about environment in your earlier
classes. Environmental studies deal with the sum of all
social, economical, biological, physical and chemical
interrelations with our surroundings. In this unit the focus
will be on environmental chemistry. Environmental
chemistry deals with the study of the origin, transport,
reactions, effects and fates of chemical species in the
environment. Let us discuss some important aspects of
environmental chemistry.
14.1 ENVIRONMENTAL POLLUTION
Environmental pollution is the effect of undesirable changes
in our surroundings that have harmful effects on plants,
animals and human beings. A substance, which causes
pollution, is known as pollutant. Pollutants can be solid,
liquid or gaseous substances present in greater
concentration than in natural  abundance and are
produced due to human activities or due to natural
happenings. Do you know, an average human being
requires nearly 12-15 times more air than the food. So,
even small amounts of pollutants in the air become
significant compared to similar levels present in the food.
Pollutants can be degradable, like discarded vegetables
which rapidly break down by natural processes. On the
other hand, pollutants which are slowly degradable, remain
in the environment in an unchanged form for many
decades. For example, substances such as dichlorodi-
phenyltrichloroethane (DDT), plastic materials, heavy
metals, many chemicals, nuclear wastes etc., once released
into the environment are difficult to remove. These
The world has achieved brilliance without wisdom, power
without conscience. Ours is a world of nuclear giants and
ethical infants.
© NCERT
not to be republished
ENVIRONMENTAL CHEMISTRY 399
pollutants cannot be degraded by natural
processes and are harmful to living organisms.
In the process of environmental pollution,
pollutants originate from a source and get
transported by air or water or are dumped into
the soil by human beings.
14.2 ATMOSPHERIC POLLUTION
The atmosphere that surrounds the earth is
not of the same thickness at all heights.  There
are concentric layers of air or regions and each
layer has different density.  The lowest region
of atmosphere in which the human beings
along with other organisms live is called
troposphere. It extends up to the height of
~ 10 km from sea level.  Above the troposphere,
between 10 and 50 km above sea level lies
stratosphere. Troposphere is a turbulent,
dusty zone containing air, much water vapour
and clouds. This is the region of strong air
movement and cloud formation. The
stratosphere, on the other hand, contains
dinitrogen, dioxygen, ozone and little water
vapour.
Atmospheric pollution is generally studied
as tropospheric and stratospheric pollution.
The presence of ozone in the stratosphere
prevents about 99.5 per cent of the sun’s
harmful ultraviolet (UV) radiations from
reaching the earth’s surface and thereby
protecting humans and other animals from its
effect.
14.2.1 Tropospheric Pollution
Tropospheric pollution occurs due to the
presence of undesirable solid or gaseous
particles in the air. The following are the major
gaseous and particulate pollutants present in
the troposphere:
1. Gaseous air pollutants: These are oxides
of sulphur, nitrogen and carbon, hydrogen
sulphide, hydrocarbons, ozone  and other
oxidants.
2. Particulate pollutants: These are dust,
mist, fumes, smoke, smog etc.
1. Gaseous air pollutants
(a) Oxides of Sulphur: Oxides of sulphur
are produced when sulphur containing fossil
fuel is burnt. The most common species,
sulphur dioxide, is a gas that is poisonous to
both animals and plants. It has been reported
that even a low concentration of sulphur
dioxide causes respiratory diseases e.g.,
asthma, bronchitis, emphysema in human
beings. Sulphur dioxide causes irritation to
the eyes, resulting in tears and redness. High
concentration of SO
2
 leads to stiffness of flower
buds which eventually fall off from plants.
Uncatalysed oxidation of sulphur dioxide is
slow. However, the presence of particulate
matter in polluted air catalyses the oxidation
of sulphur dioxide to sulphur trioxide.
2SO
2
 (g) +O
2
 (g) ? 2SO
3
(g)
The reaction can also be promoted by
ozone and hydrogen peroxide.
SO
2
 (g) +O
3
 (g) ? SO
3
(g) + O
2
 (g)
SO
2
(g)  + H
2
O
2
(l) ? H
2
SO
4
(aq)
(b) Oxides of Nitrogen: Dinitrogen and
dioxygen are the main constituents of air.
These gases do not react with each other at a
normal temperature. At high altitudes when
lightning strikes, they combine to form oxides
of nitrogen. NO
2 
is oxidised to nitrate ion, 
3
NO
-
which is washed into soil, where it serves as a
fertilizer. In an automobile engine, (at high
temperature) when fossil fuel is burnt,
dinitrogen and dioxygen combine to yield
significant quantities of nitric oxide (NO) and
nitrogen dioxide ( NO
2 
) as given below:
N
2 
(g) +  O
2
 (g)     
1483K
???? ?
    2NO(g)
NO reacts instantly with oxygen to give NO
2
2NO (g) +  O
2
 (g) ? 2NO
2
 (g)
Rate of production of NO
2
 is faster when
nitric oxide reacts with ozone in the
stratosphere.
NO (g) +  O
3
 (g) ? NO
2
 (g) + O
2
 (g)
The irritant red haze in the traffic and
congested places is due to oxides of nitrogen.
Higher concentrations of NO
2
 damage the
leaves of plants and retard the rate of
photosynthesis.  Nitrogen dioxide is a lung
irritant that can lead to an acute respiratory
disease in children. It is toxic to living tissues
also. Nitrogen dioxide is also harmful to
various textile fibres and metals.
© NCERT
not to be republished
Page 3


CHEMISTRY 398
UNIT 14
ENVIRONMENTAL CHEMISTRY
After studying this unit, you will be
able to
• • • • • understand the  meaning of
environmental chemistry;
• • • • • define atmospheric pollution, list
reasons for global warming. green
house effect and acid rain;
• • • • • identify causes for ozone layer
depletion and its effects;
• • • • • give reasons for water pollution
and know about international
standards for drinking water;
• • • • • describe causes of soil pollution;
• • • • • suggest and adopt strategies
for control of environmental
pollution;
• • • • • appreciate the importance of green
chemistry in day to day life.
You have already studied about environment in your earlier
classes. Environmental studies deal with the sum of all
social, economical, biological, physical and chemical
interrelations with our surroundings. In this unit the focus
will be on environmental chemistry. Environmental
chemistry deals with the study of the origin, transport,
reactions, effects and fates of chemical species in the
environment. Let us discuss some important aspects of
environmental chemistry.
14.1 ENVIRONMENTAL POLLUTION
Environmental pollution is the effect of undesirable changes
in our surroundings that have harmful effects on plants,
animals and human beings. A substance, which causes
pollution, is known as pollutant. Pollutants can be solid,
liquid or gaseous substances present in greater
concentration than in natural  abundance and are
produced due to human activities or due to natural
happenings. Do you know, an average human being
requires nearly 12-15 times more air than the food. So,
even small amounts of pollutants in the air become
significant compared to similar levels present in the food.
Pollutants can be degradable, like discarded vegetables
which rapidly break down by natural processes. On the
other hand, pollutants which are slowly degradable, remain
in the environment in an unchanged form for many
decades. For example, substances such as dichlorodi-
phenyltrichloroethane (DDT), plastic materials, heavy
metals, many chemicals, nuclear wastes etc., once released
into the environment are difficult to remove. These
The world has achieved brilliance without wisdom, power
without conscience. Ours is a world of nuclear giants and
ethical infants.
© NCERT
not to be republished
ENVIRONMENTAL CHEMISTRY 399
pollutants cannot be degraded by natural
processes and are harmful to living organisms.
In the process of environmental pollution,
pollutants originate from a source and get
transported by air or water or are dumped into
the soil by human beings.
14.2 ATMOSPHERIC POLLUTION
The atmosphere that surrounds the earth is
not of the same thickness at all heights.  There
are concentric layers of air or regions and each
layer has different density.  The lowest region
of atmosphere in which the human beings
along with other organisms live is called
troposphere. It extends up to the height of
~ 10 km from sea level.  Above the troposphere,
between 10 and 50 km above sea level lies
stratosphere. Troposphere is a turbulent,
dusty zone containing air, much water vapour
and clouds. This is the region of strong air
movement and cloud formation. The
stratosphere, on the other hand, contains
dinitrogen, dioxygen, ozone and little water
vapour.
Atmospheric pollution is generally studied
as tropospheric and stratospheric pollution.
The presence of ozone in the stratosphere
prevents about 99.5 per cent of the sun’s
harmful ultraviolet (UV) radiations from
reaching the earth’s surface and thereby
protecting humans and other animals from its
effect.
14.2.1 Tropospheric Pollution
Tropospheric pollution occurs due to the
presence of undesirable solid or gaseous
particles in the air. The following are the major
gaseous and particulate pollutants present in
the troposphere:
1. Gaseous air pollutants: These are oxides
of sulphur, nitrogen and carbon, hydrogen
sulphide, hydrocarbons, ozone  and other
oxidants.
2. Particulate pollutants: These are dust,
mist, fumes, smoke, smog etc.
1. Gaseous air pollutants
(a) Oxides of Sulphur: Oxides of sulphur
are produced when sulphur containing fossil
fuel is burnt. The most common species,
sulphur dioxide, is a gas that is poisonous to
both animals and plants. It has been reported
that even a low concentration of sulphur
dioxide causes respiratory diseases e.g.,
asthma, bronchitis, emphysema in human
beings. Sulphur dioxide causes irritation to
the eyes, resulting in tears and redness. High
concentration of SO
2
 leads to stiffness of flower
buds which eventually fall off from plants.
Uncatalysed oxidation of sulphur dioxide is
slow. However, the presence of particulate
matter in polluted air catalyses the oxidation
of sulphur dioxide to sulphur trioxide.
2SO
2
 (g) +O
2
 (g) ? 2SO
3
(g)
The reaction can also be promoted by
ozone and hydrogen peroxide.
SO
2
 (g) +O
3
 (g) ? SO
3
(g) + O
2
 (g)
SO
2
(g)  + H
2
O
2
(l) ? H
2
SO
4
(aq)
(b) Oxides of Nitrogen: Dinitrogen and
dioxygen are the main constituents of air.
These gases do not react with each other at a
normal temperature. At high altitudes when
lightning strikes, they combine to form oxides
of nitrogen. NO
2 
is oxidised to nitrate ion, 
3
NO
-
which is washed into soil, where it serves as a
fertilizer. In an automobile engine, (at high
temperature) when fossil fuel is burnt,
dinitrogen and dioxygen combine to yield
significant quantities of nitric oxide (NO) and
nitrogen dioxide ( NO
2 
) as given below:
N
2 
(g) +  O
2
 (g)     
1483K
???? ?
    2NO(g)
NO reacts instantly with oxygen to give NO
2
2NO (g) +  O
2
 (g) ? 2NO
2
 (g)
Rate of production of NO
2
 is faster when
nitric oxide reacts with ozone in the
stratosphere.
NO (g) +  O
3
 (g) ? NO
2
 (g) + O
2
 (g)
The irritant red haze in the traffic and
congested places is due to oxides of nitrogen.
Higher concentrations of NO
2
 damage the
leaves of plants and retard the rate of
photosynthesis.  Nitrogen dioxide is a lung
irritant that can lead to an acute respiratory
disease in children. It is toxic to living tissues
also. Nitrogen dioxide is also harmful to
various textile fibres and metals.
© NCERT
not to be republished
CHEMISTRY 400
(c) Hydrocarbons: Hydrocarbons are
composed of hydrogen and carbon only and
are formed by incomplete combustion of fuel
used in automobiles. Hydrocarbons are
carcinogenic, i.e., they cause cancer. They
harm plants by causing ageing, breakdown of
tissues and shedding of leaves, flowers and
twigs.
(d)  Oxides of Carbon
(i ) Carbon monoxide: Carbon monoxide (CO)
is one of the most serious air pollutants. It is a
colourless and odourless gas, highly
poisonous to living beings because of its ability
to block the delivery of oxygen to the organs
and tissues. It is produced as a result of
incomplete combustion of carbon.  Carbon
monoxide is mainly released into the air by
automobile exhaust.  Other sources, which
produce CO, involve incomplete combustion
of coal, firewood, petrol, etc. The number of
vehicles has been increasing over the years all
over the world. Many vehicles are poorly
maintained and several have inadequate
pollution control equipments resulting in the
release of greater amount of carbon monoxide
and other polluting gases.  Do you know why
carbon monoxide is poisonous?  It binds to
haemoglobin to form carboxyhaemoglobin,
which is about 300 times more stable than the
oxygen-haemoglobin complex. In blood, when
the concentration of carboxyhaemoglobin
reaches about 3–4 per cent, the oxygen
carrying capacity of blood is greatly
reduced. This oxygen deficiency, results into
headache, weak eyesight, nervousness and
cardiovascular disorder. This is the reason why
people are advised not to smoke. In pregnant
women who have the habit of smoking the
increased CO level in blood may induce
premature birth, spontaneous abortions and
deformed babies.
(ii) Carbon dioxide: Carbon dioxide (CO
2
) is
released into the atmosphere by respiration,
burning of fossil fuels for energy, and by
decomposition of limestone during the
manufacture of cement. It is also emitted
during volcanic eruptions. Carbon dioxide gas
is confined to troposphere only. Normally it
forms about 0.03 per cent by volume of the
atmosphere. With the increased use of fossil
fuels, a large amount of carbon dioxide gets
released into the atmosphere. Excess of CO
2
in the air is removed by green plants and this
maintains an appropriate level of CO
2
 in the
atmosphere. Green plants require CO
2
 for
photosynthesis and they, in turn, emit oxygen,
thus maintaining the delicate balance. As you
know, deforestation and burning of fossil fuel
increases the CO
2
 level and disturb the balance
in the atmosphere. The increased amount of
CO
2
 in the air is mainly responsible for global
warming.
Global Warming and Greenhouse Effect
About 75 %  of the solar energy reaching the
earth is absorbed by the earth’s surface, which
increases its temperature. The rest of the heat
radiates back to the atmosphere. Some of the
heat is trapped by gases such as carbon
dioxide, methane, ozone, chlorofluorocarbon
compounds (CFCs) and water vapour in the
atmosphere. Thus, they add to the heating of
the atmosphere. This causes global warming.
We all know that in cold places flowers,
vegetables and fruits are grown in glass
covered areas called greenhouse. Do you
know that we humans also live in a
greenhouse? Of course, we are not surrounded
by glass but a blanket of air called the
atmosphere, which has kept the temperature
on earth constant for centuries. But it is now
undergoing change, though slowly. Just as
the glass in a greenhouse holds the sun’s
warmth inside,  atmosphere traps the sun’s
heat near the earth’s surface and keeps it
warm. This is called natural greenhouse
effect because it maintains the temperature
and makes the earth perfect for life. In a
greenhouse, solar radiations pass through
the transparent glass and heat up the soil
and the plants. The warm soil and plants emit
infrared radiations. Since glass is opaque to
infrared radiations (thermal region), it partly
reflects and partly absorbs these radiations.
This mechanism keeps the energy of the
sun trapped in the greenhouse. Similarly,
carbon dioxide molecules also trap heat as
they are transparent to sunlight but not
to the heat radiation. If the amount of
© NCERT
not to be republished
Page 4


CHEMISTRY 398
UNIT 14
ENVIRONMENTAL CHEMISTRY
After studying this unit, you will be
able to
• • • • • understand the  meaning of
environmental chemistry;
• • • • • define atmospheric pollution, list
reasons for global warming. green
house effect and acid rain;
• • • • • identify causes for ozone layer
depletion and its effects;
• • • • • give reasons for water pollution
and know about international
standards for drinking water;
• • • • • describe causes of soil pollution;
• • • • • suggest and adopt strategies
for control of environmental
pollution;
• • • • • appreciate the importance of green
chemistry in day to day life.
You have already studied about environment in your earlier
classes. Environmental studies deal with the sum of all
social, economical, biological, physical and chemical
interrelations with our surroundings. In this unit the focus
will be on environmental chemistry. Environmental
chemistry deals with the study of the origin, transport,
reactions, effects and fates of chemical species in the
environment. Let us discuss some important aspects of
environmental chemistry.
14.1 ENVIRONMENTAL POLLUTION
Environmental pollution is the effect of undesirable changes
in our surroundings that have harmful effects on plants,
animals and human beings. A substance, which causes
pollution, is known as pollutant. Pollutants can be solid,
liquid or gaseous substances present in greater
concentration than in natural  abundance and are
produced due to human activities or due to natural
happenings. Do you know, an average human being
requires nearly 12-15 times more air than the food. So,
even small amounts of pollutants in the air become
significant compared to similar levels present in the food.
Pollutants can be degradable, like discarded vegetables
which rapidly break down by natural processes. On the
other hand, pollutants which are slowly degradable, remain
in the environment in an unchanged form for many
decades. For example, substances such as dichlorodi-
phenyltrichloroethane (DDT), plastic materials, heavy
metals, many chemicals, nuclear wastes etc., once released
into the environment are difficult to remove. These
The world has achieved brilliance without wisdom, power
without conscience. Ours is a world of nuclear giants and
ethical infants.
© NCERT
not to be republished
ENVIRONMENTAL CHEMISTRY 399
pollutants cannot be degraded by natural
processes and are harmful to living organisms.
In the process of environmental pollution,
pollutants originate from a source and get
transported by air or water or are dumped into
the soil by human beings.
14.2 ATMOSPHERIC POLLUTION
The atmosphere that surrounds the earth is
not of the same thickness at all heights.  There
are concentric layers of air or regions and each
layer has different density.  The lowest region
of atmosphere in which the human beings
along with other organisms live is called
troposphere. It extends up to the height of
~ 10 km from sea level.  Above the troposphere,
between 10 and 50 km above sea level lies
stratosphere. Troposphere is a turbulent,
dusty zone containing air, much water vapour
and clouds. This is the region of strong air
movement and cloud formation. The
stratosphere, on the other hand, contains
dinitrogen, dioxygen, ozone and little water
vapour.
Atmospheric pollution is generally studied
as tropospheric and stratospheric pollution.
The presence of ozone in the stratosphere
prevents about 99.5 per cent of the sun’s
harmful ultraviolet (UV) radiations from
reaching the earth’s surface and thereby
protecting humans and other animals from its
effect.
14.2.1 Tropospheric Pollution
Tropospheric pollution occurs due to the
presence of undesirable solid or gaseous
particles in the air. The following are the major
gaseous and particulate pollutants present in
the troposphere:
1. Gaseous air pollutants: These are oxides
of sulphur, nitrogen and carbon, hydrogen
sulphide, hydrocarbons, ozone  and other
oxidants.
2. Particulate pollutants: These are dust,
mist, fumes, smoke, smog etc.
1. Gaseous air pollutants
(a) Oxides of Sulphur: Oxides of sulphur
are produced when sulphur containing fossil
fuel is burnt. The most common species,
sulphur dioxide, is a gas that is poisonous to
both animals and plants. It has been reported
that even a low concentration of sulphur
dioxide causes respiratory diseases e.g.,
asthma, bronchitis, emphysema in human
beings. Sulphur dioxide causes irritation to
the eyes, resulting in tears and redness. High
concentration of SO
2
 leads to stiffness of flower
buds which eventually fall off from plants.
Uncatalysed oxidation of sulphur dioxide is
slow. However, the presence of particulate
matter in polluted air catalyses the oxidation
of sulphur dioxide to sulphur trioxide.
2SO
2
 (g) +O
2
 (g) ? 2SO
3
(g)
The reaction can also be promoted by
ozone and hydrogen peroxide.
SO
2
 (g) +O
3
 (g) ? SO
3
(g) + O
2
 (g)
SO
2
(g)  + H
2
O
2
(l) ? H
2
SO
4
(aq)
(b) Oxides of Nitrogen: Dinitrogen and
dioxygen are the main constituents of air.
These gases do not react with each other at a
normal temperature. At high altitudes when
lightning strikes, they combine to form oxides
of nitrogen. NO
2 
is oxidised to nitrate ion, 
3
NO
-
which is washed into soil, where it serves as a
fertilizer. In an automobile engine, (at high
temperature) when fossil fuel is burnt,
dinitrogen and dioxygen combine to yield
significant quantities of nitric oxide (NO) and
nitrogen dioxide ( NO
2 
) as given below:
N
2 
(g) +  O
2
 (g)     
1483K
???? ?
    2NO(g)
NO reacts instantly with oxygen to give NO
2
2NO (g) +  O
2
 (g) ? 2NO
2
 (g)
Rate of production of NO
2
 is faster when
nitric oxide reacts with ozone in the
stratosphere.
NO (g) +  O
3
 (g) ? NO
2
 (g) + O
2
 (g)
The irritant red haze in the traffic and
congested places is due to oxides of nitrogen.
Higher concentrations of NO
2
 damage the
leaves of plants and retard the rate of
photosynthesis.  Nitrogen dioxide is a lung
irritant that can lead to an acute respiratory
disease in children. It is toxic to living tissues
also. Nitrogen dioxide is also harmful to
various textile fibres and metals.
© NCERT
not to be republished
CHEMISTRY 400
(c) Hydrocarbons: Hydrocarbons are
composed of hydrogen and carbon only and
are formed by incomplete combustion of fuel
used in automobiles. Hydrocarbons are
carcinogenic, i.e., they cause cancer. They
harm plants by causing ageing, breakdown of
tissues and shedding of leaves, flowers and
twigs.
(d)  Oxides of Carbon
(i ) Carbon monoxide: Carbon monoxide (CO)
is one of the most serious air pollutants. It is a
colourless and odourless gas, highly
poisonous to living beings because of its ability
to block the delivery of oxygen to the organs
and tissues. It is produced as a result of
incomplete combustion of carbon.  Carbon
monoxide is mainly released into the air by
automobile exhaust.  Other sources, which
produce CO, involve incomplete combustion
of coal, firewood, petrol, etc. The number of
vehicles has been increasing over the years all
over the world. Many vehicles are poorly
maintained and several have inadequate
pollution control equipments resulting in the
release of greater amount of carbon monoxide
and other polluting gases.  Do you know why
carbon monoxide is poisonous?  It binds to
haemoglobin to form carboxyhaemoglobin,
which is about 300 times more stable than the
oxygen-haemoglobin complex. In blood, when
the concentration of carboxyhaemoglobin
reaches about 3–4 per cent, the oxygen
carrying capacity of blood is greatly
reduced. This oxygen deficiency, results into
headache, weak eyesight, nervousness and
cardiovascular disorder. This is the reason why
people are advised not to smoke. In pregnant
women who have the habit of smoking the
increased CO level in blood may induce
premature birth, spontaneous abortions and
deformed babies.
(ii) Carbon dioxide: Carbon dioxide (CO
2
) is
released into the atmosphere by respiration,
burning of fossil fuels for energy, and by
decomposition of limestone during the
manufacture of cement. It is also emitted
during volcanic eruptions. Carbon dioxide gas
is confined to troposphere only. Normally it
forms about 0.03 per cent by volume of the
atmosphere. With the increased use of fossil
fuels, a large amount of carbon dioxide gets
released into the atmosphere. Excess of CO
2
in the air is removed by green plants and this
maintains an appropriate level of CO
2
 in the
atmosphere. Green plants require CO
2
 for
photosynthesis and they, in turn, emit oxygen,
thus maintaining the delicate balance. As you
know, deforestation and burning of fossil fuel
increases the CO
2
 level and disturb the balance
in the atmosphere. The increased amount of
CO
2
 in the air is mainly responsible for global
warming.
Global Warming and Greenhouse Effect
About 75 %  of the solar energy reaching the
earth is absorbed by the earth’s surface, which
increases its temperature. The rest of the heat
radiates back to the atmosphere. Some of the
heat is trapped by gases such as carbon
dioxide, methane, ozone, chlorofluorocarbon
compounds (CFCs) and water vapour in the
atmosphere. Thus, they add to the heating of
the atmosphere. This causes global warming.
We all know that in cold places flowers,
vegetables and fruits are grown in glass
covered areas called greenhouse. Do you
know that we humans also live in a
greenhouse? Of course, we are not surrounded
by glass but a blanket of air called the
atmosphere, which has kept the temperature
on earth constant for centuries. But it is now
undergoing change, though slowly. Just as
the glass in a greenhouse holds the sun’s
warmth inside,  atmosphere traps the sun’s
heat near the earth’s surface and keeps it
warm. This is called natural greenhouse
effect because it maintains the temperature
and makes the earth perfect for life. In a
greenhouse, solar radiations pass through
the transparent glass and heat up the soil
and the plants. The warm soil and plants emit
infrared radiations. Since glass is opaque to
infrared radiations (thermal region), it partly
reflects and partly absorbs these radiations.
This mechanism keeps the energy of the
sun trapped in the greenhouse. Similarly,
carbon dioxide molecules also trap heat as
they are transparent to sunlight but not
to the heat radiation. If the amount of
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ENVIRONMENTAL CHEMISTRY 401
carbon dioxide crosses the delicate proportion
of 0.03 per cent, the natural greenhouse
balance may get disturbed. Carbon dioxide is
the major contributor to global warming.
Besides carbon dioxide, other greenhouse
gases are methane, water vapour, nitrous
oxide, CFCs and ozone. Methane is produced
naturally when vegetation is burnt, digested
or rotted in the absence of oxygen. Large
amounts of methane are released in paddy
fields, coal mines, from rotting garbage dumps
and by fossil fuels. Chlorofluorocarbons (CFCs)
are man-made industrial chemicals used in
air conditioning etc. CFCs are also damaging
the ozone layer (Section 14.2.2). Nitrous oxide
occurs naturally in the environment. In recent
years, their quantities have increased
significantly due to the use of chemical
fertilizers and the burning of fossil fuels. If
these trends continue, the average global
temperature will increase to a level which may
lead to melting of polar ice caps and flooding
of low lying areas all over the earth. Increase
in the global temperature increases the
incidence of infectious diseases like dengue,
malaria, yellow fever, sleeping sickness etc.
Acid rain
We are aware that normally rain water has a
pH of 5.6 due to the presence of H
+
 ions formed
by the reaction of rain water with carbon
Fig. 14.1  Acid deposition
Think it Over
What can we do to reduce the rate of global
warming?
If burning of fossil fuels, cutting down
forests and trees add to greenhouse gases
in the atmosphere, we must find ways to
use these just efficiently and judiciously.
One of the simple things which we can do
to reduce global warming is to minimise the
use of automobiles. Depending upon the
situation, one can use bicycle, public
transport system, or go for carpool. We
should plant more trees to increase the
green cover. Avoid burning of dry leaves,
wood etc.  It is illegal to smoke in public
places and work places, because it is
harmful not only for the one who is smoking
but also for others, and therefore, we should
avoid it. Many people do not understand
the greenhouse effect and the global
warming. We can help them by sharing the
information that we have.
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