ENVIRONMENTAL CHEMISTRY

Chapter 14

ENVIRONMENTAL CHEMISTRY 1:

     THE ATMOSPHERE

SOLVED TEST YOURSELFS

Test Yourself 14.1 (COMPOSITION OF ATMOSPHERE)

  1. No. i. What do you mean by atmosphere?

Answer. Atmosphere is the envelope of different gases around-the Earth. It extends continuously from the Earth’s surface outwards without any boundary. About 99% of atmospheric mass lies within 30 kilometers of the surface and 75% lies within the lowest 11 kilometers.

  1. No. ii. What is difference between atmosphere and environment?

Answer. Environment refers to biosphere and hydrosphere whereas atmosphere refers to air and its components. Atmosphere is related to weather and environment is concerned with climate.

  1. No.iii. Name the major constituents of troposphere.

Answer. Troposphere is the lowest region of the atmosphere. It extends up to 12km upwards from the Earth’s surface. It consists of different gases. The major gases present in troposphere are nitrogen (78%) and oxygen (21%). These two gases comprise 99% by valance of the Earth’s atmosphere.

 No.iv. How the temperature of atmosphere is maintained?

Answer. Solar radiations reach up to the Earth and are absorbed by it. Then, this solar energy is (visible light) radiated as heat energy (infrared radiations) by the Earth. Keep in mind, wave length of visible light is shorter than that of infrared radiations radiated by the Earth. Due to this, visible light can pass through a medium from which infrared radiations cannot pass.

Carbon dioxide and water vapours present in the atmosphere are good infrared radiations absorber. Both of these gases allow the visible light to pass through it that reaches up to the Earth. But when Earth radiates the heat the form of infrared radiations, these gases absorb much of the outgoing infrared radiation that keeps the atmosphere warm. Thus presence of carbon dioxide and water vapours is responsible for maintaining the temperature of the atmosphere.

  1. No. v. Where the ozone layer exists?

Answer. Ozone gas is found in stratosphere. Stratosphere region lies above the troposphere region and covers from 12 to 50 km of the atmosphere. It is considered that in the middle of the stratosphere formation of ozone takes place by the combination of oxygen molecule and atomic oxygen such as.

O2 + O ————> O3

Hence, formation of ozone in this region results in formation of ozone layer. Thus ozone layer exists about 25km to 30km away from the Earth’s surface.

  1. No.vi. Why the temperature of upper stratosphere is higher?

Answer. Stratosphere is further divided into three layers upper, middle and lower region. The presence of ozone in stratosphere region is responsible for the rise of temperature due to absorption of radiations. Within this region, temperature increases as altitude increases, such as lower layer temperature is about -58°C and upper layer is about 2°C. It is because ozone’s upper layer absorbs high energy ultraviolet radiation coming from the son while lower layer receives very low UV- radiations.

Test Yourself 14.2 (POLLUTANTS)

  1. No. i. What do you mean by an air pollutant?

Answer. The harmful substances present in air are called air pollutants. Even a beneficial substance beyond a specific concentration may be harmful. Air pollutants change the weather, badly affects the human health, damage the plants and destroy buildings.

  1. No. ii. Name three primary air pollutants.

Answer. Primary pollutants are the waste or exhaust products driven out because of combustion of fossil fuels and organic matter. These are oxides of sulphur (SO2 and SO3); oxides of carbon (CO2 and CO); oxides of nitrogen (especially nitric oxide NO) hydrocarbon (CH4); ammonia and compounds of fluorine.

  1. No. iii. Identify as primary or, secondary air pollutant, SO2, CH4, HNO3, NH3, H2SO4 O3.

Answer. Primary air pollutant are; SO2, CH4, HNO3

Secondary air pollutant is: NH3, H2SO4 O3.

  1. No.iv. Why COs is called a greenhouse gas?

Answer. The CO2 in the atmosphere acts like a glass wall of the greenhouse. It has the capability of absorbing some of the infrared radiations emitted by the Earth. Hence, increased concentration of CO2 layer absorbs the infrared radiations emitted by the Earth’s surface and prevents heat energy escaping from the atmosphere. As a result, less heat energy is lost from the atmosphere. Therefore, the average temperature of the atmosphere gradually increases. So CO2 is responsible for increasing the atmospheric temperature. This is called the greenhouse effect. Therefore, CO2 is called as greenhouse gas.

  1. No. v. Why the flood risks are increasing?

Answer. Accumulation of carbon dioxide in air is resulting in increasing atmospheric temperature about 0.05 °C every year. It is causing major changes in atmospheric circulation resulting in changing weather patterns. It melts glaciers and snow caps that are increasing flood risks and intense tropical cyclones.

  1. No.vi. Comment: Why burning in open air is preferred?

Answer. Combustion in limited supply of, oxygen produces CO is air due to incomplete burning. Being colorless and odourless, its presence cannot be noticed easily and readily. When inhaled, it binds with the hemoglobin most strongly than that of oxygen. Thus, hindering the supply of oxygen in body. Exposure to higher concentration of CO causes headache and fatigue. If inhaled for a longer time it results in breathing difficulties and ultimately death. To avoid incomplete combustion burning is prefered in open air. In open air complete combustion takes place that produces CO2 instead of CO.

  1. No.vii. How sulphur containing compounds are emitted naturally?

Answer. Naturally occurring sulphur containing compounds are emitted in the bacterial decay of organic matter, in volcanic gases and forest fires. But the concentration of sulphur containing compounds in the atmosphere because of natural sources is very small as compared to the concentration of those compounds emitted by fossil fuel combustion in automobiles and industrial units.

  1. No. viii. How combustion of fossil fuels in internal combustion engine produces oxides of nitrogen?

Answer. Combustion of fossil fuels in internal combustion engines in thermal power stations and factories where huge amount of coal is burnt NO is formed by the direct combination of nitrogen and oxygen as shown in figure.

N2 + O2           2NO

However, it quickly reacts with air to form nitrogen dioxide. NO is highly toxic gas.

2NO + O2 ——— > 2NO

Formation of NO & NO2 on combustion of fossil fuel and causing air pollution

Mixture of these gases represented as NOX enters in the air through automobile exhaust and chimneys of thermal power stations and factories. It irritates breathing passage.

Test yourself 14.3 (ACID RAIN AND ITS EFFECTS)

  1. No. i. How acid rain is produced?

Answer. Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOX to HNO2 and HNO3. Normal rain water is weakly acidic because it consists of dissolved CO2 of the air. Its pH is about 5.6 to 6. But

rain water on dissolving air pollutants (acids) becomes more acidic and its pH reduces to 4. Thus, acid rain is formed on dissolving acidic air pollutants such as sulphur dioxide and nitrogen dioxide by rain water.

  1. No. ii. Why acid rain damages buildings?

Answer. Acid rain attacks the calcium carbonate present in the marble and limestone of buildings and monuments. Thus, these buildings are getting dull and eroded day by day because of acid rain attack.

  1. No.iii. How aquatic life is affected by acid rain?

Answer. Acid rain on soil and rocks leaches heavy metals (Al, Hg, Pb, Cr, etc) with it and discharges these metals into rivers and lakes. Aquatic life present in lakes, suffers because of high concentration of these metals. Especially high concentration of aluminum metal clogs the fish gills. It causes suffocation and ultimately death of fish. In this way aquatic life is affected by acid rain as shown in figure.

  1. No.iv. Plants are dying day by day comment.

Answer. Acid rain increases the acidity of the soil. Many crops and plants cannot grow properly in such soil. Even old trees are being affected due to acidity of soil. Their growth is retarded. They get dry and die. Acid rain directly damages the leaves of trees and plants, thus limiting their growth. Depending upon the severity of the damage, plants growth can be hampered. Plants capability to resist cold or diseases reduces and ultimately they die as shown in figure.

Effect of acid rain on plants

Test Yourself 14.4 (OZONE DEPLETION AND ITS EFCTS)

  1. No. i. Justify, ozone is beneficial for human kind.

Answer. Ozone layer surrounds the globe and protects Earth like a shield from harmful ultraviolet radiations of sunlight. Otherwise, ultraviolet radiations would reach to the Earth and cause skin cancer. Thus ozone layer in stratosphere is beneficial for life on the Earth.

  1. No. ii. Why ozone is depleting in atmosphere?

Answer. The ozone molecule absorbs solar radiations and dissociate readily,   i.e.,   self   dissociation   of   ozone   takes   place.   However chlorofluorocarbons (CFCs) (used as refrigerants in air conditioners! and refrigerators) are major cause of depletion of ozone layer. These compounds   leak   in   one   way   or   other;   escape   and   diffuse   to stratosphere. There ultraviolet radiations break the C-C1 bond in CFCl3 and generate chlorine free radicals as

 

CFCl3             CFCl2+Cr

These free radicals are very reactive. They react with ozone to form oxygen as;

O3+ Cl       O2 + OCl·

OCl·           O· + Cl·

O· + O·         O2

A single chlorine free radical released by the decomposition of CFCs is capable of destroying up to 1 lac ozone molecules.

  1. No. iii. What do you mean by ozone hole?

Answer. Ozone hole means the region of atmosphere in which ozone layer has been depleted. Signs of ozone depletion were first noticed over Antarctica in 1980s. since 1990s depletion have also been recorded over the Arctic as well.

  1. No.iv. Where ozone layer is found?

Answer. Ozone is an allotropic form of oxygen consisting of three oxygen atoms. It is formed in atmosphere by the, association of an oxygen atom with an oxygen molecule in the mid of stratosphere.

Ozone is present throughout the atmosphere. But its maximum concentration called ozone layer lies in stratosphere region about 25 to 30 km away from the Earth’s surface. This layer surrounds the globe and protects Earth like a shield from harmful ultraviolet radiations of sunlight as shown in figure.

Ultraviolet radiations absorbed by ozone layer

 MULTIPLE CHOICE QUESTIONS

Put a () on the correct answer

  1. about 99% atmosphere’s mass lies within:
  2. 30km b. 35km
  3. 15 km               d. 11 km
  1. Depending upon temperature   variation,   atmosphere is divided into how many regions
  2. One b. two
  3. Three             d. four
  1. Just above the Earth’s surface is:
  2. mesosphere b. stratosphere
  3. thermosphere          d. troposphere
  1. A group of gases that maintains temperature of atmosphere is;
  2. carbon dioxide and water vapours
  3. nitrogen and carbon dioxide,
  4. oxygen and water vapours,
  5. nitrogen and oxygen.
  1. The Earth’s atmosphere is getting hotter because of
  2. increasing concentration of CO.
  3. increasing concentration of CO2.
  4. increasing concentration of O3.
  5. increasing concentration of SO2.
  1. Which one of the followings is not a Greenhouse Effect?
  2. increasing atmospheric temperature
  3. increasing food chains
  4. increasing flood risks
  5. increasing sea-level
  1. Normally rain water is weakly acidic because of
  2. SO3 gas. b. CO2 gas.
  3. SO2 gas.             d. NO2 gas.
  1. Buildings are being damaged by acid rain because it attacks:
  2. calcium sulphate b. calcium nitrate,
  3. calcium carbonate   d. calcium oxalate.
  1. Acid rain affects the aquatic life by clogging fish gills because of:
  2. lead, metal. b. chromium metal,
  3. mercury metal. d. aluminum metal.
  1. Ozone is beneficial for us as it:
  2. absorbs infra-red radiations.
  3. absorbs ultra-violet radiations,
  4. absorbs chlorofluorocarbons.
  5. absorbs air pollutants.
  1. Which one of the following, is not an air pollutant?
  2. nitrogen  b. carbon monoxide
  3. nitrogen dioxide  d. ozone
  1. Iron and steel structures are, damaged by
  2. carbon monoxide. b. sulphur dioxide,
  3. methane.                d. carbon dioxide
  1. Infrared radiations emitted by the Earth dm absorbed by:
  2. CO2andH2O b. N2and02
  3. CO2andN2.    d. O2 and CO2.
  1. Global warming causes rising of the sea level. The cause of global warming is:
  2. CO2 gas. b. SO2 gas.
  3. NOX gases.                 d. O3 gas.
  1. Which gas protects the Earth’s surface from ultraviolet radiations?
  2. CO2 b. CO
  3. N2 d. O3
  1. Effects of ozone depletion are following except the one;
  2. increases infectious diseases,
  3. increases crops production,
  4. can cause skin cancer,
  5. can cause climatic changes.
  1. Which one of these pollutants is not found in car exhaust fumes?
  2. CO b. O3
  3. NO2 d. SO2
  1. The process by which atmospheric nitrogen is turned into nitrates in the soil is called:
  2. nitration. b. fixing
  3. oxidation.              d. adduction.
  1. Global warming is because of:
  2. absorption of infrared radiations emitted by the Earth’s

surface.

  1. absorption of infrared radiations coming from the Sun.
  2. absorption of ultraviolet radiation coming from the Sun.

d,   emission of ultraviolet radiation from the Earth’s surface.

  1. Carbon monoxide i$ harmful to us because:
  2. it paralyses the lungs. .
  3. it damages lungs tissues.
  4. it reduces oxygen carrying ability of hemoglobin.
  5. it makes the blood coagulate.

ANSWER MCQS

1.a       2.d       3.d       4.a       5.b       6.b       7.b       8.c    9. d     10.b     11.a     12.b     13.a     14.a     15.d   16.b  17.b     18.b  19.a 20. c

ANSWERS TO THE SHORT QUESTIONS

  1. No. 1.Explain the phenomenon of decreasing temperature in troposphere.

Answer. Troposphere lies just above the Earth’s surface and extends upward up to 12 km. Although concentration of carbon dioxide and water vapours in negligible in this region, but both of these gases absorb much of the outgoing infrared radiation that keeps the atmosphere warm. As concentration of these gases gradually decreases upward, correspondingly temperature also decreases at a rate of 6°C per kilometer. In this region temperature decreases upwards in a regular way from 17°C to -58°C. Thus carbon dioxide and water vapours are responsible for decreasing of temperature in troposphere.

  1. No. 2.Differentiate between primary and secondary air pollutants.

Answer. Primary pollutants are the Waste or exhaust products driven out because of combustion of fossil fuels and organic matter. These are oxides of sulphur (SO2 and SO3); oxides of carbon (CO2 and CO); oxides of nitrogen (especially nitric oxide NO); hydrocarbon (CH4); ammonia and compounds of fluorine.

Secondary pollutants are produced by various reactions of primary pollutants. These are sulphuric acid, carbonic acid, nitric acid, hydrofluoric acid, ozone arid peroxy acetyl nitrate (PAN).

  1. No.3.State the major sources of CO and CO2 emission

Answer. The major sources of CO and CO2 emission are volcanic eruption and decomposition of organic matter naturally. However, the major source for the emission of these gases is combustion of fossil fuels. Forest fires and burning of wood also emit CO2 and CO. Especially, when supply of oxygen is limited, emission of CO dominates

  1. No.4. CO2 is responsible for heating up atmosphere, how?

Answer. The CO2 forms a layer around the Earth like an envelope. It allows the heat rays of the Sun to pass through it, and reaches up to the Earth. These rays are reflected from the Earth surface and go back to upper atmosphere as shown in figure. Normal concentration of CO2 layer retains enough heat to keep the atmosphere warm. So, normal 1 concentration of CO2 is necessary and beneficial for keeping the temperature    warm.    Otherwise,    the    Earth    would    have   been uninhabitable. The Earth’s average temperature would be about -20°C, rather than presently average temperature 15°C.

The natural process of heat going out.

  1. No.5. CO is a hidden enemy, explain its action.

Answer. CO is a health hazard being highly poisonous gas. Being colorless and odourless, its presence cannot be noticed easily and readily. When inhaled, it binds with the hemoglobin most strongly than that of oxygen. Thus hindering the supply of oxygen in body. Exposure to higher concentration of CO causes headache and fatigue. If inhaled for a longer time it results in breathing difficulties and ultimately death.

  1. No. 6.What threats are to human health due to SO gas as air pollutant?

Answer. SO2 is a colorless gas having irritating smell. It causes suffocation, irritation and severe respiratory problems to asthmatic people.

  1. No. 7.   Which air pollutant   is produced on   anaerobic decomposition of organic matter?

Answer. Anaerobic decomposition of organic matter produces CO2 and CO gases.

 No. 8.How acid rain increases the acidity of soil?

Answer. Normal rain water is weakly acidic because it consists of dissolved CO2 of air. Its pH is about 5.6 to 6. But rain-water on dissolving air pollutants (acidic gases SO2 and NOx) becomes more acidic and its pH decreases. As a result, normal rain water changes into acidic rain. When this acid rain falls on soil it increases the acidity of soil.

  1. No. 9.Point out two serious effects of ozone depletion.

Answer.

(i)       Depletion of ozone enables ultraviolet radiations of Sun to reach to the Earth that can cause skin cancer to human beings and other animals.

(ii)     Decreased ozone layer will increase infectious diseases like malaria.

  1. No. 10.How ozone layer forms in stratosphere?

Answer. As majority of the ultraviolet radiations coming from the sun have been absorbed in the upper stratosphere Therefore, the mid stratosphere has less UV light passing through it. Here O and O2 recombine to form ozone which is an exothermic reaction. Ozone formation in this region results in formation of ozone layer. Thus, ozone layer exists in mid stratosphere as shown in figure.

O2

03

Ultraviolet radiations absorbed by ozone layer

  1. No. 11.Why the 75% of the atmospheric mass lies within the troposphere?

Answer. Atmosphere consists of gases. That means atmospheric mass is because of presence of gases; nitrogen, oxygen, carbon dioxide etc. The two major gases are nitrogen and oxygen that comprise 99% by volume of the atmosphere. About 99% of atmospheric mass lies within 30 kilometer of the surface and 75% lies within the lowest 11 kilometers that is the troposphere.

  1. No. 12.How ozone layer is being depleted by Chlorofluocarbons?

Answer. Chlorofluorocarbons (CFCs) used as refrigerants in air conditioners and refrigerators are major cause of depletion of ozone layer. These compounds leak in one way or other; escape and diffuse to stratosphere. There ultraviolet radiations break the C-C1 bond in CFCl3 and generate chlorine free radicals.

UV

CFCl2               CFCl2

These free radicals are very reactive. They react with ozone to form oxygen as

O3+ Cl           O2 + OCl·

OCl·          O· + Cl·

O· + O·         O2

A single chlorine free radical released by the decomposition of CFCs is capable of destroying up to 1 lac ozone molecules as shown in figure.

Depletion of ozone layer by chlorofluoro carbons

ANSWERS TO THE LONG QUESTIONS

  1. No.1. Write down the significance of atmospheric gases.

Answer. The study of composition of atmosphere provides us the knowledge about significance of gases present in the atmosphere. Atmosphere is divided into four regions. Each region has its natural characteristics. Atmosphere is the envelope of different gases around the Earth. Major concentration of Na, O2 and Ar are present in atmosphere. The presence of CO2 and water vapours play a significant role in maintaining temperature of the atmosphere. Both of these gases are transparent to visible light but absorb infrared radiations emitted by the Earth’s surface. Therefore, these gases absorb much of the outgoing radiations and warm the atmosphere. The presence of O2 in stratosphere saves us from dangerous UV radiations.

  1. No. 2.Give the characteristics of troposphere. Why temperature decreases upwards in this sphere?

Answer. Followings are the characteristics of troposphere.

(i)       Troposphere is lowest region of the atmosphere.

(ii)      It extends from the surface of the Earth up to 12 km.

(iii)     In this region temperature decreases from 17°C to -58°C i.e. at the rate of about 6°G per Km.

(iv)     The major constituents of troposphere are nitrogen and oxygen gases that constitute 99% by volume of the Earth’s atmosphere.

(v)      In this region all weathers occurred almost all aircrafts fly in this region.

Decreasing of temperature upwards

Although concentration of carbon dioxide and water vapours is negligible in atmosphere, yet they play a significant role in maintaining temperature of the atmosphere. Both of these gases allow visible light to pass through but absorb infrared radiations emitted by the Earth’s surface. Therefore, these gases absorb much of the outgoing radiations and warm the atmosphere. As the concentration of gases decreases gradually with the increase of altitude, correspondingly temperature also decreases at a rate of 6 K per kilometer.

  1. No. 3.What is the characteristics of stratosphere? Why temperature increases upwards in this sphere?

Answer. Following are characteristics of stratosphere,

  • Stratosphere lies above the troposphere,

(ii)    It extends from 12 km to 50 km,

(iii)    In this region temperature increases from -58°C to 20.i.e. there is gradual increases of temperature by 60°C.

(iv)    This region is further subdivided into three regions; lower, middle and upper region.

(v)      Ozone gas is found in this region which is responsible for the rise of temperature in this region.

(vi)     Ozone layer is formed in middle of the stratosphere.

This region is next to troposphere and extends up to 50 kilometers. In this region, temperature rises gradually up to 2 °C. The presence of ozone (due to absorption of radiation) in this region is responsible for the rise of temperature in stratosphere. Within this region, temperature increases as altitude increases, such as lower layer temperature is about -58 °C and upper layer is about 2°C. Thus, stratosphere is layered in temperature as shown in figure.

Altitude (km)

Since ozone in the upper layer absorbs high energy ultraviolet radiations from the Sun, it breaks down into monoatomic arid diatomic oxygen O2 .This phenomenon results in increasing of temperature. The mid stratosphere has less UV light passing through it. Here O and O2 recombine to form ozone which is an exothermic reaction. Thus temperature of mid stratosphere is also higher than low stratosphere. The lower stratosphere receives very low UV radiations. Because of it temperature of this layer is lowest of the above layers.

  1. No.4. CO2 is necessary for plants but why its increasing concentration is alarming for us?

Answer. It is an essential gas for plants as O2 is essential for animals. Plants consume CO2 in photosynthesis process and produce O2.While animals use O2 in respiration and give out CO2. In this way a natural balance exists between these essential gases as represented here.

6O:

But this balance is being disturbed by emitting more and more CO2 in air through different human activities. It traps some of the infrared radiations emitted by the Earth. Hence, increased concentration of CO2 layer absorbs the infrared radiations emitted by the Earth’s surface and prevents heat energy escaping from the atmosphere. It helps to stop surface, from cooling down during night. As the concentration of CO2 in air increases, less heat energy is lost from the atmosphere as shown in figure.

The natural process of heat going out.

The increased concentration of

CO, due to human activities

results in trapping of heat

causes green house effect.

Greenhouse effect

Therefore, the average temperature of the atmosphere gradually increases. Greater is amount of CO2, more is trapping of heat or warming.

It is alarming because it has the following harmful effects:

(i)       Accumulation of carbon dioxide in air is resulting in increasing atmospheric temperature about 0.05 °C every year.

(ii)      It is causing major changes in atmospheric circulation resulting in changing weather patterns. Extreme weather events are occurring more commonly and intensely than previously,

(iii)     It melts glaciers and snow caps that are increasing flood risks and intense tropical cyclones.

(iv)     Sea-level is rising due to which low lying areas are liable to be submerged, turning previously populated areas no longer habitable

 

  1. No.5. Why CO is considered a health hazard?

Answer. CO is an air pollutant. It is a health hazard being highly poisonous gas. Being colorless and odourless, its presence cannot be noticed easily and readily. When inhaled, it binds with the hemoglobin most strongly than that of oxygen. Thus stop the supply of oxygen in body. Exposure to higher concentration of CO causes headache and fatigue. If inhaled for a longer time it results in breathing difficulties and ultimately death. It is the reason burning is not allowed in closed places. It is advised to switch off coal or gas heaters, cooking range, etc., before going to sleep.

  1. No. 6.Define acid rain. How it forms and what are its effects?

Answer. Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOX to HNO2 and HNO3. Normal rain water is weakly acidic because it consists of dissolved CO2 of the air. Its pH is about 5.6 to 6. But rain water on dissolving air pollutants (acids) becomes more acidic and its pH reduces to 4. Thus, acid rain is formed on dissolving acidic air pollutants such as sulphur dioxide and nitrogen dioxide by rain water. Following figure shows how oxides of sulphur and nitrogen are converted into acids. These acids dissolve in rain water and damage soil, animals, plants and aquatic life.

Acid rain formation and its effects

(i)       Acid rain on soil and rocks leaches heavy metals (Al, Hg, Pb, Cr, etc) with it and discharges these metals into rivers and lakes. This water is used by human beings for drinking purpose. These metals accumulate in human body to a toxic level. On the other hand, aquatic life present it lakes also suffers because of high concentration of these metals. Especially high concentration of aluminum metal clogs the fish gills. It causes suffocation and ultimately death of fish.

(ii)      Acid rain attacks the calcium carbonate present in the marble and limestone of buildings and monuments. Thus, these buildings are getting dull and eroded day by day.

(iii)    Acid rain increases the acidity of the soil. Many crops and plants cannot grow properly in such soil. It also increases the toxic metals in the soil that poisons the vegetation. Even old trees are being affected due to acidity of soil. Their growth is retarded. They get dry and die.

(iv)   Acid rain directly damages the leaves of trees and plants, thus limiting their growth. Depending upon the severity of the damage, plants growth can be hampered. The capability of plants to resist cold or diseases reduces and ultimately they die.

  1. No.7. Compounds of sulphur are air pollutants. Describe the sources of these compounds along with their effects.

Answer. Naturally occurring sulphur containing compounds are emitted in the bacterial decay of organic matter, in volcanic gases and forest fires. But the concentration of sulphur containing compounds in the atmosphere because of natural sources is very small as compared to the concentration of those compounds emitted by fossil fuel combustion in automobiles and industrial units. About 80% of the total SO2 is released by the combustion of coal and petroleum products as shown here

 

Fossil fuels contain Sulphur

Effects of SO2

(i)        SO2 is a colorless gas having irritating smell. It causes suffocation, irritation and severe respiratory problems to asthmatic people.

(ii)      SO2 forms sulphuric acid which damages buildings and vegetations.

  1. No. 8. Where does ozone layer lie in atmosphere? How it is depleting and how we can prevent its depletion?

Answer. Ozone is present throughout the atmosphere. But its maximum concentration called ozone layer lies in stratosphere region about 25 to 30 Km away from the Earth’s surface.

Under normal conditions ozone concentration in stratosphere remains nearly constant through a series of complex atmospheric reactions. Two reactions that maintain a balance in ozone concentrations are as follows:

O2 + O                     O3   (formation)

O2 + O                       2O2 (decomposition)

But this ozone layer is being depleted through various chemical reactions, such as:

(i)       The ozone molecule absorbs solar radiations and dissociate readily i.e., self dissociation of ozone takes place.

(ii).     However, chlorofluorocarbons (CFCs) (used as refrigerants in air conditioners and refrigerators) are major cause of depletion of ozone layer. These compounds leak in one way or other; escape and diffuse to stratosphere. There ultraviolet radiation breaks the C-C1 bond in CFCl3 and generates chlorine free radicals as

UV

CFCI3                         CFCI2+Cl·

These free radicals are very reactive. They react with ozone to form oxygen as

O3+ Cl            O2 + OCl·

OCl·            O· + Cl·

O· + O·            O2

A single chlorine free radical released by the decomposition of CFCs is capable of destroying up to 1 lac ozone molecules. The region in which ozone layer depletes is called ozone hole. Depletion of ozone layer can be prevented by controlling the use of CFCs.

  1. No. 9. Oxides of nitrogen cause air pollution. Describe the sources of these compounds.

Answer. Combustion of fossil fuels in internal combustion engines, in thermal power stations and factories where huge amount of coal is burnt, NO is formed by the direct combination of nitrogen and oxygen as shown in figure.

N2 + O2                     2NO

However, it quickly reacts with air to form nitrogen dioxide. NO is highly toxic gas.

2NO + O2               2NO2

Nitrogen in the air is normally

unreactive

Formation of NO & NO2 on combustion of fossil fuel and causing air

Pollution.

Mixture of these gases represented as NOX enter in the air through automobile exhaust and chimneys of thermal power station

and factories. It irritates breathing passage.

  1. No.10. What is the main cause of air pollution? Explain the role of Government to control the pollution.

Answer. Causing air pollution through auto exhaust is almost the most common air polluting act which an average driven commits daily for hours without considering its consequences. One is poisoning the air, creating a big problem that has local, regional and global effects.

Government should do short term as well as long term planning to preserve the natural world. Because without a healthy natural environment, there will be no healthy human, plant, or animal.

(i)       First of .all quality of fuel must be improved by adding anti-knocking agents in fuels. At the same time, automobiles combustion engines must be efficient so that they should burn the fuel completely. No unburned hydrocarbon molecules (fuel) should come out of the exhaust. The hydrocarbons present in polluted air are very harmful They can cause liver damage and even cancer. So government must guide the people to use converters in auto exhausts.

(ii)      Fossil fuels produce a number of air pollutants because of impurities and complex molecule nature of hydrocarbons. Government should promote the use of alternative fuels such as methanol, ethanol and bio-diesel. These fuels are less polluting than hydrocarbons fuel, as their molecules are simple, and burn completely in the engine. Their burning produces less carbon monoxide, soot and other pollutants”

(iii)     The government must plan to avoid using carbon dioxide producing fuels as it is a greenhouse gas. It should go to battery-powered electric vehicles.

(iv)     Government should provide efficient transport in the big cities, so that people should avoid using their own vehicles.

 

 

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