CHAPTER 14 ENVIRONMENTAL CHEMISTRY 1: THE ATMOSPHERE – 10th Class Chemistry Notes

CHAPTER 14 ENVIRONMENTAL CHEMISTRY 1: THE ATMOSPHERE – 10th Class Chemistry Notes

 

Question: Give the characteristics of troposphere. Why temperature decreases upwards in this sphere? What are the characteristics of stratosphere? Why temperature increases upwards in this sphere? Write down the significance of atmospheric gases?

Answer: COMPOSITION OF ATMOSPHERE:

Atmosphere: Atmosphere is the envelope of different gases around the Earth. The study of Earth’s atmosphere and its processes is called atmospheric science (aerology).

 

Composition of Atmosphere: Atmosphere extends continuously from the Earth’s surface outwards without any boundary. About 99% of atmospheric mass lies within 30 kilometres of the surface and 75% lies within the lowest 11 kilometres.

Percentage composition of atmosphere by volume is shown in Table.

Table:  Composition of dry air

 

Gas % by Volume
Nitrogen 78.09
Oxygen 20.94
Argon 0.93
Carbon dioxide 0.03

 

Do you know

 

· • Sunlight has short wavelength radiations.

· • Solar energy absorbed by the Earth surface is transformed into heat energy which is of longer wavelength.

· • On the average, there is total 32% reflection of light: 6% being reflected from the Earth’s surface and 26% being reflected back into space because of clouds, gases and dust particles in the atmosphere. 18% of sunlight is absorbed by atmospheric gases.

· • The remaining 50% reaches upto the Earth and is absorbed by it.

• This energy is radiated as heat energy of longer wavelength which is absorbed by water vapours and CO2 in atmosphere.

· • Sunlight has short wavelength radiations.

· • Solar energy absorbed by the Earth surface is transformed into heat energy which is of longer wavelength.

· • On the average, there is total 32% reflection of light: 6% being reflected from the Earth’s surface and 26% being reflected back into space because of clouds, gases and dust particles in the atmosphere. 18% of sunlight is absorbed by atmospheric gases.

· • The remaining 50% reaches upto the Earth and is absorbed by it.

• This energy is radiated as heat energy of longer wavelength which is absorbed by water vapours and CO2 in atmosphere.

 

LAYERS OF ATMOSPHERE: Based on temperature variation atmosphere is divided into four layers. This division is called atmospheric stratification.

 

1) Troposphere:

1) Troposphere: (Greek tropein means to change, mix or circulate. The troposphere is the lowest layer of Earth’s atmosphere. It extends from Earth’s surface to an average height of about 12 km, although this altitude actually varies from about 9 km (30,000 ft) at the poles to 17 km (56,000 ft) at the equator, with some variation due to weather.

The major constituents of troposphere are nitrogen and oxygen gases. These two gases comprise 99 % by volume of the Earth’s atmosphere. Although, concentration of carbon dioxide and water vapours is negligible in this layer, 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°C per kilometre. This is the region where all weathers occur. Almost all aircrafts fly in this region.

 

2) Stratosphere:

 

This region is next to troposphere and extends upto 50 kilometres.

 

Temperature variation: In this region, temperature rises gradually upto 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.

 

Upper stratosphere and Ozone Layer or Ozonosphere: Ozone is a triatomic molecule or allotrope of oxygen. Ozone layer is a layer of ozone in the mostly present in mid stratosphere. Since its very less amount is present in upper stratosphere.  Since ozone in the upper layer absorbs high energy ultraviolet radiations from the Sun, it breaks down into monoatomic (O) and diatomic oxygen (O2 ). This is called destruction, depletion or decomposition of ozone.

 

Mid stratosphere: 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.

 

Lower Stratosphere: The lower stratosphere receives very less UV radiations, thus monoatomic oxygen is not found here and ozone is not formed nor deplete here due to very low temperature.

 

Mesosphere:

The mesosphere is the third layer of Earth’s atmosphere. The mesosphere is directly above the stratosphere and below the thermosphere. It extends from about 50 to 85 km (31 to 53 miles) above our planet.

Temperature variation: Temperature decreases with height throughout the mesosphere from 2 OC to -93 OC.  The coldest temperatures in Earth’s atmosphere, about -90° C (-130° F), are found near the top of this layer.

The mesosphere is difficult to study, so less is known about this layer of the atmosphere than other layers. Weather balloons and other aircraft cannot fly high enough to reach the mesosphere. Satellites orbit above the mesosphere and cannot directly measure traits of this layer. Scientists use instruments on sounding rockets to sample the mesosphere directly, but such flights are brief and infrequent. Since it is difficult to take measurements of the mesosphere directly using instruments, much about the mesosphere is still mysterious.

 

Thermosphere:

The thermosphere is the highest layer of Earth’s atmosphere extends from 85 KM to 120 KM.  The temperature of the thermosphere gradually increases with height. Temperature of this layer can rise as high as 1500 °C (2700 °F), though the gas molecules are so far apart that its temperature in the usual sense is not very meaningful. The air is so rarefied that an individual molecule (of oxygen, for example) travels an average of 1 kilometre (0.62 mi; 3300 ft) between collisions with other molecules.[10] Although the thermosphere has a high proportion of molecules with high energy, it would not feel hot to a human in direct contact, because its density is too low to conduct a significant amount of energy to or from the skin.

 

Question: Why lower troposphere has relatively high temperature than the upper?

Answer: Although variations do occur, the temperature usually declines with increasing altitude in the troposphere because the troposphere is mostly heated through energy transfer from the earth surface. Thus, the lowest part of the troposphere (i.e. Earth’s surface) is typically the warmest section of the troposphere.

 

Interesting information: Ozone is quite a well-known gas. Photocopiers and any other source of static electricity, can cause it to form from oxygen. You may have noticed a strange bitter smell near photocopiers; this is ozone. It is a poisonous gas and is formed on hot days in badly polluted cities.

 

 

Pollution: Any undesirable change in the physical, chemical or biological characteristics of environment (Earth, Water, Air, etc) as a result of human activities that effect the life is called pollution.

 

POLLUTANTS:

A pollutant is a waste material that pollutes air, water or soil.

 

Severity of pollutants: Three factors determine the severity of a pollutant.

a) Chemical nature.

b) Concentration.

c) Persistence.

 

Air pollution: The presence of substances in air that harm human beings and other organisms.

Air pollutant: 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.

Types of air Pollutants: Major air pollutants are classified as primary pollutants and secondary pollutants.

 

Primary pollutants: 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 (specially nitric oxide NO); hydrocarbon (CH4 ); ammonia and compounds of fluorine (HF).

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

Sources of Air Pollutants:

As you know 99% of atmosphere consists of N2 and O2 . Although, other gases are minor constituents, they can have major effects on our environment. Because atmosphere determines the environments in which we live. So, these minor constituents are safe upto a concentration limit. But in some areas this limit has been crossed considerably during the last 60 years because of human activities. Different sources of air pollutants are described as:

 

(i) Oxides of Carbon (CO2 and CO)

Sources of oxides of carbon are as follows:

  • Both of these gases are emitted due to volcanic eruption and decomposition of organic matter naturally.
  • However, the major source for the emission of these gases is combustion of fossil fuels (coal, petroleum and natural gas). Fossil fuels burnt in combustion engine of any type of automobile, kiln of any industry or open air fires emit CO2and CO.
  • Forest fires and burning of wood also emit CO2and CO. Especially, when supply of oxygen is limited, emission of CO dominates.

 

CO2 is responsible to maintain the temperature of atmosphere in balance: 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 upto the Earth. These rays are reflected from the Earth surface and go back to upper atmosphere. Normal concentration of CO2 layer retains enough heat to keep the atmosphere warm. So, normal 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.

 

CO2 is essential for plants to carry out process of photosynthesis: CO2 is not an air pollutant. Rather, 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. But this balance is being disturbed by emitting more and more CO2 in air through different human activities.

 

 

Greenhouse Effect and Global Warming:

Although, CO2 is not a poisonous gas, yet its increasing concentration due to burning of fossil fuels in different human activities is alarming. Because CO2 in the atmosphere acts like a glass wall of a green house. It allows UV radiations to pass through it but does not allow the IR radiations to pass through it. 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 that 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 surface of the Earth. Therefore, the average temperature of the surface gradually increases. This is called greenhouse effect. This effect is proportional to amount of CO2 in air. Greater is amount of CO2 , more is trapping of heat or warming. Due to increased warming this phenomenon is also called global warming.

 

Effects of global warming

  1. Accumulation of carbon dioxide in air is resulting in increasing atmospheric temperature about 0.05 °C every year.
  2. It is causing major changes in weather patterns. Extreme weather events are occurring more commonly and intensely than previously.
  3. It melts glaciers and snow caps that are increasing flood risks and intense tropical cyclones.
  4. Sea-level is rising due to which low lying areas are liable to be submerged, turning previously populated areas no longer habitable.

 

Why is CO considered a health hazard?

CO is an air pollutant. It is a health hazard being highly poisonous gas. Being colourless and odourless, its presence cannot be noticed easily and readily. When inhaled, it binds with the haemoglobin 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. 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.

Do you know

 

Converters should be used in automobile exhaust so that theyconvert CO to CO2and oxides of nitrogen NOx to N2 . before it enters in air. Catalytic converters as shown in figure 14.5 are attached to automobile exhausts. When hot gases pass through the converters, harmful pollutants are converted to harmless substances. Such as, carbon monoxide is oxidized to carbon dioxide, unburn hydrocarbons are oxidized to carbon dioxide and water, while oxides of nitrogen are reduced to nitrogen.

 

 

Question: Compounds of sulphur are air pollutants. Describe the sources of these compounds along with their effects.

Sulphur Compounds:

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.

 

Effects of So2:

(i) SO2 is a colourless 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.

 

To control pollution because of SO2 , it is necessary to remove sulphur from fossil fuels before they are burnt.

 

Question: Oxides of nitrogen cause air pollution. Describe the sources of these compounds.

 

Nitrogen Compounds (NOx):

Naturally occurring oxides of nitrogen, mainly nitric oxide (NO), is produced by the electrical lightening in air.

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. .

 

However, it quickly reacts with air to form nitrogen dioxide. NO2 is highly toxic gas.

 

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. These oxides form nitric acid combining with water vapours in air. Nitric acid is a component of acid rain. Its effects will be discussed in section.

 

Role of Government to control pollution:

Causing air pollution through auto-exhaust is almost the most common air polluting act which an average citizen 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.

1. 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. So government must guide the people to use converters in auto- exhausts.

2. 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.

3. 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.

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

 

Question: Define acid rain. How it forms and what are its effects?

ACID RAIN AND ITS EFFECTS

Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOx to HNO3 and HNO2 . 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.

The oxides of acids dissolve in rain water and damage soil, animals, plants and aquatic life.

 

Effects of acid rain

  1. 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 in lakes also suffers because of high concentration of these metals. Especially high concentration of aluminium ions clogs the fish gills. It causes suffocation and ultimately death of fish.
  2. 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.
  3. 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.
  4. 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 die as shown in figure.

 

Question: Where does ozone layer lie in atmosphere? How is it depleting and how we can prevent its depletion?

OZONE DEPLETION AND ITS EFFECTS

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 14.10. Otherwise, ultraviolet radiations would cause skin cancer. Thus ozone layer in stratosphere is beneficial for life on the Earth.

 

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:

 

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

 

1)  The ozone molecule absorbs solar radiations and thus self-ionization  of ozone takes place.

2) 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. Ultraviolet radiations break the C-Cl bond in CFCI3 and generates chlorine free radicals as:

 

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

 

A single chlorine free radical released by the decomposition of CFCs is capable of destroying up to many thousands of ozone molecules. The region in which ozone layer depletes is called ozone hole.

 Signs of ozone depletion were first noticed over Antarctica in 1980s. Since 1990s depletion have also been recorded over the Arctic, as well.

 

Effects of Ozone Depletion

Even minor problems of ozone depletion can have major effects.

  1. Depletion of ozone enables ultraviolet radiations of Sun to reach to the Earth, that can cause skin cancer to human beings and other animals.
  2. Decreased ozone layer will increase infectious diseases like malaria.
  3. It can change the life cycle of plants disrupting the food chain.
  4. It can change the wind patterns, resulting in climatic changes all over the
    world. Especially, Asia and Pacific will be the most affected regions, facing climate induced migration of people crisis.

 

 

 

Incineration of waste material causes air pollution

Incineration is a waste treatment process that involves the burning of solid waste at high temperatures between 650° C to 1100° C in incinerators. Incinerators reduce the solid mass of the original waste by 80-85% and convert the waste materials into ash, flue gas and heat. Although, the volume of solid waste is reduced effectively by incineration, it produces highly poisonous gases and toxic ash. The flue gas includes, dioxins, furans, sulphur dioxide, carbon dioxide, carbon monoxide, hydrochloric acid and a large amount of particulate matter.

 

 

Difference between Pollutants and Contaminants: So the pollutants are those substances which cause pollution. While contaminants are those substances that make something impure.

 

Key Points

 

  • Atmosphere is the envelope of different gases around the Earth.
  • Atmosphere is divided into four regions; troposphere, stratosphere, mesosphere and thermosphere.
  • Troposphere is just above Earth’s surface and extends upto 12 kilometer.
  • Stratosphere is next to troposphere and extends upto 50 km. In this region, temperature rises upwards because of presence of ozone layer.
  • Mesosphere is next to stratosphere and extends up to 85 km. Thermosphere lies beyond mesosphere.
  • Natural sources of air pollutants are volcanic eruption and decomposition of organic matter.
  • Source of air pollutants because of human activities are burning of fossil fuel in combustion engines of automobiles, kiln of industries, open air fires and forest fires.
  • CO2forms a layer around Earth, that absorbs the infrared radiations emitted by Earth surface. Thus, heating up atmosphere is called greenhouse effect.
  • CO is highly poisonous gas so it is health hazard.
  • SO2is also health hazard and forms sulphuric acid by combining with water vapors in air. It is also a component of acid rain.
  • Acid rain consists of H2SO4and HNO3that reduces the pH of rain water to.
  • Ozone layer lies in stratosphere about 25 to 30 km away from Earth’s surface.
  • Ozone layer protects Earth like a shield from harmful ultraviolet radiations of sunlight.
  • Chlorofluorocarbons destroy ozone molecules, depleting the ozone called ozone hole.
  • Depletion of ozone enables ultraviolet radiations of the Sun to reach the Earth; causing infectious diseases; changing the life cycle of plants, wind patterns.

 

Exercise

 

Short Answer Questions

  1. Explain the phenomenon of decreasing temperature in troposphere.

Answer: Although, concentration of carbon dioxide and water vapours is negligible in this layer, 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°C per kilometre. This is the region where all weathers occur. Almost all aircrafts fly in this region.

 

  1. Differentiate between primary and secondary air pollutants.

Answer: Primary pollutants: 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 (specially nitric oxide NO); hydrocarbon (CH4 ); ammonia and compounds of fluorine (HF).

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

 

  1. State the major sources of CO and CO2emission?

Answer: Oxides of Carbon (CO2 and CO)

Sources of oxides of carbon are as follows:

  • Both of these gases are emitted due to volcanic eruption and decomposition of organic matter naturally.
  • However, the major source for the emission of these gases is combustion of fossil fuels (coal, petroleum and natural gas). Fossil fuels burnt in combustion engine of any type of automobile, kiln of any industry or open air fires emit CO2and CO.
  • Forest fires and burning of wood also emit CO2and CO. Especially, when supply of oxygen is limited, emission of CO dominates.

 

  1. CO2is responsible for heating up atmosphere, how?

Answer: Concentration of carbon dioxide and water vapours is negligible in troposphere, 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.

 

  1. CO is an hidden enemy, explain its action.

 

Answer: CO considered a health hazard Or CO is a hidden enemy

CO is an air pollutant. It is a health hazard being highly poisonous gas. Being colourless and odourless, its presence cannot be noticed easily and readily. When inhaled, it binds with the haemoglobin 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. 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. What threats are to human health due to SO2gas as air pollutant?

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

 

 

  1. Which air pollutant is produced on anaerobic decomposition of organic matter?

Answer: Oxides of Sulphur are produced in anaerobic decomposition of organic matter

 

 

  1. How acid rain increases the acidity of soil?

Answer: Acid rain contain acidic components like sulphuric acid and nitric acid. When acid rain reaches the soil it increases the acidity of the soil that result in death of organisms.

 

 

  1. Point out two serious effects of ozone depletion.

Answer: Effects of Ozone Depletion

Even minor problems of ozone depletion can have major effects.

  1. Depletion of ozone enables ultraviolet radiations of Sun to reach to the Earth, that can cause skin cancer to human beings and other animals.
  2. Decreased ozone layer will increase infectious diseases like malaria.
  3. It can change the life cycle of plants disrupting the food chain.
  4. It can change the wind patterns, resulting in climatic changes all over the
    world. Especially, Asia and Pacific will be the most affected regions,facing climate induced migration of people crisis.

 

  1. How ozone layer forms in stratosphere?

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.

 

 

  1. Why the 75% of the atmospheric mass lies within the troposphere?

Answer: About 75% of atmospheric mass lies in troposphere because in this region most of the chemical reactions take place and lot of gases release here. Due to presence of these gases troposphere contain 75% of atmospheric mass.

 

  1. How ozone layer is being depleted by chlorofluorocarbons?

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. Ultraviolet radiations break the C-Cl bond in CFCI3 and generates chlorine free radicals as:

 

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

 

A single chlorine free radical released by the decomposition of CFCs is capable of destroying upto many lacs of ozone molecules. The region in which ozone layer depletes is called ozone hole.

 

 

Long Answer Questions

  1. Write down the significance of atmospheric gases.

Answer: Significance of atmospheric gases: The earth atmospheric gases along with the abundant water vapours play important role in the maintaining life on the earth surface. That is the reason our planet is unique than other.

1)Role of CO2 and O2 Or Atmospheric Gases are Indispensable for Life on Earth: Atmospheric gases like Carbon dioxide and oxygen play important role in presence of life on earth. Oxygen is necessary for respiration while carbon dioxide is necessary for photosynthesis.

 

2) Maintaining Temperature of Erath: Carbon dioxide along with water vapours maintain the temperature of the universe by absorbing infrared radiation coming back from the earth surface.

3) Protection from harmful UV radiations: Ozone in the stratosphere absorb high energetic UV radiations and protect us from cancer and other serious effects of these rays.

4) Burning process: Oxygen gas play a key role in burning process.

 

5) Medium for waves to travel to earth: Atmospheric gases provide medium to radiation and in this way they reach the earth.

6) Role of Nitrogen: Nitrogen gas has strong triple bond between the two nitrogen atoms, it is almost inert. Nitrogen present 78% by volume in the atmosphere mainly inhibits the interaction of oxygen with substances present on the surface of the earth. It reduces the interaction between oxygen and fuel during burning and thus help in protect us from harmful effects of burning.

 

 

  1. Give the characteristics of troposphere. Why temperature decreases upwards in this sphere?

Answer: 1) Troposphere:

1) Troposphere: (Greek tropein means to change, mix or circulate. The troposphere is the lowest layer of Earth’s atmosphere. It extends from Earth’s surface to an average height of about 12 km, although this altitude actually varies from about 9 km (30,000 ft) at the poles to 17 km (56,000 ft) at the equator, with some variation due to weather.

The major constituents of troposphere are nitrogen and oxygen gases. These two gases comprise 99 % by volume of the Earth’s atmosphere. Although, concentration of carbon dioxide and water vapours is negligible in this layer, 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°C per kilometre. This is the region where all weathers occur. Almost all aircrafts fly in this region.

 

 

  1. What are the characteristics of stratosphere? Why temperature increases upwards in this sphere?

Answer: Stratosphere:

 

This region is next to troposphere and extends upto 50 kilometres.

 

Temperature variation: In this region, temperature rises gradually upto 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.

 

Upper stratosphere and Ozone Layer or Ozonosphere: Ozone is a triatomic molecule or allotrope of oxygen. Ozone layer is a layer of ozone in the mostly present in mid stratosphere. Since its very less amount is present in upper stratosphere.  Since ozone in the upper layer absorbs high energy ultraviolet radiations from the Sun, it breaks down into monoatomic (O) and diatomic oxygen (O2 ). This is called destruction, depletion or decomposition of ozone.

 

(Destruction or depletion of Ozone)

 

 

Mid stratosphere: 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.

 

Lower Stratosphere: The lower stratosphere receives very less UV radiations, thus monoatomic oxygen is not found here and ozone is not formed nor deplete here due to very low temperature.

 

  1. CO2is necessary for plants but why its increasing concentration is alarming for us?

Answer: CO2 is essential for plants to carry out process of photosynthesis: CO2 is not an air pollutant. Rather, 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. But this balance is being disturbed by emitting more and more CO2 in air through different human activities.

 

Increase concentration of CO2 is alarming Or CO2 is the cause of global warming: CO2 is alarming is alarming o Although, CO2 is not a poisonous gas, yet its increasing concentration due to burning of fossil fuels in different human activities is alarming. Because CO2 in the atmosphere acts like a glass wall of a green house. It allows UV radiations to pass through it but does not allow the IR radiations to pass through it. 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 that 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 surface of the Earth. Therefore, the average temperature of the surface gradually increases. This is called greenhouse effect as shown in figure 14.3(b). This effect is proportional to amount of CO2 in air. Greater is amount of CO2 , more is trapping of heat or warming. Due to increased warming this phenomenon is also called global warming.

 

 

  1. Why is CO considered a health hazard?

See Answer of Short question number 5.

 

 

  1. Define acid rain. How it forms and what are its effects?

Answer: ACID RAIN AND ITS EFFECTS

Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOx to HNO3 and HNO2 . 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.

 

Effects of acid rain

  1. 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 in lakes also suffers because of high concentration of these metals. Especially high concentration of aluminium ions clogs the fish gills. It causes suffocation and ultimately death of fish.
  2. 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.
  3. 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.
  4. 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 die.

 

  1. Compounds of sulphur are air pollutants. Describe the sources of these compounds along with their effects.

Answer: Sulphur Compounds:

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.

 

Effects of So2:

(i) SO2 is a colourless 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.

 

To control pollution because of SO2 , it is necessary to remove sulphur from fossil fuels before they are burnt.

 

  1. Where does ozone layer lie in atmosphere? How is it depleting and how we can prevent its depletion?

Answer: OZONE DEPLETION AND ITS EFFECTS

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 14.10. Otherwise, ultraviolet radiations would cause skin cancer. Thus ozone layer in stratosphere is beneficial for life on the Earth.

 

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:

 

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

 

1)  The ozone molecule absorbs solar radiations and thus self-ionization  of ozone takes place.

2) 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. Ultraviolet radiations break the C-Cl bond in CFCI3 and generates chlorine free radicals as:

 

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

 

A single chlorine free radical released by the decomposition of CFCs is capable of destroying upto many lacs of ozone molecules. The region in which ozone layer depletes is called ozone hole.

 Signs of ozone depletion were first noticed over Antarctica in 1980s. Since 1990s depletion have also been recorded over the Arctic, as well.

 

Effects of Ozone Depletion

Even minor problems of ozone depletion can have major effects.

  1. Depletion of ozone enables ultraviolet radiations of Sun to reach to the Earth, that can cause skin cancer to human beings and other animals.
  2. Decreased ozone layer will increase infectious diseases like malaria.
  3. It can change the life cycle of plants disrupting the food chain.
  4. It can change the wind patterns, resulting in climatic changes all over the
    world. Especially, Asia and Pacific will be the most affected regions,facing climate induced migration of people crisis.

 

  1. Oxides of nitrogen cause air pollution. Describe the sources of these compounds.

Answer: Nitrogen Compounds (NOx):

Naturally occurring oxides of nitrogen, mainly nitric oxide (NO), is produced by the electrical lightening in air.

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 .

 

However, it quickly reacts with air to form nitrogen dioxide. NO2 is highly toxic gas.

 

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. These oxides form nitric acid combining with water vapours in air. Nitric acid is a component of acid rain.

 

Test yourself 14.1

  1. What do you mean by atmosphere?

Answer: Atmosphere: Atmosphere is the envelope of different gases around the Earth. The study of Earth’s atmosphere and its processes is called atmospheric science (aerology).

 

  1. What is difference between atmosphere and environment?

Answer: Atmosphere is the envelope of different gases around the Earth while the surrounding of living organisms in which they live is called environment.

 

  1. Name the major constituents of troposphere.

Answer: The major constituents of the troposphere are nitrogen and oxygen. These gases constitute 99% of earth atmosphere.

 

  1. How is the temperature of atmosphere maintained?

Answer: The temperature of the atmosphere is maintained by CO2 and water vapors. These gases absorbs much of the infrared radiations coming back from the earth surface and thus maintains the temperature of the atmosphere.

 

  1. Where the ozone layer exists?

Answer: The ozone layer exist in the middle stratosphere.

 

  1. Why is the temperature of upper stratosphere higher?

Answer: The presence of ozone in this region is responsible for its high temperature.

 

Test yourself 14.2

  1. 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.

 

  1. Name three primary air pollutants.

Answer: These are oxides of sulphur (SO2 and SO3 ); oxides of carbon (CO2 and CO); oxides of nitrogen (specially nitric oxide NO); hydrocarbon (CH4 ); ammonia and compounds of fluorine (HF).

 

  1. Identify as primary or secondary air pollutant.
    SO2, CH4, HNO3,NH3,H2SO4,O3

Answer:

Primary air pollutant Secondary air pollutant
1) SO2 HNO3
2) CH4 H2SO4
3) NH3 O3

 

 

  1. Why is CO2called a greenhouse gas?

Answer: A chamber or house made to trap infrared radiation is called green house. CO2 is called greenhouse gas because it has the ability to absorb infrared radiation and thus heat the earth atmosphere and is the main cause of global warming or green house effect.

 

  1. Why are the flood risks increasing?

Answer: Due to global warming temperature of earth atmosphere increases. This results in increase of temperature of earth surface and its atmosphere. The high temperature melts the glaciers and snow caps and increasing the flood risks.

 

  1. Comment: burning in open air is preferred.

Answer: In a closed system burning produces poisonous called CO which binds with haemoglobin and thus reduces the oxygen carrying capacity of haemoglobin. As burning open air produce CO2 which is beneficial, so burning in open place is preffered.

 

  1. How are sulphur containing compounds 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. How combustion of fossil fuels in internal combustion engine produces oxides of nitrogen?

Answer: combustion of fossil fuels in internal combustion engine produces nitrogen monoxide by the combination of nitrogen with oxygen.

N2 + O2 → 2NO

However it quickly combine with another oxygen and form a highly poisonous gas NO2.

2NO + O2 → 2NO2

 

Test yourself 14.3,

 

  1. How is acid rain produced?

Answer: Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOx to HNO3 and HNO2 . 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. 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.

 

  1. How is aquatic life 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. 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 in lakes also suffers because of high concentration of these metals. Especially high concentration of aluminium ions clogs the fish gills. It causes suffocation and ultimately death of fish.

  1. Why are plants dying day by day? Comment.

Answer: 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. 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 die.

 

Test yourself 14.4

  1. 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 cause skin cancer. Thus ozone layer in stratosphere is beneficial for life on the Earth.

 

  1. Why is ozone depleting in atmosphere?

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

 

1)  The ozone molecule absorbs solar radiations and thus self-ionization  of ozone takes place.

2) 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. Ultraviolet radiations break the C-Cl bond in CFCI3 and generates chlorine free radicals.

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

 

iii. What do you mean by ozone hole?

Answer: A single chlorine free radical released by the decomposition of CFCs is capable of destroying upto many lacs of ozone molecules. The region in which ozone layer depletes is called ozone hole.

 

  1. Where is the ozone layer found?

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.

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