Chemistry Part Ii Chapter 14 Environmental Chemistry

The environment comprises of four components - atmosphere, hydrosphere, lithosphere and biosphere.

Environmental chemistry deals with the study of the origin, transport, reactions, effects and fates of chemical species in the environment.

They act as nuclei for cloud formation. 

Primary pollutants are those which after their formation remains as such e.g. NO. Secondary pollutants are those which are formed as a result of the reaction between primary pollutants e.g. peroxyacetylnitrate (PAN).

Mesosphere and thermosphere.

Gaseous ions like  and atoms of N and O. 

During respiration, sulphuric acid droplets are drawn into lungs, causing bronchitis and respiratory problems leading to death.

Ozone and peroxyacetylnitrate (PAN).

Pollution of sea water due to the discharge of wastes into it is called marine pollution. such as oil, plastic, industrial and agricultural waste and chemical particles into the ocean.

CFCs (chlorofluorocarbons), halons, CH₃CCl₃ (Methyl chloroform), CCl₄ (Carbon tetrachloride), HCFCs (hydrochlorofluorocarbons), hydro Bromo fluorocarbons and methyl bromide are found to have a direct impact on the depletion of the ozone layer.

Green house effect i.e. global warming. 

The gases which allow the solar radiations to pass through but prevent the escape of heat radiations are known as greenhouse gases. The important greenhouse gases and vapours are carbon dioxide, methane, nitric oxides, water vapours, chlorofluorocarbons (CFC) and ozone.

(i) Oceans dissolve CO₂ in the form of carbonates and bicarbonates.
(ii) Living green plants which use CO₂ for photosynthesis.

The major sources of CO₂ production are vehicles, plants and industries. 

Small sized solid particles and liquid droplets are collectively formed as particulates. Of various particles emitted into the atmosphere, carbon (soot) particles are common ones.

Oil pollution effect the sea- birds.

The soil is made up of mineral, organic matter, biological systems(algage, bacteria etc.), soil water and soil air.

Mixing of soil or rock particles into water is called citation. 

The gas is carbon monoxide. 

The branch of chemistry that emphasises on the processes and production that would bring about minimum pollution or deterioration to the environment.

Environmental chemistry is that branch of science which deals with the study of various chemical phenomena occurring in the environment. In the environment, we deal with the study of the origin, transport, reactions, effect and fate of certain chemical species of air, water and soil. 

The addition of any undesirable material to air, water and soil from a natural source or due to human activity which adversely affects the quality of the environment is called environmental pollution. The undesirable material thus added to the environment is called a pollutant. Environmental pollution affects human beings, animals, plants and materials.
Main causes of pollution: Main causes of pollution are:
(i) Fast population growth
(ii) Use of pesticides in agriculture
(iii) Rapid industrialisation
(iv) Excessive industrialisation.

Pollutant: If the concentration of a substance already present in nature or of new substance increases by natural events or human activities to undesirable proportions so that it spoils the environment and reduces the quality of life or affects health, then it is regarded as a pollutant and this phenomenon is known as pollution. Sulphur dioxide, nitrogen oxides, carbon monoxide, pollen dust etc. are pollutants.

Contaminant: A substance which does not occur in nature but is introduced by human activity into the atmosphere and reduces the quality of life and affects our health is called contaminant. 
Pyrosulphuric acid (H₂S₂O₇) does not occur in nature. Pyrosulphuric acid leaked from a defective tank killed some persons and caused skin and breathing problems to many in Delhi some time ago. This man-made substance, because of its dangerous effects, is a contaminant.

(i) Source: The site from which the pollutants or contaminants come is called source. 
(ii) Sink: The material or medium which consumes or interacts with a long-lived pollutant is called sink. A marble deposit acts as a sink for atmospheric sulphuric acid and in the process gets damaged.
 

Oceans are sinks for atmospheric carbon dioxide.
(iii Receptor: Anything that is affected by the pollutant is called receptor. For example, the human being is the receptor of smog as it causes irritation in eyes and breathing problems.

Air pollution is defined as the addition of undesirable materials into the atmosphere either due to the natural phenomenon or due to human activity on the earth which adversely affects the quality of air and hence causing harm to humans, other animals, vegetation and materials. The chemical substance causing pollution is called air pollutant. About 90% of air pollution problems are caused by the pollutants such as carbon monoxide, sulphur dioxide, sulphuric acid, nitrogen oxides, volatile organic compounds and suspended particulate matter.
The natural sources of air pollution are volcanic eruptions, vegetable decay, forest fires (caused by lightning), evaporation of volatile organic compounds from leaves and wind erosion of soil.
Man made pollutants are carbon dioxide (CO₂), oxides of nitrogen (NO, NO₂), sulphur dioxide (CO₂), hydrocarbons, aerosols etc.
Air is also polluted by burning of fuels (coal, oil, gasoline, etc.) in power and industrial plants and in motor vehicles.
Types of air pollutants: There are two types of air pollutants:
(i) Primary air pollutants
(ii) Secondary air pollutants

(i) Primary air pollutants: A primary air pollutant is a harmful chemical substance that directly enters the air as a result of natural events or human activities. For example carbon oxides (CO and CO₂), nitrogen oxides (NO, NO₂), sulphur oxides (SO₂), hydrocarbons and suspended particulate matter.
(ii) Secondary air pollutants: A secondary air pollutant is a harmful material which is formed in the air due to a chemical reaction between two or more air components or a primary pollutant and one or more air components in the atmosphere. For example sulphur dioxide (primary pollutant) reacts with oxygen gas (in the atmosphere) to form secondary pollutant sulphur trioxide (SO₃).

Sulphur trioxide formed may react with water vapours in air to form sulphuric acid.

Sulphuric acid is also a secondary pollutant. Other secondary pollutants are SO₃, H₂SO₄, NO₂, HNO₃, H₂O₂ etc.

Tropospheric (air pollution) pollution refers to that part  of atmosphere which is nearest to the earth's surface extending to a height of about 80 km. It includes:
(i) Gases such as oxides of sulphur, CO, oxides of nitrogen and hydrocarbons.
(ii) Particulate matter such as dust, smoke, fumes etc. 
(iii) Radioactive materials and many others.
It should be noted that these substances are naturally present in the atmosphere in low concentration, but they become pollutants when their concentration is relatively high.
Most of these pollutants can cause damage to human health and some pollutants (such as SO₂) can produce a corrosive effect on building materials.

Carbon monoxide is a colourless and tasteless gas and is not soluble in water. It is formed in a number of ways:
(i) By the incomplete combustion of fuels and substances containing carbon.

(ii) By the reaction between CO₂ and carbon at high temperature particularly in a blast furnace.

(iii) By the dissociation of carbon dioxide at high temperature.

(iv) Some natural processes like volcanic activity, natural gas emission, seed germination, electrical discharge during storm etc release some amount of CO into the atmosphere.
(v) Carbon monoxide is also released by the combustion of diesel, petrol etc. in the engines of automobiles and as a result of certain industrial processes.

Harmful effects:
(i) Effect of CO on human health: Carbon monoxide when inhaled passes through the lungs into the blood where it reacts with haemoglobin (Hb) of the red blood corpuscles (RBC) to form a stable compound known as carboxy haemoglobin (Hb – CO).

The latter is not in a position to transport the inhaled oxygen to various parts of the body. This will cause suffocation and ultimately lead to death. 
(ii) Effect of CO on plants: A high concentration of carbon monoxide will harmfully affect the plants causing leaf drop, a decrease in leaf size and premature ageing of the plants.

Carbon monoxide is produced as a result of incomplete combustion of carbon. The largest portion of carbon monoxide comes from the exhaust of motor vehicles.
Carbon monoxide is poisonous because it binds to haemoglobin much more strongly (about 200 times) than oxygen. The presence of carbon monoxide, therefore, reduces the amount of haemoglobin available in the blood for the transport of oxygen to the body cells and, thus with less O₂ levels, the normal metabolism is impaired.

The main source of CO pollution due to human activity is the use of internal combustion engine in the automobiles. These engines emit a mixture of CO, NO_x, hydrocarbons and particulates. The following measures may be adopted to check CO pollution:
1. By using antipollution catalysts: Automobile exhaust is treated with the antipollution catalyst before discharging into the atmosphere. It consists of two steps:
(a) In the first step, oxides of nitrogen are reduced to ammonia and nitrogen by using finely divided platinum. The quantity of NH₃ is made minimum through suitable conditions.
(b) In the second step, air or oxygen is introduced to oxidise CO and hydrocarbons into CO₂ and H₂O in the presence of finely divided Pt catalyst. In modern automobiles, catalytic exhaust reactors are used in which excess air is pumped into the exhaust gas and the mixture is passed through a catalytic converter in the exhaust system.

2. By modification of internal combustion engines: Internal combustion engines are modified to reduce the amount of pollutants formed during fuel combustion. The presence of excess air ensures complete combustion of CO and hydrocarbons to CO₂ and water.
Automobile carburettors are adjusted and cleaned regularly to the proper fuel-air ratios.

3. By using substitute fuel: Instead of gasoline, the use of CNG (Condensed natural gas) and LNG (Liquefied natural gas) have been used as these are pollution-free fuels.

Source of CO₂: The main sources of CO₂ are:
(i) Complete combustion of fuels and carbon compounds and carbonates. 

(ii) Auto-exhaust and many industrial processes also produce a lot of carbon dioxide.
(iii) Carbon dioxide is also produced by the biological decay of plants.
Sinks of CO₂:
(i) The most important sink of CO₂ is the ocean; ocean contains most of the dissolved CO₂ in the form of carbonates, bicarbonates and organic compounds.
(ii) Another sink of CO₂ is the living green plants where photosynthesis process occurs which results in the removal of CO₂ in the atmosphere.
CO₂ as a pollutant.
The increasing concentration of CO₂ (presently 325 ppm) can lead to increase in the earth’s temperature i.e. green house effect occurs. A slight increase in the earth’s temperature can cause havoc in times of change in world climate, melting of glaciers, flooding of the coastal plains and change in biological activity of the ocean.
Plants maintain a balance in the atmospheric CO₂ level by using it in photosynthesis

Control of CO₂ pollution:
CO₂ pollution can be controlled by using following steps:
(i) The release of CO₂ from various industrial processes should be controlled. 
(ii) The production of CO₂ from vehicles exhaust should be checked.
(iii) To adjust the balance of CO₂ in the atmosphere, the main sink of CO₂ i.e. forests should be developed.

A number of oxides of nitrogen such as NO, N₂O, NO₂, N₂O₃ and N₂O₅ are introduced into the atmosphere due to the natural sources and due to human activity. NO and NO₂ are considered as pollutants and denoted by the general formula NO_x.
The source of NO_x. Nitric oxide (NO) is a colourless gas and nitrogen dioxide (NO₂) is reddish brown gas having a pungent smell and is suffocating in nature. 
(i) Natural sources: Natural bacterial action is the only natural source which discharges NO_X mainly in the form of NO into the atmosphere in large quantity. Lightning discharge also results in the combination of N₂ and O₂ to form NO.
(ii) Man-made sources: The major man-made sources of NO_x are combustion of coal, oil, natural gas and gasoline. The basic reactions are:

(iii) Chemical industries as a source: Chemical industries like sulphuric acid and nitric acid industries produce NO_x as by-products which are discharged into the air.
Sinks of NO_x: NO_x (i.e. NO and NO₂) in the atmosphere are converted into nitric acid through the following reactions in which ozone also takes part:

Nitric acid acts as a temporary sink and comes down in the form of acid rain or precipitates as nitrate salts after reacting with bases such as ammonia, lime etc.
Harmful effects of NO_x pollution:
(i) Nitric oxide binds to haemoglobin and decreases oxygen transport efficiency of blood. 
(ii) Acid rain (HNO₃) can cause the pH of the water to drop to 4 or 5. This can affect vegetation and building materials. 
(iii) The sunlight reacts with nitrogen dioxide to produce highly active oxygen atoms.

The active oxygen immediately reacts with traces of hydrocarbons in the air and produces irritates called photochemical smog. This is a health hazard.
(iv) Oxides of nitrogen have a harmful effect on nylon, rayon and cotton yarns and also cause of fading of dyes used for textiles. 
(v) Nitrogen dioxide (NO₂) results in respiratory problems in human beings and leads to bronchitis. 

(i) NO₂ gas is acidic in nature. So, it can be removed by using an alkaline solution containing Ca(OH)₂ and Mg(OH)₂.
(ii) The flue gases emitted from power plants and industrial units contain oxides of nitrogen along with SO₂ gas etc. These gases can be removed by scrubbing the flue gases with H₂SO₄. The following reaction takes place first. scrubbing the flue gases with H₂SO₄.
The following reaction takes place first:

Then NO and NO₂ gases react to form N₂O₃ which when scrubbed by H₂SO₄ gives NOHSO₄.

Thus both NO₂ and SO₂ gases get removed from flue gases.
These cleaned flue gases are harmless and are released to the atmosphere. 
(iii) The oxides of nitrogen present in the exhaust gases of internal combination engine are reduced to N₂ and NH₃ by the use of finely divided platinum as a catalyst.

The two oxides of sulphur i.e. SO₂ and SO₃ (i.e. SO_x) are the most serious pollutants of the atmosphere. Both are colourless gases with a pungent smell.
(i) Natural sources: Volcanic erruption is the natural source for the release of SO₂ into the atmosphere.
(ii) Man-made sources:
(a) Combustion of sulphur containing coal and fuel oil in the thermal power plants.

(b) A significant amount of SO₂ is formed from the roasting of metallic sulphide ores.

(c) A small amount of CO₂ undergoes photolytic oxidation in air to form SO₃.

Sinks:
(i) Sulphur dioxide in the air can undergo several reactions. It can combine with water droplets to form sulphurous acid.

(ii) Sulphur dioxide also reacts with O₃ in the air by photolytic and catalytic processes to form SO₃ When then combines with water vapour to form droplets of H₂SO₄.

Sulphuric acid comes down in the form of acid rain which then reacts with ammonia emitted from the biological process and limestone or marble. 

Hence marble statues or structures, limestone mineral and sources of ammonia act as sinks for the atmospheric oxide of sulphur.
Harmful effects of SO_x:
(i) SO₂ at a concentration of 5 ppm causes throat and eye irritation.
(ii) SO₂ at a concentration of 1 ppm causes severe discomfort.
(iii) SO₂ has harmful effects on buildings and statues made up of marble and limestone (CaCO₃). The gas released from Mathura refinery is a constant threat to the Taj Mahal at Agra.
(iv) Atmospheric SO₂ is also harmful to plants. It damages vegetables, crops and affects plant growth and nutrient quality of plant products. 
(v) High concentration of SO₂ may cause lung diseases and lung cancer. 

(i) Removal of sulphur from the fuel before combustion: A number of processes are employed. The pyrites from coal can be eliminated by physical techniques such as grinding followed by washing. Organic sulphur in coal can be removed by carbonization, liquefaction or gasification.
(ii) Removal of SO_x from flue gases: A number of methods are employed as.
(a) Limestone method: CaCO₃ is added directly to the boiler where quick lime is formed.

Quicklime (CaO) reacts with SO_x in the flue gases to form calcium sulphite and calcium sulphate. 

Calcium sulphite and calcium sulphate are removed and disposed of.
(b) Lime slurry method: Sulphur dioxide is passed through lime slurry where calcium sulphite is formed. 

The waste CaSO₃ can be used to prepare gypsum by oxidation of CaSO₃.

(c) Magnesium oxide slurry method: Sulphur dioxide may be removed by using magnesium oxide for scrubbing.

(iv) Sodium hydroxide method: Sulphur dioxide can be removed by first treating with sodium hydroxide and then with calcium hydroxide. 

Incomplete combustion of fossil fuel in industry and thermal power plants and the exhaust of automobiles release unburnt hydrocarbons (such as methane, ethylene etc.) constantly into the atmosphere causing pollution.
Harmful effects of hydrocarbons:
(i) Aromatic hydrocarbons irritate various membranes, cause irritation in eyes, nose, lungs etc. Benzene and benzpyrene cause cancer.
(ii) Hydrocarbons react with ozone and oxides of nitrogen to form aldehydes and peroxy acyl nitrate (PAN) which constitute photo- chemical smog.

(i) Reactions occurring in the troposphere: All biological activities occur in the troposphere which is the lowest region and lies just above the surface of the earth (0-11 km). The most important reactions taking place in this region are those involving CO₂and H₂O vapours. When sunlight enters troposphere, CO₂ molecules present in this region absorb a large part of the energy and get excited.
   
other molecules and the excess energy present in them is converted into heat and temperature of the atmosphere may rise. Similar behaviour is also shown by the water vapours in this region.
(ii) Reactions occurring in the stratosphere: The most important reaction taking place in this region (11-50 km) the formation and dissociation of ozone. It involves two steps process:
(i) First step: The U.V. radiations coming from the sun have the efficient energy to split dioxygen (O₂) into two oxygen atoms.

(ii) Second step:  These oxygen atoms react with more of dioxygen to form ozone.

The ozone thus formed on absorbing U.V. radiations dissociates into dioxygen and an oxygen atom. Heat is given off which warms up the stratosphere. 

Thus ozone cycle is completed in the stratosphere. 
Also in the stratosphere, freons undergo photochemical decomposition, giving chlorine atoms. 

These reactive chlorine atoms then destroy the ozone through the following steps of reactions:

(iii) Reactions occurring in the mesosphere and thermosphere (collectively called ionosphere): In the ionosphere, free ions and electrons are formed due to photochemical reactions. For example,
. 
These free ions and electrons are not present as such below the mesosphere but immediately collide with other ions, atoms or molecules to form neutral species. But in the upper atmosphere, they do not react with other species.

The formation of ozone in the stratosphere (11-50 km) occurs in two steps:
First step: The UV radiations coming from the sun have sufficient energy to split the dioxygen into oxygen atoms.
 

Second step: The oxygen atoms react with more dioxygen to form ozone. 
The ozone thus formed on absorbing U.V. radiations dissociate into dioxygen and an oxygen atom. In this process, heat is given out which warms up the stratosphere. 

As a result, this stratosphere is a zone of increasing temperature. In this way 'Ozone cycle' is completed in the stratosphere. 
At an altitude of 25-30 km, we have a layer in which the concentration of ozone is about 10ppm. It is called ozone layer. This region is also called ozonosphere. It does not allow the harmful UV radiations coming from the sun to reach the surface of the earth and thus protects life on the earth. This ozone layer is also called earth protective umbrella.

The ozonosphere region i.e. presence of ozone layer in the upper atmosphere prevents the harmful U.V. radiations from reaching our earth. But ozone depleting substances (ODS) are being released in the atmosphere by high-flying jets and rockets. These are converting ozone into oxygen. Actually, two types of compounds like nitric oxide and chlorofluorocarbons are responsible for depleting the ozone layer and creating a hole in it.
(i) Nitric oxide as ozone depleting substance: It is produced:
(a) at the ground level due to human activity or natural sources
or
(b) in large amounts in the exhaust gases by the engine of supersonic transport planes.
This NO is introduced directly into the stratosphere. NO reacts with ozone thereby decreasing the concentration of ozone and forms NO₂.

NO₂ then reacts with oxygen atoms available in the stratosphere (due to the decomposition of ozone and oxygen) producing back NO.

Thus no NO is consumed but O₃ gets depleted.
(ii) chlorofluorocarbons (Freons) as ozone depleting substance: These are produced from:
(a) aerosol sprays in which they function as propellants and
(b) refrigerating equipment in which they act as coolants.
These are introduced in the stratosphere where they first undergo photochemical decomposition to give chlorine atoms.
            
The reactive chlorine atoms then destroy the ozone layer through the following sequence of reactions:

It has been found that one molecule of CFC can destroy one lakh O₃ molecules in the stratosphere. Due to depletion of ozone, a large hole has been created in the ozone layer.

(ii) Effect on the immune system: The ultraviolet radiations are also likely to suppress the immune system. The immunity against certain diseases is likely to decrease. This will lead to herpes and other diseases.

(iii) Loss of sight: The ultraviolet radiations damage the cornea and lens of the eye, thereby resulting in weakening of eyesight and may lead to blindness.

We know that chlorofluorocarbons (CFC) commonly known as freons and oxides of nitrogen are responsible for depleting ozone layer from the atmosphere. So, in order to preserve ozone layer, the emission of oxides of nitrogen and chlorofluorocarbons should be checked. The emission of oxides of nitrogen can be easily checked by controlling the NO_x pollution. For checking chlorofluorocarbons, the use of chlorocarbons as solvents, propellants in aerosol spray cans, fire fighting reagents, refrigerants etc. shall have to be banned. 

The combination of smoke and fog is called smog. It is of two types:
(a) London smog or sulphurous smog or classical smog: London smog formation is initiated by a mixture of SO_2, particulate and high humidity in the atmosphere. The chemicals present in the particulates catalyse the conversion of SO₂ to SO₃ which then combines with H₂O of the humidity forming a fog of sulphuric acid droplets. These then condense on the surface of the particulate.

This type of smog is seen in the early morning hours of winters months. 
(b) Photochemical smog or Los Angeles smog: It is formed when the air contains NO₂and hydrocarbons and the mixture is exposed to sunlight. As the reaction occurs in the presence of sunlight to form the smog, it is called photochemical smog) This type of smog is formed in the months of summer (strong sunlight) during the daytime when NO₂ and hydrocarbons are present in very large amounts due to heavy vehicular traffic.

(i) During the early morning i.e. before the sunrise, the automobile exhaust emits CO, hydrocarbons and oxides of nitrogen. The NO reacts with oxygen to produce NO₂ gas.
(ii) In the presence of sunlight (during sunrise), NO₂ undergoes photolysis (ultraviolet and visible radiation) to form NO and atomic oxygen,

(iii) Some of these oxygen atoms combine with O₂ in the air to produce ozone gas.

(iv) Ozone reacts with NO to form NO₂ and O₂.

The NO again  absorbs ultraviolet radiation and the entire cycle starts again. Reactive oxygen atoms also combine with hydrocarbons coming out of the exhaust of vehicles to form organic free radicals. The formation of organic free radicals results into a number of chain reactions producing many undesirable compounds (organic peroxides, organic hydroperoxides, peroxy acyl nitrates etc.) which constitute photochemical smog.

As this type of smog contains O₃ and NO₂, it is oxidising in character.

Control of photochemical smog: Photochemical smog can be controlled or suppressed by:
(i) Installation of efficient catalytic converters in the automobiles to reduce photochemical smog i.e. it prevents the release of nitrogen oxides and hydrocarbon to the atmosphere.

(ii) Certain compounds which trap free radicals. When these compounds are sprayed in the atmosphere, they generate free radicals which readily combine with free radical precursors of photochemical smog.

When fossil fuels are burnt, a large amount of CO₂ enters the atmosphere. The gaseous CO, dissolves in water droplets to produce carbonic acid (weakly acidic).

Rainwater normally is not acidic but the weak carbonic acid can give water a pH value of about 5.6. If no other acid comes in contact with water, the pH of the water should remain 5.6 and 7.0. But actually acidity of rain water has increased significantly in many regions of the words. This is due to a large quantity of SO₂ and oxides of nitrogen present in the air. These gases react to produce HNO₃ and H₂SO₄.

Nitric acid and sulphuric acid are brought down from the atmosphere during rain/snow and can lower the pH of the water to 4 or even less that that.
Harmful effects of acid rain:
(i) Acid rain damages iron and steel structures.
(ii) Acid rain causes extensive damage to buildings and sculptural materials made of marble, limestone, slate, mortar etc.

Taj mahal is threatened by acid vapour from Mathura Refinery:
(iii) Acid rain increases the acidity of water in the lakes which are lethal for the fishes. Thus some of the lakes have become fishless and rain water has greatly affected the fish population. 
(iv) Acid rain damages leave of trees and plants and retards the growth of forests. 

Rain -water normally has a pH of 5.6 due to the formation of carbon dioxide present in the atmosphere. 

When the pH of the rain drops below 5.6, it becomes acidic. Actually, the acidity of rain water is increased due to the large quantity of SO₂ and oxides of nitrogen present in the air.

Acid rain is caused by the presence of oxides of sulphur and nitrogen in the atmosphere. Oxides of sulphur are released into the environment mostly due to fossil fuel combustion, ore smelting and petroleum refining. Nitrogen oxides are released into the atmosphere largely due to automobile exhaust and fossil fuel combustion. SO₂ and NO₂ after oxidation and reduction with water form sulphuric acid and nitric acid respectively. These acids are major contributors to acid rain.


Harmful effects of acid rain. Acid rain:
(i) is toxic to vegetation and aquatic life. 
(ii) damages leaves of trees and plants and retards the growth of forests. 
(iii) damages buildings and statues,
(iv) corrode water conduct pipes resulting in the leaching of heavy metals such as iron, lead and copper into drinking water.

The air where Taj Mahal (in Agra) is located, contains very high levels of sulphur and nitrogen oxides. The resulting acid rain reacts with marble (CaCO₃) of Taj Mahal. 

As a result, the wonderful monument is being slowly eaten away and the marble is getting discoloured and lustreless.

i) The earth receives a large amount of energy from the sun. Out of the total energy received, about 66% is absorbed by the earth while the rest 34% is reflected (or scattered) back into space.
ii) The reason for this energy reaching the earth is that visible and ultraviolet radiations (short wavelength) can pass through CO₂ and water vapours present in the atmosphere. Ultraviolet radiations are absorbed by ozone layer but visible light reaches the earth. As a result, the temperature of the earth increases. However, when the earth cools, the energy is re-emitted from the earth’s surface in the form of infra-red radiations (longer wavelength and have a heating effect). I.R. radiations are heat radiations. These I.R. radiations can be absorbed by CO₂ and H₂O vapours but can not pass through them.
iii) The heat thus absorbed by CO₂ and H₂O vapours cannot go to the upper atmosphere and is radiated back to the surface of the earth. In this way, additional heat is kept within the lower atmosphere and warming of the earth occurs. This warming of the earth leads to the warming of air (due to the presence of green -house gases) is called global warming. This warming of the earth or global warming due to re-emission of the sun’s energy absorbed by the earth followed by its absorption by CO₂ molecules and H₂O vapours present near the earth’s surface and then heat radiated back to the earth by CO₂ and H₂O vapours is called ‘green- house effect.’

(i)  The green -house gases (namely carbon dioxide, methane, nitric oxides, water vapours, chlorofluorocarbons and ozone) are useful in keeping the earth warm with an average temperature of about 15°C. In the absence of these green- house gases, the average surface temperature will fall to about–18°C. As a result, this temperature will freeze all water and destroy most of the living beings. The escape of green -house gases in the atmosphere is equally harmful.
(ii)  Further, the amount of CO₂ increases in the atmosphere as a result of combustion of the fossil fuels or by human activities. Thus heat radiated back to the earth by CO₂ and H₂O vapours will increase. Consequently, the temperature of the earth’s surface will increase. Thus evaporation of surface water will increase which further helps in the rise of temperature.
This increase in temperature may result in the melting of glaciers and polar ice caps and level of sea water may rise thereby flooding the coastal lands and thus have a drastic effect on the climate. Also presence of green house gases has led to the warming of air called global warming.

Water pollution is defined as the contamination of water by foreign substances which make it harmful for the health of animals or plants or aquatic life and make it unfit for domestic, industrial and agricultural use. The principal sources of water pollution may be classified as:
(i) Organic matter
(ii) Organic chemicals
(iii) Oils
(iv) Plant nutrients
(v) Sediments
(vi) Heat
(vii) Inorganic chemicals and minerals
(viii) Pesticides
(ix) Detergents
(x) Radioactive wastes.

BOD: BOD stands for biochemical oxygen demand. It is a measure of the dissolved oxygen that would be needed by the microorganism to oxidise some of the inorganic and organic compounds present in the polluted water. BOD, therefore, is a measure of the contamination caused by the totality of these compounds which can be oxidised in the presence of micro-organisms. The BOD is taken as the measure of water quality. Clean water has BOD value of less than 5 ppm whereas highly polluted water has BOD value of 17ppm or more.
Determination of BOD: The water sample is first saturated with oxygen. It is then kept at constant temperature (usually 20°C) for five days. In this period of time, the micro-organisms in the water sample oxidise pollutants. The remaining amount of dissolved oxygen is determined and BOD is obtained by subtraction.
COD: COD is another parameter called the chemical oxygen demand. It is expressed in terms of amount of oxygen in ppm that would be required to oxidise the contaminants.
Determination of COD: The water sample is treated with a known quantity of an oxidising agent such as potassium dichromate in acidic medium. This oxidises most of the polluting substances, including those which are resultant to microbial oxidation. The remaining K₂Cr₂O₇ is determined by back titration using standard solution of Mohr’s salt (reducing agent). From the concentration of K₂Cr₂O₇ used up, the amount of oxygen used in the oxidation may be calculated from the following chemical equation:

The results are expressed in terms of amount of oxygen in ppm that would be required to oxidise the contaminants. This is called COD.

The concentration of dissolved oxygen in water is a useful tool for the support of aquatic life. The fish growth is inhibited if the dissolved concentration of oxygen in water is below 6 ppm.
Oxygen reaches water through two sources:
The first source is that oxygen dissolves at the surface of the water from the atmosphere. Still, water takes up oxygen slowly whereas turbulent water takes it up more rapidly since bubbles are often submerged.
The second source is from photosynthesis, where there are many aquatic green plants present, the water often becomes super saturated with oxygen during daylight. At night, photosynthesis stops but the plants continue to respire and actually reduce the amount of dissolved oxygen.

Deoxygenation of water takes place through the following processes:
(i) Oxidation of organic matter: The dissolved oxygen in water is consumed rapidly by microorganisms to oxidise organic matter of sewage.
 

(ii) Bio-oxidation of the nitrogenous matter: Oxygen in water may be consumed by the bio-oxidation of the nitrogenous material. 

(iii) Oxidation of chemical reducing agent: The dissolved oxygen in water may be consumed by the chemical or biochemical oxidation of chemical reducing agents. 

There are two major sources of water pollution i.e. sewage and industrial wastes. 
Treatment of sewage:
(i) Sewage must be churned by machines and churned sewage is passed into a tank with a gentle slope. Heavier particles settle and the water flowing down is relatively pure.
(ii) Water must be sterilised with the help of chlorination. It kills microbes of sewage fungus as well as some pathogens, spores.
(iii) Treatment of water with lime, alum etc also helps in its purification.
Treatment of industrial waste: The chemical substances present in the industrial waste products dissolved in water can be precipitated by suitable chemical reactions and removed later on from water. Photocatalysis and ion-exchanges have been developed for the treatment of industrial wastes. 

The following parameters must be followed before water is supplied for drinking purposes:
(i) It must be colourless and free from any odour or smell.
(ii) Its pH should be between 6·0 to 9·0.
(iii) It must be free from harmful chemicals.
(iv) It should be free from micro-organisms which lead to many diseases.
(v) Its turbidity should be less than 10 ppm.
(vi) It should be reasonably soft.

Soil pollution is due to the dumping of solid and semi-solid waste from agriculture, industry and urban areas in the soil. Soil pollution is different from air and water pollution in following respects:
(i) Soil pollutants do not get dispersed unlike air and water pollutants. 
(ii) The pollutants remain at a particular place for a longer period. 
(iii) The pollution is localised unlike air and water pollution. 

(i) Industrial wastes: Industrial wastes like scarp flash and effluents get mixed with soil. These are mostly toxic due to the presence of certain cyanides, chromates, acids, alkalies and metals like nickel, mercury etc. 
(ii) Urban wastes: Urban wastes such as domestic refuse, commercial wastes, rubbish from construction sites, waste paper, rags, fibres, broken glass articles slowly mix up with the soil. They lead to pollution such as foul smell.
(iii) Faulty agricultural practices: Fertilisers, pesticides weedicides etc. are chemical substances and from the soil they pass to ground water and are harmful to aquatic animals.
(iv) Radioactive pollutants: Dumping of the nuclear wastes from the nuclear power plants into the soil has been one of the greatest sources of radio active pollution of the soil. All the radio active wastes from atomic and hydrogen bombs emit radiations which are disastrous for the life on the earth.
Control of soil pollution:
(i) Use of manures: Manure prepared from animal dung and another farm refuse is added to the soil to maintain its fertility.
(ii) Use of biofertilizers: Biofertilizers such as nitrogen fixing bacteria and blue-green algae are inoculated in order to bring about the enrichment of the soil.
(iii) Proper sewage system: This system must be employed and sewerage recycling must be installed in all towns and cities. 

In the Union Carbide Ltd. Plant in Bhopal, three tanks that stored methyl isocyanate, pressure rose in one of the tanks and blasted. As a result, methyl isocyanate escaped into the atmosphere. Actually, a small amount of water seeped into the storage tank and initiated the following hydrolysis reaction:

Further the water that entered the tank was not pure and impurities present initiated the following polymerisation reaction:

Due to simultaneous occurrence of the above two exothermic reactions, the extremely toxic methyl isocyanate gas from the reservoir escaped into the atmosphere. 

Domestic waste comprises of two types of materials:
(i) Biodegradable materials
(ii) Non-biodegradable materials
(i) Biodegradable materials include leaves, rotten food etc. The biodegradable waste should be deposited in the landfills. With the passage of time, it is converted into manure. 
(ii) Non-biodegradable materials include plastics, glass, metal scrap etc. The non-biodegradable waste is sent to industry for recycling. 

A Large number of compost producing pits are being set up different places in an agricultural field or garden to protect ourselves from files and bad odour. These should be kept covered for minimising the bad colour and also preventing the entry of flies into it. The recyclable materials like glass, plastics, newspapers etc. should be sold to the vendor who further sells it to the dealer. The dealer further supplies it to the industry involved in recycling process where these wastes are converted into good produce.

Pesticides: The chemical substances which are used to kill or block the reproductive process in harmful organisms such as rodents, mites, bacteria, fungi etc. Synthetic (man-made) pesticides are broad-based in their action, that is they harm much more organisms than these intended. These substances are non-biodegradable. Pesticides can be divided into three types namely insecticides, herbicides and fungicides.
Herbicides: Herbicides or weedicides are used to kill weeds. Weeds are undesirable plants which grow along with the main crops. Common herbicides are:
(i) 2, 4-D (2, 4-dichloro phenoxy acetic acid)
(ii) MPCA(2- methyl-4-chloro-1-phenoxyacetic acid)
(iii) Butachlor. 
The herbicides called triazines are widely used to kill weeds in corn fields.

The solar energy radiated back from earth surface is absorbed by the green- house gases (such on CO₂, CH₄, O₃, CFC’s and water vapour) present near the earth's surface. As a result, the green- house gases heat up the atmosphere near the earth surface and keep it warm (Global warming). Thus, they keep the temperature of the earth constant and permit the growth of plants and existence of life on the earth. On the other hand, in the absence of greenhouse gases, the vegetation and life on the earth have no meaning at all. 

By green chemistry, we mean producing the chemicals of our daily needs using such reactions and chemicals processes which neither use toxic chemicals nor emit such chemicals into the atmosphere. 
Green chemistry uses mild and environmental friendly reagents such as sunlight, microwaves, sound waves and enzymes. These are not toxic reagents. Recently many photochemical reactions are carried out using sunlight and ultraviolet light. Many known reactions have also been carried out by using microwave arrangement. Instead of microwaves, sound waves have also been used to carry out certain known chemical reactions. It is evident that if continuous efforts are made to develop green chemistry, it will certainly help us to keep our environment pollution free.

Water pollution arises as a result of various human activities. This includes discharges from wastewater treatment plants, runoff from agricultural fields, storm-water drainage, etc. Pollutants from these sources enter the water bodies, thereby contaminating the water and rendering it impure. Industries and chemical factories discharge toxic, heavy metals as Fe,Mn,Al, etc,. along with organic wastes into water. Domestic sewage and animal excreta are also responsible for pathogenic contamination of water.These pollutants make water unfit for drinking. therefore, all industrial and chemical discharges should be made free from toxic metals before allowing them to enter a water body.The concentration of these pollutants should be checked regularly. Compost should be preferred over chemical fertilisers in gardens and agricultural fields to avoid harmful chemicals from entering ground water.

BOD (Biochemical Oxygen Demand): BOD stands for biochemical oxygen demand. It is a measure of the dissolved oxygen that would be needed by the microorganism to oxidise some of the inorganic and organic compounds present in the polluted water. BOD, therefore, is a measure of the contamination caused by the totality of these compounds which can be oxidised in the presence of micro-organisms. The BOD is taken as the measure of water quality. Clean water has BOD value of less than 5 ppm whereas highly polluted water has BOD value of 17 ppm or more. 

we have observed soil pollution in nearby agricultural fields. This is due to the use of a lot of fertilisers are chemical substances which help the soil to become fertile. But an excess of fertilisers pass to the groundwater from the soil and as runoff to lakes rivers and seas and are harmful to aquatic animals.

The control of soil pollution can be made by using manure and biofertilizers instead of fertilisers:

Use of manure : manure is a semi-decayed organic matter which is added to the solid to maintain fertility. These are mostly prepared from animal dung and other farm refuse. These are much better than the commonly used fertilisers.
Use of biofertilizers: These organisms are inoculated in water to bring nutrient enrichment of the solid, e.g., nitrogen-fixing bacteria and blue-green algae.

A.

Methyl isocyanate

Methyl isocyanate CH₃-N=C=O (MIC Gas) gas leaked from the storage tank of the union carbide plant in Bhopal gas tragedy.

C.

only F^– 

C.

Ozone layer does not permit infrared radiation from the sun to reach the earth

Ozone layer does not allow ultraviolet radiation from sun to reach earth.

C.

Oxides of sulphur and nitrogen

NO, NO₂, SO₂ and SO₃ are responsible for smog.

D.

[3Ca₃(PO₄)₂. CaF₂]

$3C{a}_3(P{O}_4{)}_2. Ca(OH{)}_2] + 2F^{-} \stackrel{ }{\to }\underset{Harder Teeth enamel}{3C{a}_3(P{O}_4{)}_2.Ca{F}_2] }+2O{H}^{-}$

A.

Ozone

Among the given chlorofluorocarbons are the compounds that are responsible for ozone depletion which degrades ozone into molecular oxygen. It is not a component of photochemical smog. while other given compounds are the main components of photochemical smog.

D.

Photochemical smog does not cause irritation in eyes and throat. 

Photochemical smog is formed in warm and sunny climate during day time by the action of sunlight on primary pollutants. It contains nitrogen oxides, ozone, PAN etc. , which are oxidising in nature. so, photochemical smog is an oxidising agent in character. it causes irritation in eyes and throat. 

B.

reduce the use and production of hazardous chemicals

Green chemistry involves such reactions which reduce the use and production of hazardous or toxic chemical to reduce pollution from the environment.

B.

Micro-organisms present in the soil

Micro-organisms present in the soil is a sink for CO

D.

N₂O

Nitrous oxide (N₂O) occurs naturally in the environment.

In an automobile engine, when fuel is burnt dinitrogen and dioxygen combine to yield NO and NO₂

B.

Troposphere

Meso-sphere is the coldest region having about - 100^o C temperature.