CHEMISTRY - Concept of Emission and Absorption Spectra

An instrument used to separate the radiations of different wavelengths is called Spectroscope or a spectrograph. A spectrograph consist of a prism or a diffraction grating for the dispersion of radiations and a telescope to examine the emergent radiations with the human eye. However, if in a spectroscope , the telescope is replaced by a photographic film, the instrument is called a spectrograph and the photograph of the emergent radiation recorded on the film is called a spectrogram or simply a spectrum of the given radiation. The branch of science dealing with the study of spectra is called spectroscopy.
The spectra are broadly classified into :-
1. Emission Spectra.
2. Absorption Spectra.

1. Emission Spectra :- When the radiation emitted from the some sources, e.g. from the sun or by passing electric discharge through a gas at low pressure or by heating some substances to high temperature etc. is passed directly through the prism and then received on the photographic plate, the spectrum obtained is called 'Emission spectrum'.

Depending upon the source of radiation, the emission spectra are mainly of two types :-

(A) Continuous spectra :- When white light from the any source such as sun, a bulb or any hot glowing body is analysed by passing through a prism , it is observed that it splits up into seven different wide bands of colours from violent to red, ( like rainbow ). These colours are so continuous that each of them merges into the next. Hence the spectrum is called continuous spectrum.
It may be noted that on passing through the prism , red colour with the longest wavelength is deviated least while violet colour with shortest wavelength is deviated the most.

(B) Line Spectra :- When some volatile salt is placed in the Bunsen flame or an electric discharge is passed through a gas at low pressure, light is emitted. The colour of light emitted depends upon the nature of substance. For example , sodium or its salts emit yellow light while potassium or it's gives out violet light. If this light is resolved in a spectroscope, it is found that no continuous spectrum is obtained but some isolated coloured lines are obtained on the photographic plate separated from each other by dark spaces. This spectrum is called 'Line emission spectrum' or simply Line spectrum.
Each line in the spectrum corresponds to a particular wavelength. Further, it is observed that each element gives its own characteristic spectrum, differing from those of all other elements. For example, Sodium always gives two yellow lines. Hence, the spectra of the element are described as their finger prints differing from each other like the finger prints of the human beings.

2. Absorption Spectra :- When white light from any source is first passed through the solution or vapours of a chemical substance and then analysed by the spectroscope, it is observed that some dark lines are obtained in the otherwise continuous spectrum. These dark lines are supposed to result from the fact that when white light ( Containing radiations of many wavelengths ) is passed through the chemical substance , radiations of certain wavelengths are absorbed, depending upon the nature of the element. This shows that the wavelengths absorbed were same as were emitted in the emission spectra. The spectrum thus obtained is , therefore , called 'absorption spectrum' .

Discovery of Neutron

Moseley , in 1913 , performed experiments to determine the exact quantity of charge present on the nucleus. The number of units of positive charge present on the nucleus was called the atomic number of the element. Since each unit of positive charge corresponds to one proton, therefore , atomic number was equal to the number of protons present in the nucleus. Further , since each proton has one unit mass on the atomic mass scale , therefore , the mass of the atom was calculated from the number of protons present in the nucleus (remembering that electrons have negligible mass).
Chadwick , in 1932 , performed some scattering experiments in which he bombarded some light elements like beryllium and boron with fast moving alpha - particles. He found that some new particles were emitted which carried no charge I.e. were neutral but had a mass nearly equal to that of proton. This particle was termed 'Neutron'.

Hence , the neutron may be defined as :- A neutron may be defined as that fundamental particle which carries no charge but has a mass nearly equal to that of hydrogen atom or proton.

Discovery of Proton - Study of Anode Rays

Since the atom as a whole is electrically neutral and the presence of negatively charge particles in it was established , therefore it was thought that some positively charged particles must also be present in the atom. For this purpose, Goldstein in 1886, performed discharge tube experiments in which he took perforated cathode and a gas at low pressure was kept inside the tube. 
On passing high voltage between the electrodes , it was found that some rays were coming from the side of the anode which passed through the holes in the cathode and produced green fluorescence on the opposite glass wall coated with zinc sulphide. These rays were called anode rays or canal rays or positive rays.

Origin of Anode Rays :- These rays are believed to be produced as a result of the knock out of the electrons from the gaseous atoms by the bombardment of high speed electrons of the cathode rays on them. Thus, anode Rays are not emitted from the anode but are produced in the space between the anode and the cathode.

Properties of Anode Rays :- In actual there are the same method of we discuss in cathode rays. The anode rays were found to possess the following main properties :-

1. They travel in the straight lines. However, their speed is much less than that of the cathode rays.

2. They are made up of material particles.

3. They are positively charged, as indicated by the direction of deflection of these rays in the electric and magnetic fields. That is why they are called 'positive rays' 

4. Unlike cathode rays, the ratio, charge/mass is found to be different for the particles constituting anode rays when different gases are taken inside the discharge tube. In other words, the value of charge/mass depends upon the nature of the gas taken inside the discharge tube.

5. The value of the charge on the particles constituting the anode rays is also found to be depends upon the nature of gas taken inside the discharge tube. However, the charge on these particles is found to be a whole number multiple of the charge present on the electrons. In other words , these positively charged particles may carry one, two or three units of positive charged , depending upon the number of electrons knocked out.

6. The mass of the particles constituting the anode rays is also found to be different for different gases taken in the discharge tube. However, it's value is found to be nearly equal to be that of the atom of the gas.

Hence, a proton may be defined as :- A proton may be defined as that fundamental particle which carries one unit positive charge and has a mass nearly equal to that of hydrogen atom. 

It may be mentioned here that in case of any other gas taken inside the discharge tube, the mass of the positively charged particles was found to be nearly a whole number multiple of the mass of the proton. Hence, it led to the belief that porton must be fundamental particles present in all atoms and that the atoms of gases other than hydrogen contained some whole number of protons.

Discovery of Electron - Study of Cathode Rays

The electrical nature of matter had been indicated in very early experiments on the production of frictional electricity.  This was further confirmed by the experiments on the electrolysis of substances by Davy, Stoney and Faraday in the first half of the nineteenth century. However, the discovery of electrons came as a result of the study of conduction of electricity through gases as explained :-

William Crookes, in 1879, study the conduction of electricity through gases at low pressure. For this purpose, he took a discharge tube which is a long glass tube , about 60cm long, sealed at both the ends and fitted with the two metal electrodes. It has a side tube fitted with a stop cock.
The tube is connected to a vacuum pump and the pressure inside the discharge tube is reduced to at low as 0.01 mm. Now, when a high voltage is applied between the electrodes, it is found that some invisible rays are emitted from the cathode. The presence of these rays is detected from the fact that the glass wall of the discharge tube opposite to the cathode begins to glow with a faint greenish light. Obviously, this must be due to the bombardment of the wall by some rays emitted from the cathode. These rays were cathode rays.

Properties of Cathode Rays :- From the various experiments carried out by J.J. Thomson (1897) and others, the cathode rays have been found to posses the following properties :-

1. They produce a sharp shadow of the solid object placed in their path. This shows that cathode rays travel in straight lines.

2. If a light paddle wheel mounted on the axle is placed in their path, the wheels begins to rotate. This shows that cathode rays are made up of material particles.

3. When an electric field is applied on the cathode rays , they are deflected towards the positive plate of the electric field. This shows that cathode rays carry negative charge.

4. When cathode rays strike a metal foil , the latter becomes hot. This indicates that cathode rays produce heating effect.

5. They cause ionization of the gas through which they pass.

6. They produce X-rays when they strike against the surface of hard metals like tungsten, molybdenum etc.

7. They produce green fluorescence on the glass walls of the discharge tube as well as on certain other substances such as zinc sulphide.

8. They affect the photographic plates. 

9. They possess penetrating effect. They are stopped only by metal foils of greater thickness.

Hence, the electron may be defined as :-  An electron is that fundamental particle which carries one unit negative charge and has a mass nearly equal to 1/1837 th of that of hydrogen atom.

Origin of Cathode Rays :- The cathode rays are first produce from the material of the cathode and then from the gas inside the discharge tube due to bombardment of the gas molecules by the high speed electrons emitted first from the cathode.

CHEMISTRY - Thermistors

Thermistors :- A thermistor is a heat sensitive device whose resistivity changes very rapidly with the change of temperature. The temperature co-efficient of resistivity of a thermistor is very high, which may be positive or negative.
Thermistor are usually prepared from oxides of various metals such as nickel, iron, copper, cobalt etc. These compounds are also semi-conductor. The size of thermistor is very small. The thermistors are generally in the form of beads, discs or rods. A pair of platinum leads are attached at the two ends for electric connections. This arrangement is sealed in a small glass bulb.
A thermistor can have a resistance in the range of 0.1 ohm to 10 power 7 ohm, depending upon its composition. A thermistor can be used over a wide range of temperature.
A thermistor differs from an ordinary resistance in the following ways :-
1. The temperature coefficient of resistivity of a thermistor is a very high.
2. The temperature coefficient of resistivity of a thermistor can be both positive and negative.
3. The resistance of thermistor changes very rapidly with change in temperature.

Important application of thermistor :-

1. Thermistors are used to detect small temperature changes and to measure very low temperature. A typical thermistor can be easily measure a change in temperature of 10 power -3 degree celcious

2. Thermistor are used to safe-guard the filaments of the picture tube of a television set against the variation of current.

3. Thermistor are used to temperature control units of industries.

4. Thermistors are used in the protection of windings of generators, transformers and motors.

5. Thermistors are used for voltage stabilisation and remote sensing.

Super-Conductivity :- Professor K. Onnes in 1911 discovered that certain metals and alloys at very low temperature lose their resistance considerably. This photochemical is known as super-conductivity. As the temperature decreases , the resistance of the material also decreases , but when the temperature reaches a certain critical value ( called critical temperature or transition temperature ) , the resistance of the material completely disappeares I.e it becomes zero. Then the material behaves as if it is a super-conductor and there will be flow of electrons without any resistance whatsoever. The critical temperature is different for different materials.
Super-conductivity is a very interesting field of research all over the world these days. The scientists have been working actively to prepare a room temperature super conductor and have met with some success only.

Applications of super-conductors :-

1. Super conductors are used for making very strong electromagnets

2. Super conductivity is playing an important role in material science research and high energy particles physics.

3. Super conductivity is used to produce very high speed computers.

4. Super conductors are used for the transmission of electric power.

Particulates - as Pollutants

Small solid particles and liquid droplets suspended in the air are collectively are called as particulates. Some important characterstics of these particulates are as under :-

* Size. Their size varies from 5nm to 500000 nm.
* Mass. Their mass varies from 60ug to 2000ug per cubic meter in urban and industrial areas.
* Concentration. Their number varies from few hundred per cubic cm in clear air to polluted air.
* Surface Area. They have large surface Area due to which they have good sorption for organic as well as inorganic matter.
* Chemical nature. Different types of particulates have a different chemical nature.

Sources of Particulates :-

* Natural Sources :- These include volcanic eruptions , grinding , blowing of dust and soil by the wind , spraying of salts by seas and oceans etc.

* Man-made sources :-

(A) Soot :- These are the smallest particulates. They are the most common particulates introduced in the air due to incomplete combustion of fossil fuel such as coal wood , fuel oil , natural gas etc.

(B) Fly Ash :- These are ash particles coming alongwith furnace flue gases and are the biggest particulates. They are produce due to high ash fossil fuels as in the thermal power plants or in smelting and mining processes.

(C) Inorganic particulates :- A few common inorganic particulates are :-
* Metallic particles :- These are discharging into the air during finishing of metals.
* Metal oxides :- They are released into the atmosphere during the combustion of fuels containing metals compounds.
* Lead halides :- Tetraethyl lead is added to gasoline to act as an anti - knocking agent.
* Asbestos dust :- Asbestos is a fibrous silicate mineral which is introduced in the atmosphere by the industries manufacturering asbestos sheets, ropes etc.
* Sulphuric acid and nitric acid :- These particulates in the form of droplets produce in the atmosphere when oxides of sulphur or nitrogen come in contact with the moisture.

(D) Organic Particulates :- Organic Particulates include paraffins, olefins and aromatic compounds , particularly polycyclic aromatic hydrocarbons. These originate from the combustion of static fuels or automobiles fuels or from petroleum refineries. The polycyclic hydrocarbon are easily absorbed on the soot particles and prove to be great health hazards.

Harmful effects of particulates pollutants :-

* Effect on human :- Different type of lung disease are causes by different type of particulates. e. g asbestos cause asbestosis , dust containing free silica cause silicosis etc.
Lead particulates are highly poisonous and have a serious effect on a children's brain and cause cancer.
* Effacts on Plants :- Particulates deposit on the leaves of the plants thereby blocking the stomata and retarding and transmission of mineral from the soil.

* Effects on material :- Particulates damage a large number of material e.g suit , dust , fumes , clothes , building , soil , etc. They also accelerates corrosion of metals.

* Effect on Visibility :- The light scattered by particulates result in poor visibility of the object in front of us as it happened at night when we are driving a car or scooter especially on foggy nights. They have a very harmful effects on eye retina and harmful effects on Visibility of a human beings which is a very dangerous for any one of human beings.

Effect on Climate :- Particulates act as a nuclei for cloud formation and hence effect the climate. Further , they scattered and reflected back the heat of the sunlight and controlling the warming up of the earth surface during to increase in carbon dioxide I.e. they counteract the green house effect.

Control of Particulates Pollution :- To reduced the pollution in the air , the removal of Particulates is very essential. This is done by using the following techniques which are based on the settling or washing away of the particulates.

* Gravity settling chamber :- It consist of a horizontal rectangular tank. It is so large that the effluented gases which are allowed to enter into the slow down.

* Cyclone Collector :- In this techniques, the gas is allowed to flow at tight circular spiral. Due to the centrifugal force , the particulates move towards the wall and start settling down.

* Wet Scrubbers :- In this techniques, spray towers and chamber are used into which a suitable liquid is introduced in the form of a fine spray which washes away the particulates.

* Electrostatic precipitation :- This is the most effective and efficient method for the removal of Particulates. About 99% of the particulates can be removed by this method.

Oxides of Sulphur :- as Pollutant

The two oxides of sulphur, I.e. SO2 and SO3 are the most serious pollutants of the atmosphere. Both are colourless gases with pungent smell.

Natural Source of release of sulphur dioxide into the atmosphere is the volcanic eruptions. It accounts for about 67% of the total amounts of sulphur dioxide present on the globe. The remaining 33% is discharge into the atmosphere due to human activity.

Man-Made Sources mainly include the following :-
* Combustion of sulphur containing coal and fuel oil in the thermal power plants.
* Roasting of sulphide ores in the smelting industries.
* A small amounts of sulphur dioxide undergoes photolytic oxidation in the air to form SO3.
* In the presence of water vapours, SO3 reacts to form sulphuric acid. Which remains suspended in the air as droplets or comes down in the form of acid rain.

Harmful Effects of Oxides of Sulphur :-
* SO2 and SO3 are both strongly irritating to the respiratory tract. SO2 at concentration of 5 ppm cause throat and eye irritating. It causes breathlessness and affects larynx, I.e voice box. SO3 is more harmful even at a concentration of 1 ppm and causes severe discomfort. Elderly persons and those with heart and lung disease are most seriously affected.
* Even a very low concentration of sulphur dioxide has a very damaging effects on the plants. If exposed for a Long time , a few days or weeks , it shows down the formation of chlorophyll resulting in the injury to the leaf including loss of green colour. This is called chlorosis.
* Sulphur oxides as such or after beings converted into sulphuric acid damage a number of material :

(a) They damage building material like especially marble.

(b) They corrodes metals, particularly iron and steel.

(c) They bring about fading and deterioration of fabrics leather and paper.

(d) They affect the colour of the paints.

Control of Oxides of Sulphur pollution :- The sulphur oxides pollution can be controlled in a number of ways as follows :-

* By use of low sulphur or sulphur free fuels like natural gas.

* By removing sulphur from the fuel before burning.

* By making sulphur free liquefied gaseous fuel from coal.

* By using alternate sources of energy , e.g hydroelectric power plants and nuclear plants.

* By removal of Oxides of sulphur from flue gases.

Oxides of Nitrogen - as Pollutants

A number of oxides of nitrogen such as NO, N2O, N2O3 are introduced into the atmosphere due to natural sources and due to human activities. Out of these only two oxides , I.e NO and NO2 are considered as Pollutants. Nitric oxides is colorless and odourless gas whereas nitrogen dioxide is reddish Brown gas having pungent smell and is suffocating in nature.

Sources of oxides of nitrogen production :-
(a) Natural sources :- Natural bacterial action is the only natural source which discharges oxides of nitrogen , mainly in the form of NO, into the atmosphere in huge amounts. Besides , lightning discharge also results in the combination of N2 and O2 to form NO.

(b) Man-Made Sources :-

* Combustion of fossil fuel both in motor vehicle and stationary sources or powe plants produces so high temperature at which oxidation of the atmospheric nitrogen takes place forming NO and NO2.

The first reaction takes place at a temperature of 1200-1750*C. The reverse reaction is prevented due to rapid cooling of NO. The second reaction takes places about 1100*C.

* Chemical industries like sulphuric and nitric acid industries produce oxides of nitrogen as by - products which are discharge into the air. 

Sinks of Oxides of nitrogen pollutant :- The average residences time of NO is 4 days while that of NO2 is 3 days. During this time, they are converted into nitric acid through the reactions in which ozone plays an important role.

Harmful effects of Oxides of nitrogen pollution :-
* Neither NO nor NO2 is a health hazards at the present pollution levels. Out of the two , NO2 is more dangerous as it is a toxic gas and affects a respiratory system and damages the lungs. It's concentration at 100ppm may be fatal for animals and human beings. Though NO has about 1500 times greater affinity than CO for hemoglobin but fortunately, it is unable to enter the blood stream from the atmosphere. But it's presence in large amount may result in the increase of NO2 by oxidation and thus become harmful.
* Some sensitive Orr ornamental plants are affected by the presence of Oxides of nitrogen in the ambient air even at low concentration. Their photosynthetic activities decreases  and they start shedding their leaves and fruits.
* In the presence of Oxides of nitrogen , textiles like cotton , rayon etc. start fading.
* The acid rain causes the pH of the soil to decreased to 4 or 5 and thus reduced the fertility of the soil.
* The most serious hazards of nitrogen oxides is in the formation of an unpleasant mixture of gases and particulates that make up photochemical smog.

Control of oxides of nitrogen :-
* Just as in case of CO , the catalytic converters are used in the automobiles exhaust which is in the first stage convert the oxides of nitrogen to free N2 or to a small amount of NH3.

* The flue gases coming from the power plants or industrial units and containing NO2 and SO2 are free from these gases by scrubbing them with sulphuric acid.

Green Chemistry

One of the major cause of environmental pollution is the rapid industrilization particularly the development of those industries which either produce or use toxic chemicals. One way to protect our environment from chemical effluents and wastes is to use Green Chemistry. By green chemistry , we mean producing the chemicals of our daily needs using such reactions and chemical processes which neither use toxic chemicals nor emit such chemicals into the atmosphere. Although it is a challenging task but some initial efforts have already been made to acheive this goal.

Techniques Used :- Green Chemistry does not employ toxic reagent or solvents and severe conditiin but use mild and environmental friendly reagent such as sunlight , microwaves , sound waves , and enzymes. Each of these is briefly described below :-

1. Use of sun light and microwaves :- The use of sunlight and ultraviolet light have given birth to photochemistry. During the past forty years, a number of photochemical reactions have been studied and their condition standardized. One major advantage of these reaction is that sometimes the prroduct of photochemical reactions are such that they cannot obtained by usual chemical method. Recently, many known reaction have also been carried out using microwave ovens. In these reaction , no toxic solvent are used but only the reactant in proper ratio are mixed on a solid support such a alumina and exposed to microwaves. The reaction is complete with in minutes and yields are not only comparable but in some cases, they are even higher than those obtained under serve conditions using toxic solvents. Thus the uses of microwave have not only reduced the time of reaction but have also increased the yields. Efforts are , however, needed to develop the solid support and optimization the dose of microwave radiation to realise the desired results.

2. Use of Sound Waves :- Instead of microwaves , sound wave have also been used to carry out certain known chemical reaction with encouraging results. This branch of chemistry is called sonochemistry. Here again efforts are needed to developed conditions to achieve the desired goals.

3. Use Of Enzymes :- One of the most throughly investigated environmental friendly reagents are enzymes. These work in aqueous solution and at ambient temperature. Using these enzymes many biochemical method have been developed to prepare precursors and intermediates of certain medicine and antibiotics.

Green Chemistry in Day-to-Day Life :- A few examples are of the chemical used/produced and the techniques employed in green chemistry are given below :-

1. Dry cleaning of clothes and laoundary :- Earlier tetrachloroethene was used as solvent for dry cleaning. This compound is suspected to be carcinogenic and contaminated the ground water. It's use has been replaced by liquefied carbon dioxide along with a suitable detergent which is less harmful. Similarly, for bleaching of clothes in the laundary, hydrogen peroxide is beings used which gives better results and is not harmful. Moreover, it saves as a lot of water.

2. Bleaching of paper :- Earlier , chlorine gas was used for bleaching paper which is highly toxic chemical. It's use has been replaced by hydrogen peroxide along with a suitable catalyst.

3. In synthesis of chemicals :- For examples ,  ethanal is now-a-days being prepared on a commercial scale using environment - friendly chemical and conditions. The yields is about 90% .

From the above discussion , it is evident that if concerted efforts are made to develop green chemical products, it will certainly help us to keep our environment pollution free.

CHEMISTRY - Troposphere Pollution

The five major pollutants present in the air :-

1. Carbon monoxide
2. Hydrocarbon
3. Oxides of nitrogen
4. Oxides of sulphur
5. Particulates

Now, we shall discuss the sources, harmful effect and the method to control each of these pollutant. It is colourless, odourless and tasteless gas. It is 96.5% as heavy as air.

* Sources of Carbon monoxide production :-

(a) Due to human activity :-   

*  Incomplete combustion of fossil fuels or hydrocarbon fuels in the automobiles.
* Forest fires or agricultural wastes burning.
* Reaction of carbon dioxide with carbon containing material at high temperature.

(b) Natural sources :-
* From conversion of methane produced by the bacterial decay ogre living or organic matter (80%).
* From synthesis and decay of chlorophyll (3%).
* From oceans and other undetermined sources (10%).

Harmful Effect of Carbon monoxide :- carbon monoxide is poisonous because it combines with hemoglobin of the red blood corpuscles about a 200 times more easily than does oxygen to form carboxyhemoglobin reversibly as follows :-
                Hb. + CO - HbCO
The normal function of the hemoglobin is to combines with Oxygen in the lungs to form hemoglobin.
The oxyhemoglobin travels to the different body cells where it gives up oxygen and takes up carbon dioxide for returns to the lungs which is then exhaled out. However, if large amounts of carbon monoxide is present in the surrounding air, it combines more readily with the hemoglobin. Thus, transport of oxygen to the differents parts of the body is inhibited. In other words, the body becomes oxygen-starved.

Control of Carbon monoxide pollution :- The main source of CO pollution due to human activities is the use of internal combustion engine in the automobiles. These engine emit a mixture of CO , Oxides of nitrogen and particulates. However, any method used to control any one of these pollutants result in the increase in the other pollutants. A few method of controlling a CO Pollution are as follows :-
* The carbarettor is adjusted so as to give a proper air fuel ratio.
* The exhaust system is developed in such a way that it brings about complete combustion in the fuel so that negligible CO is formed.
* Catalytic converters can be fitted into the exhaust pipe which may convert the poisonous gases like CO into harmless products before they are thrown into the air.
* Modifications of internal combustion engine have been done by certain companies like Honda and Toyota of Japan instead of catalytic converters.
* Instead of gasoline , the use of CNG ( Condensed Natural gas ) and LNG ( Liquefied natural gas ) has been started as they are pollution free fuels.

2. Hydrocarbons as pollutants :- Many differents hydrocarbons are present in the air. Most of these hydrocarbons are of low molecular mass and are gases or volatile liquids at ordinary temperature. Methane the simplest hydrocarbons is the most abundant hydrocarbon carbon pollutant. In Los Angeles ( in USA ) where the vehicular traffics is very high , its amounts in the air was found to be 3ppm in 1965.

Sources of Hydrocarbons :-
(a) Natural Sources :-
* Trees emit large amounts of hydrocarbons in the air.
* Methane is released into the air in large quantities by the anaerobic decomposition of organic matter in soil, water and sediments.
* Domestic animals contributed a very huge amount of methane into the air every years.

(b) Man-Made sources :-
* Just as in case of CO , the largest man-made source of hydrocarbons pollutant is the internal combustion engine. About a 15% of the total hydrocarbons present in the air is from this source.
* Some hydrocarbon pollution also takes places due to burning of stationary fuel , I.e coal , wood, kerosene oil etc.
* Evaporation or organic solvent like benzene, toluene etc. during industrial operations add sub- stantially to the total hydrocarbon pollution.

Harmful Effects of Hydrocarbon Pollution :-
* At low concentrations, as they exist in through air at present, there are no harmful effects of the hydrocarbon on human beings. However , if their concentration rises to 500-1000 ppm, they have carcinogenic effect on the lungs and cause cancer.
* Due to their photochemical reactions with oxygen and oxides of nitrogen , they form photochemical oxidants and photochemical smog which have a strong damaging effect on the human beings as well as plants.

Control of Hydrocarbon Pollution :- As automobiles are the main sources of hydrocarbon pollution, all those steps taken to control CO pollution are needed for controlling hydrocarbon pollution. 

3. Oxides of Nitrogen .

4. Oxides of Sulphur .

5. Particulates .

ORGANIC CHEMISTRY - Resonance

The phenomenon of resonance is said to occur if for a molecule, we can write two or more than two Lewis structures which differ in the position of electrons and not in the relative position of atoms. The various Lewis structures are called canonical structures or the resonance structures. The real structure of the molecule is not represented by any one of the canonical structures but is a resonance hybrid of the various resonance structures. The various resonance structures are separated by a double headed arrow. This arrow should be clearly distinguished from that used for an equilibrium.
Resonance hybrid is always more stable than any of its canonical structures. Thus, the actual molecule of benzene is more stable than either of two Kekule structures. The magnitude of stability conferred for a molecule as a result of resonance is expressed in terms of resonance energy or delocalization energy. It is defined as the difference in internal energy of the resonance hybrid and that of the most stable canonical form.

Heat of hydrogenation :- Heat of hydrogenation is the amounts of heat evolved when one mole of an unsaturated compound is hydrogenrated.

Halogenation :- The halogenation of benzene and some others arenes is carried out at low temperatures, in the absence of sun light and in presence of Lewis acid such as anhydrous ferric or aluminium chloride as catalyst.

CHEMISTRY - Organic Compound

The simplest organic compounds containing only carbon and hydrogen are called hydrocarbons. They are considered to be parent organic compounds while all the organic compounds are thought to have been derived from them by replacement of one or more of their hydrogen atoms by other atoms or group's of atoms. As the study of ore chemistry advanced, the number of organic compounds became very large. As a result, it became rather inconvenient to study the chemical behavior of these compounds individually. Therefore, in order to simplify the systematize the study of organic chemistry, all the known organic compounds have been broadly divided into two categories depending upon the nature of their carbon skeleton. These are :-

1. Acyclic or open chain compounds.
2. Cyclic or closed chain compounds.

1. Acyclic or Open chain compounds :- These compounds contain open chains of carbon atoms in there molecules. The carbon chains may be either straight chains or branched chains.
Open chain compounds are also called alphabetic compounds since the earlier compounds of this class were obtain either from animal or vegetable fats.

2. Cyclic or closed chain or ring compounds :- These compounds contain one or more closed chain or rings of atoms in their molecules. Depending upon the constitution of the ring, these are further divided into the following two categories :-

* Homocyclic or Carbocyclic compounds :- These compounds contain rings which are made up of only one kind of atoms, I.e carbon atoms. These are further divided into the following two sub-classes.

(a) Acyclic compounds :- Carbocyclic compounds which are resemble alphabetic compounds in most of their properties are called alicyclic compounds.

(b) Aromatic compounds :- These are special type of cyclic unsaturated compounds. The name aromatic for this class of cyclic unsaturated compounds has been derived from the Greek word aroma meaning fragrant smell simple since most of the compounds discovered earlier had pleasant smell.

* Heterocyclic Compounds :- Cyclic compounds containing one or more heteroatoms in their ring are called heterocyclic compounds.
The heteroatoms commonly found in these compounds are oxygen , nitrogen and sulphur but occasionally phosphorus, boron, silicon and some metal atoms like tin, selenium etc. may also be present. Depending upon the chemical behavior , they are further classified into the following two categories :-

(a) Alicyclic Heterocyclic Compounds :-  Aliphatic cyclic compounds containing one or more heteroatoms in their ring are called alicyclic heterocyclic compounds.

(b) Aromatic heterocyclic compounds :- Aromatic cyclic compounds containing one or more heteroatoms in their molecule are called aromatic heterocyclic compounds.

Green House Effect and Global Warming

Carbon dioxide present in normal concentration in the air is not an air pollutant. Rather it helps to maintain the temperature on the earth required for the existence of living organisms. However, due to human activities such as burning of fossil fuel like coal, natural gas, petroleum etc. The amount of carbon dioxide in the air is increasing. This result in warming of the earth surface and thus bring about drastic changes in the climate as explained below :-

Out of sun's energy entering the atmosphere about 34% is scattered and reflected back by the particulates and the remaining 66% is absorbed by the earth. The reason for this energy reaching the earth is that visible and ultraviolet radiation can pass through carbon dioxide and water vapour present in the atmosphere. Ultraviolet radiation are absorbed by ozone layer but visible light reaches the earth and heats it up. However, when the earth cools, the energy is 're-emitted from the earth surface in the form of infrared radiations. These radiation can be absorbed by carbon dioxide and water vapours but cannot pass through them. The heats thus absorbed by carbon dioxide and water vapours cannot go to the upper atmosphere and is radiated back to the surface of the earth. In this way, additional heat is kept with in the lower atmosphere and warming of the earth occurs.
The phenomenon has been named as green house effect because it is similar to a green house for plants. A green house is a place where plants are grown on the soil but there are walls and roof made of glass. The sun's energy can enter through the glass but cannot go out of it. The energy is thus absorbed by the soil and the structure of green house. The energy thus absorbed is 're-emitted back into the green house. The same situation is observed inside a car which becomes very hot inside when it is parked in the sun for some time with its glass windows closed. In the atmosphere, carbon dioxide is playing the role of a glass of a green house. Hence it is called green house effect.

Greenhouse Effects :-

* If the rate at which solar radiation are arriving the earth remains constant but the amount of carbon dioxide in the air increases, the heat radiated back to the earth will increase. Consequently, the temperature of the earth surface will increase. As a result, evaporation of surface water will increase which will further helps in the rise of temperature. This increase in temperature will have a drastic effect on the climate and may prove even disastrous.

* Due to higher concentration of carbon dioxide in atmosphere, the plants undergoing photosynthesis will take up the gases at a faster speed. As a result, the plants in warmer climates and adequate rainfall would grow faster.

* Higher global temperature is likely to increase the incidence of infectious disease like malaria, dengue, yellow fever and sleeping sickness.

It is important to mention that earth is being saved from excessive heating due to certain opposing factors taking place simultaneously.

How to reduce rate of global warming :- As green house gases are added into the atmosphere due to burning of fossil fuel and cutting down of forest and trees , we can follow some simple method so that the green house gases are added into the atmosphere as minimum as possible. A few of these can follow s :-

* By preferring walking or using bicycles or going for car pools or taking public transport.

* By planting a new trees every month and increasing green cover.

* By avoiding burning of dry leaves etc.

* By not smoking and persuading others also not to smoke in public places.

* By importanting knowledge to those who do not know about the greenhouse effect.

CHEMISTRY - Soil Pollution

Soil is the uppermost part of the earth crust and is believe to have been formed as a result of decomposition and disintegration of surface rocks due to weathering over a very long period of time. It might have been formed at far of distance but is carried by strong winds rivers to different places.
The prosperity of a nation depends upon the quality of soil as it is the soil which provides nutrients, water and minerals for the growth of the plants. It is, infact, the heart of biosphere.
However, large scale of cutting of trees for human needs due to increase in population, digging of minerals, increasing brick-kilns, poisonous effluent of the industries and dumping of toxic and harmful wastes, increase in the use of automobiles etc. are damaging the quality of the soil and resulting into soil pollution. In India, it is sad to point out that backwardness of our people is an additional factor for soil pollution, e.g using open air lavatories by slum dwellers thereby inviting pig, crows, other vulture and files, throwing of domestic wastes including peeling of vegetables and fruits, plastics, polythene bags etc.

<b>Composition of the Soil :-</b>

1. Mineral matter :- it comprise of mineral particles of different sizes which originate from the rock and are formed by its disintegration. They are present as gravel, coarse sand, fine sand, slit, and clay.

2. Organic matter :- This is present in the soil as a result of fallen trees and mixing their parts such as roots, leaves etc. And also from the remains of dead animals and their excreta. These materials are decomposed by microorganisms present in the soil and convert them into humus. Humus has all those excellent properties which are an urgent need of the plants.

3. Biological system :- It consist of algae, bacteria, protozoa etc. Which help in the decomposition of organic matter present in the soil.

4. Soil water :- The water present in  the soil acts as a solvent for various organic and inorganic materials needed for the plants and also act as a transporting agent by carrying nutrients from the soil to different parts of the plant.

5. Soil air :- It is present in the pores of the soil, i.e the space between the soil particles. It contains more of carbondioxide and moisture than oxygen. Out ids needed for soil microorganisms and underground parts of the plants.
The soil containing almost equal amounts of sand, slit and clay alongwith humus and 34% air and 66% water is consider to the best soil for most of the crops. It is called loam soil.

<b> Sources of Soil Pollution :-</b>

1. Industrial Wastes :- It has been estimated that about a 50% of the raw material used by most of the industries becomes waste product which is either thrown into the soil nearby. This industrial waste contain huge amounts of toxic and disastrous chemical many of which are non-biodegradable. Some of the industries responsible for soil pollution are paper mills , pulp mills, sugar mills, chemical industries and petroleum industry etc.

2.Urban Wastes :- Urban wastes consist of soil refuse containing garbage and rubbish material like paper pieces, glass, plastics, used cans, leaves, polythene bags, peeling of vegetables and fruits etc. These wastes emit out poisonous gases, toxic hydrocarbons and pathogenic microbes which cause disease.

3. Agricultural pollutants :- No Doubt, the use of fertilizers, pesticides , soil conditioners , etc. have increase the yield from the crops but they have polluted the soil. Their entry into food chain has adversely affected the health of the human beings and led to a number of diseases. Some of the agricultural pollutants are :-
* Fertilizers
* Pesticides
* Insecticides
* Fungicides
* Soil conditioners
* Farm wastes

4. Radioactive Pollutants :- Dumping of the nuclear wastes from the nuclear power plants into the soil has been one of the greatest source of radioactive pollution of the soil. Nuclear tests produce nuclear dust in the atmosphere which ultimately falls into soil and pollutes it. The use of atomic and hydrogen bomb in the wars produces radioactive nuclides as the by-products.

CHEMISTRY - Water Pollution

Water is one of the basic needs of household, agriculture and industry. For most of the purposes, e.g. drinking, washing etc. We need pure and clean water. However, the natural water gets contaiminated or polluted in the following two ways :-

1. By natural processes
2. By human activity

Qater pollution is defined as the contamination of water by foreign substances which make it harmful for health of animals or plants or aquatic life and make it unfit for domestic, industrial and agricultural use.
Polluted water has any one or more of the following signs :-
* It has a bad taste to drink.
* It has an offensive odour.
* It has unpleasant colour.
* It has unchecked growth of weeds.
* It may have oil or grease floating on the surface.

Types of Water Pollution :-  Based on the sources and storage of water, water pollution may be classified into five main categories as follows :-

1. Ground Water Pollution :- Water below the surface of the earth is called ground water. Most of the water is present as ground water. The remaining presents in lakes, river, streams etc. Only about 2% is present as soil moisture above the water table which is needed for the growth of plants. Ground water collects below the surface of the earth afterpassing throughthe pores of the earthly material which act as a filter for it and is pure. It is for this reason that well water or spring water is used for domestic purposes in rural areas. However, due to disposal of domastic wastes and industrial effluent and use of fertilizers and pesticides in agriculture, a number of harmful soluble substance dissolve into the rain water and pass through tbe soil and enter into ground water and result in pollution, especially where the water table is high.

2. Surface Water Pollution :- Water present on the surface of the earth in any form is called surface water. As it is an in direct contact with the atmospheric gases, a number of gases like corbon dioxide, sulphur dioxide etc. Present in the air as pollutants dissolve into it , thereby polluting it. In addition to these atmospheric gases, the surface water also gets polluted by industrial and municipal wastes, agriculture wastes including decomposed plant and animal matter and by radioactive materials.

3. Lake Water Pollution :- Lakes cover a large part of the water area . The water in them gets contaminated in a number of way as followings :-
* Organic wastes from hills and toxic effluent from urban areas flow into them.
* Industrial effluents are thrown into them.
* Sewage treatments plants discharge toxic organic matter into them.
* Dumping of huge amounts of sediments i.e. siltation of lakes.
* Inorganic nutrients from agricultural land due to surface run-off.

4. River Water Pollution :- There are 13 major rivers in India flowing through differents states. These include Ganga, Yamuna, Narmada, Brahamputra, Krishna, Cauvery, Damodar, Godavri, Hooghly, Tapti, Gomti, Sona and Chambal besides many others. There was time when the water in these rivers was very pure and clean. Ganga jal was considered to be so pure that it could be

CHEMISTRY - Air Pollution

An addition of undesirable materials into the atmosphere either due to the natural phenomena or due to the human activity on the earth which adversely affect the quality of the air and hence affect the life on the earth. This phenomenon is known as Air pollution.
When we talk about an air pollution, we mean the pollution of air upto a height of about 50 km above the surface of the earth. This part of the air is called homosphere because the different constituents present in it form a homogeneous mixture.
The composition of pure dry air is as follows :-
Nitrogen - 78.10% , Oxygen - 20.95% , Ar. - 0.93% , Corbondioxide - 0.032%
Air has never been pure right from the day the man started burning fuel. The oxides of corbon, nitrogen, and sulphur become pollutants if their amount exceeds a certain limit ( called Tolerance Limit )

<b> Sources of Air Pollution :-</b> The main sources of air pollution may be classified into two categories as follows :-
1. Natural Sources :- Thesources which occur due to natural process. Few example are :-
* Volcanic eruption emitting poisonous gases like CO , Sulphur dioxide etc.
* Forest fires and coal-refuse fires.
* Vegetatiin decay.
* Pollen grains of flowers.

2. Sources due to human activity :- These sources occur due to the human activity is known as Sources due to human activity. Few examples are :-

* Burning Of Fossil Fuels :- Which produce some poisonous gases as by - products such as CO, oxides of nitrogen etc.
* Combustion of gasoline in the automobiles:- e.g. cars, scooter, bus, trucks etc. They emit out poisonous gases like CO, oxides of nitrogen etc.
* Increase in population :- This is one of the major cause of pollution. More, the population, more are the needs, greater are the unnatural method adopted which disturb the balance or equilibirium of the atmosphere.
* Deforestation :- Man has been cuttin trees indiscriminantely to meet his needs. This has resulted in increase in the precentage of carbon dioxide and decrease in the precentage of oxygen in the air.
* Agricultural Activities :- The pesticides added to the soil or the sprays done over the crops are carried by the wind to different parts of the town where they give a foul smell and affect the health of animals and human beings.
* Wars :- The nuclear weapons used during wars emit out radiations which adversely affect the health and prove to be fatal.
* Fast Industrialisation :- In the last few years, the number of industries in different parts of the world and their production has increased manifold. These include paper mills, sugar mills, rubber and plastic industries.

<b>Effects of Air Pollution :-</b>
* Effects on Plants.
* Effects on Materials.
* Effects on Climate.
* Effects on Human Beings.

CHEMISTRY - Polar and Non-Polar Covalent Bond

1. Non-polar Covalent Bonds - If two similar atoms come close to each other and form a bond by sharing their electrons, the shared electrons are equally attracted by the atoms as the electro negativity of the atoms is same. In other words, the shared electrons are not displaced towards any one atom. Hence, no poles are developed. This leads to the formation of completely non-polar bond. For example, if two hydrogen atoms form a bond, the electron pair will lie exactly in the middle between the two atoms and the resultant is non-polar. The important characteristics of a non-polar bond is that the electron cloud is completely systemic and there is no charge separation at all. 

2. Polar Covalent Bonds - When two dis- similar atoms, having different electro-negativities combine together to form a covalent bond, the shared pair of electrons does not lie at equal distances from the of both the bonded atoms but shifts towards the atom having greater electro negativity. Since the more electro negativity atom attracts the electrons more strongly, the distribution of electrons gets distorted, i.e. The electron cloud is displaced more towards the more electro negative atom. As a result, one end of the molecule Having more electro negative atom becomes slightly negatively charged whilethe other end acquires slightly positive charge. Thus, positive and negative roles are developed and this type of bond is called polar covalent bond. Lets consider the case of hydrogen chloride molecule. Chlorine is more electro negative than hydrogen. So the force of attraction, on the shared pair, exerted by chlorine is more than that by hydrogen. Hence, it will be shifted towards chlorine, i.e. Electron density of the electron cloud is more concentrated on chlorine side than on the hydrogen side. Molecules having polar bond are called polar molecules. In case of electro negativity difference between the two atoms bonded by a covalent bond is high, an electron can be completly free.

CHEMISTRY - Acid Rain

Acid rain is the rain water containing sulphuric acid and nitric acid ( alongwith small amount of hydrochloric acid ) which are formed from the oxides of sulphur and nitrogen present in the air as pollutant and has a pH of 4 - 5.

<b> Formation of Acid Rain :-</b> Due to natural sources such as bacterial action or volcanic eruption or due to human activities mainly involving combustion of fuel like coal, wood, petroleum products etc. Or from chemical industries, oxides of nitrogen and those of sulphur enet into atmosphere as a pollutants. The oxides of nitrogen undergo oxidation reactions. This reaction form a nitric acid . Thus, nitric acid comes down to the earth alongwith rain or react s with the bases like ammonia and lime to form nitrates.

<b> Harmful Effect of Acid Rain :- </b>

1. Acid rain causes extensive damages to buildings, statues and sculptural material, especially those made of marble, lime stone, slate, mortar etc. For example, Taj Mahal at Agra, Which is a very popular historical monument made of marble is being attacked by acid rain due to high concentration of oxides of sulphur and nitrogen in the air over Agra because of a large number of industries in the surrounding areas which are emitting these gases. As a result, marble is being eaten up causing pitting and discoloring and making it lustreless.

2. It also damages iron and steel structure.

3. It corrodes water pipes. As a result, heavy metals like iron, lead, copper are leached into drinking water which have toxic effects.

4. Acid rain increases the acidity of water in the lakes which is lethal for the fishes. For this reason, some of the lakes have become fishless. Thus it has greatly affected the fish population.

5. Acid rain damages leaves of trees and plants and retards the growth of forests. It also retards the growth of certain crops such as peas, beans, potato, raddish, carrot, spinach etc.

<b> Method of Reducing Acid Rain :-</b> Acid rain is due to emission of sulphuric dioxide and nitroge dioxide in the atmosphere. Therefore, to reduce the formation of acid rain, the emission of these gases has to be controlled. The following steps may be taken as :-

1. By using less vehicles driven by fossil fuels.

2. By using less sulphur content fossil fuels for power plants and industries, e.g., by using natural gas which is a better fuel than coal or using natural gas which is a better fuel than coal or using coal with less sulphur content.

3. By using catalytic converters in cars so that oxides of nitrogen present in exhaoust gases is converted intonitrogen at 573 K.

4. By adding powdered limestone to neutralize the acidity of soil.

CHEMISTRY - Pollutants

Pollutants are those substance which introduce harmful gases in the air or in soil or in water which harmful for living organism i.e. harm the human as well as the plants. Pollutants can be classified as :-
1. Primary and Secondary Pollutants :-

* Primary Pollutants :- Primary pollutants are those which after their formation enter the environment and remain as such. For example, nitric oxide(NO) formrd by bacterial decay or by lightning flashes ( resulting into combination of nitrogen and oxygen of the air ) becomes a pollutant if present in excessive amount.
* Secondary Pollutants :- Secondary pollutants are those harmful material which are formed by chemical reactions between the primary pollutants in the atmosphere or hydrosphere. These are the pollutant which is formed by reaction between the primary pollutants and the other elements in the atmosphere. For example, hydrocarbons and oxides of nitrogen which are primary pollutants react together in the presence of sunlight to form certain compounds which are also harmful. The compound thus formed are called secondary pollutants.

<b>2. Biodegradable and Non-Biodegradable Pollutants :-</b>

* Biodegradable Pollutants :- Biodegradable pollutants are materials such as domestic sewage, cow-dung etc. Which are easily decomposed by the microorganism either by the nature itself or by suitable treatment and thus are not harmful but if these are present in excess in the environment, they do not undergo degradation completely and thus become pollutants.

* Non-Biodegradable Pillutants :- Non-biodegradable pollutants are materials such as mercury, alluminium, DDT etc. Which do not undergo degradation or degrade very slowly but their presence even in very small amounts in the environment is very harmful for the humans as well as plants. They may react with other compounds present in the environment and produce even more toxic compounds.

Due to these pollutants our environment is going to a unbalance today . Due to the pollution the result in a depletion in the ozone layer. Which is very important for the earth all human beings and plants and animals. Ozone layer protect the human being from harmful ultrviolet radiations.

CHEMISTRY - Ozone Layer

<b>Formation of Ozone Layer :- </b> More than three billions years ago, before the evolution of life on the earth, it is believed that the atmosphere consisted mainly of methane, nitrogen, corbon dioxide, ammonia, hydrogen and water vapours and there was no oxygen. When plants started growing, they released oxygen during photosynthesis. In the stratosphere (11-50km), the oxygen started partially being converted into ozone. Now at an altitude of 25-30 km, we have a layer in which the concentration of ozone is about 10 ppm. It is called ozone layer. This region is, therefore, also called ozonosphere. It does not allow the harmful ultraviolet radiations coming from the sun to reach the surface of the earth and thus protects lifes on the earth. That is why this ozone layer is also called a earth's protective umbrella.
The formation of ozone in the stratosphere takes place in two steps. In the first step, the ultraviolet radiations coming from the sun have the sufficient energy to split the dioxygen into two oxygen atoms. In the second step, the oxygen atoms react with more dioxygen to form ozone.
The ozone thus formed absorbs the ultraviolet  radiations and is again broken into dioxygen and an oxygen atom. Heat is given out in this reaction which warms up the stratosphere.
In this way, "ozone cycle" is completed in the stratosphere.

<b> Depletion of Ozone Layer :-</b> Due to human activity, two types of compounds have been found to be most responsible for depleting the ozone layer and creating a hole into it. These two ozone-depleting agent are :-

1. Nitric Oxide(NO) :- Whch may be produced at the ground level due to human activity or natural sources or is produced in large amounts in the exhaust gases by the engine of supersonic transport planes and introduce directly into the stratosphere. NO reacts with ozone thereby decreasing the amount of ozone and forms nitrodioxide which reacts with oxygen atoms available in the stratosphere due to decomposition of ozone and oxygen producing back NO. Thus, no NO is consumed but ozone gets depleted.

2. Chlorofluorocarbons (CFC):- i.e, compounds containing chlorine, fluorine, and carbon commonly known as freons. These are introduced into the atmosphere from aerosol sprays in which function they as propellants and from refrigerating equipments, in which they act as coolants. They are also used as solvents and as blowing agent for plastic foams. They are chemically unreactve, non-toxic and odourless. However, they have a very long life time,i.e, they stay in the atmosphere for years and ultimately reach the upper layer where they start decposing in the presence of UV radiation coming from the sun.

<b> Effects of Depletion of Ozone layer :-</b>

* The most serious effect of the depletion f the ozone layer or the development of ozone hole is that the ultraviolet radiations coming from the sun can pass through the stratosphere and reach the surface of the earth. This type of radiations is known to be a cancer causing agent. It has been found that with increase in the exposure to ultraviolet radiation, the chance for occurence of skin cancer increasing.

* Exposure of eye to ultraviolet radiations damages the cornea  and lens of the eye and may cause cataract or even blindless.

* Exposure of plants to ultraviolet radiation adversely affect the plant proteins and results in the reduction of chlorophyll and harmful mutation

* Ozone depletion has a very strong effect on climate.

* Ozone depletion, if not controlled, would cause ecology disturbances, which would adversely affect man and animals.