NCERT Summary: What, Why & How | Science & Technology for UPSC CSE PDF Download

RADARS WORK

The word radar stands for radio detecting and ranging. It makes use of very short radio waves called microwaves. Radars work to find out how far away an Object is and in case of a moving object in what direction it is moving and at what speed. 

COLOR TV PICTURE PRODUCED
The picture on a television screen is nothing but a pattern of glowing dots, or pixels. The pixels are made up of fluorescent chemicals called phosphors that are coated on the back on the screen. These glow on being hit by a beam of a single phosphorus and are lit up by a single electron beam that rapidly sweeps across the screen. In a color television how-ever, each pixel contains three phosphorus each producing a different colour- green, red and yellow. Three electron beams produced by three electron guns are used to light up the different phosphorus. 

FIRE EXTINGUISHERS WORK
Fire extinguishing agents work basically in two ways – either by cooling the burning materials or by blanketing them with an inert coating that cuts off the supply of oxygen. One of the most common ones uses water which has a high heat capacity.

Foam type fire extinguishers use foaming agents that have a smothering and cooling effect on the wire. A dry chemical extinguisher sprays a very fine power of sodium bicarbonate or potassium bicarbonate or monoammonium phosphorus. These solids coat the fuel and smother the fire. A safe and effective extinguisher for all confined fires uses carbon dioxide (co₂) which acts as an inert blanket. 

LIE DETECTORS WORK
A lie detectors work on the principal that a person who tells a lie is nervous and under stress under these conditions his body undergoes some physiological changes.

These includes increases in the blood pressure, heart beat rate and perspiration on hands and soles. These changes are monitored and detected by a lie – detector. 

STORAGE BATTERIES WORK
Storage batteries are devices which act as a store house of electrical energy. The electrical energy is stored in the form of a chemical charge which is reversible. The most common storage battery is the lead acid- accumulator which uses lead as electrode and sulphuric acid as an electrolyte. Initially both the cell electrodes of the battery made of lead are coated with a layer of lead dioxide. When the battery is charged for the first time, chemical charges take place. During discharging when the battery is in use, chemical changes take place again but in the reverse way. Each cell in a lead – acid battery produces two volts and a typical car battery with six cells gives 12 volts. Nickel – iron and nickel – cadmium batteries are other kind of storage batteries which are nickel, and iron or cadmium as electrodes and potassium hydroxide as electrolyte.

AEROPLANES FLY
Aeroplanes fly by a combination of power from the engines and lift provided by the wings. These are shaped in such a way that air flowing along the upper surface takes a longer path than the air flowing along the lower surface. As a result when an aircraft moves forward, the air flowing above the wings moves faster than the air flowing below the wings, creating a low pressure above the wing according to Bernoulli’s law. This difference in pressure above and below the wings causes lift. As the speed of the aircraft on the runway increases, the lift also increases, eventually overcoming the download force of gravity. The aircraft also uses the thrust created by its engine to climb and after reaching certain height cruises along in a horizontal direction.

HELICOPTERS REMAIN STATIONARY IN MID AIR
Unlike an aeroplane, helicopters have moving wings in the shape of spinning rotor blades. Air currents passing over the upper and lower surface of the spinning blades crates low pressure above it and generate lift. The lift can be controlled by varying the pitch (angle) of the rotor blades, by increasing the pitch of the main rotor blades lift is increased and the craft climbs. Lowering the pitch of the blades lift is increased and the craft climbs. Lowering the pitch of the blades reduces lift and gravity causes the helicopter to descend. Similarly , if the pilot holds the rotor pitch in such a way the blades produce just enough lift to counteract gravity, the helicopter remains station ary in mid – air. 

NIGHT VISION GLASSES WORK
Night vision glasses, used for seeing in the pitch dark night, use the little light available to form an image of sufficient brightness to be screen. 

The image is first focused, as in a camera, on to a window which is coated with special chemicals containing sodium, potassium, cadmium and oxygen compounds which emit electrons when illuminated. 

The electrons so emitted are then accelerated by a series of powerful electric fields and made to fall into another screen coated with a fluorescing chemical which glows recreating a much brighter image of the original scene. Some night vision glasses make use of infrared radiation emitted by all objects even in the dark. These radiations are amplified in a similar way to make night vision possible.

AIR POLLUTION DETECTORS WORK
Air pollution is coused by substance that is not normally part of the atmosphere’s composition. Important air pollutants are sulpher dioxide, nitrogen oxides and carbon mono oxide usually emitted in automobile exhausts and power smoke. Pollution detectors work by making use of the chemical or physical properties of the pollutants. For example, the detection of nitrogen oxides is based on the emission of light as a result of a chemical reaction. This phenomenon is called chemilumine-scences.

If nitrogen are present, light is emitted which can be detected by a photo detected. Sulphur dioxide is detected by introducing it into a flame and then analyzing the colour produced by an instrument called flame photometer. Carbon monoxide is detected by emission of infrared radiation of a particular frequency when excited by an arc or a spark. An infrared spectrometer is used to detect the presence of this gas in air.

ATM WORK
An automatic teller machine (ATM) performs simple banking functions such as deposits withdrawal, cash dispensing, and transfers between accounts. An ATM is a terminal connected via telephone or dedicated telecommunication lines to larger computer system that identify the user’s account on the basis of data stored in a magnetic strip on the back of a plastic ATM card commonly known as the credit card. The user operates the system using an exclusive personal identification number (PIN), assigned to him or her. 

ARTIFICIAL DIAMONDS MADE
Diamond is formed in nature from graphite beneath the earth crust by the action of extreme heat and pressure. Artificial diamonds are made by mimicking the natural process by applying intense heat and pressure on graphite in the presence of iron as catalyst. A pressure of up to 1, 00,000 kg per sq. Cm. is applied and an electric furnace is used for heating the compressed mixture up to 2500 C. On cooling, the molten mass contains, tiny artificial diamonds firmly surrounded by iron. 

PEARLS CULTURED
A pearl is a biological product produced naturally by certain oysters as a defense mechanism. They are formed when a foreign body such as a sand particle enters the body of the oyster. the oysters in order to get rid of the foreign body coasts it with a material called nacre which is basically the chemical calcium carbonate. Over a period of time these layers make the pearl grow in size until they are taken out. Pearls are cultured by inserting an artificial irritant into the oyster. The oysters are then carefully tended in special beds. Cultured pearls usually take three to six years to grow to a good size. 

CRUDE OIL REFINED
Crude oil that comes out of an oil well is a thick, dark liquid containing a mixture of many organic compounds. It cannot be put to use without refining. The different constituents of crude oil boil at different temperature. Refining processes may be classified as fractional distillation separates crude oil into some of its fractions depending on their boiling points. Some components are separated by solvent extraction in which an organic solvent is used to extract the substances especially solids which are then recovered by crystallization or evaporation of the solvent. Cracking is also a process used in refiners to produce useful petrochemicals. Here heat and catalyst are used to break down some of the heavier hydrocarbons to lighter, more useful fractions.

COOKING OIL REFINED
Vegetable oils are mixtures of chemicals called glycerides or esters of glycerides and long chain fatty acids. Oil is obtained by crushing oil bearing seeds such as ground nut, sunflower seeds or rapeseed, and then pressing the oil through expellers.

These mechanically extracted oils contain impurities like gums and free fatty acids (FFA), which have to be removed to make the oils suitable cooking media. Refining of oils is done by first treating it with alkali which forms soap with the FFAs. The soaps settle out carrying with them some colouring matter. The colour and odour are removed by treating the oil with some absorbing materials like fuller’s earth. Refining can also be done by extracting the pure oil with a suitable organic solvent like hexane and then removing the solvent by distillation.

PHOTOCOPIES MADE
Photocopying makes use of materials which can be electrostatically charged and which loses the charge when exposed to light. In a photocopying machine a drum made up of selenium is first charged electrically in the dark. An image of the illuminated document to be copied is then projected on to the charged drum.

The illuminated areas in the image falling on the drum destroy the electric charge while the dark areas retain the charge. The drum is then dusted with a fine black resinous powder called toner image of the original document. The image is transferred to paper which is given the opposite charge. The toner image is finally fixed on the paper by applying heat which melts the toner and sets it into the fibers producing a permanent dry copy of the original document. The entire process is automated. 

COLOR PAINTING DONE
A printed colour picture is made up of tiny dots of three primary colours-cyan, magenta, yellow, and black. For printing, there fore, the colour original is first separated by scanning into negatives for cyan, yellow, magenta and black. During scanning, a screen of dots is also used so that the images on the four negatives are in the form of fine dots which is necessary for printing. 

The most popular colour printings process used these days is offsets printings. Offset printings employs specially treated aluminum plates on which the printings ink sticks only to selected areas. When the plate, made sensitive to light by chemical coating, is exposed to light through the film negative and washed, coated areas that repel water but accept oil based ink remain. The plate is mounted on a cylinder in printing machine and wetted so that only the printed areas are linked. The impression from the printing plate is first transferred to a cylinder covered with a rubber blanket which finally transfers the impression onto paper.

WE FALL ASLEEP
In humans the sleep wakefulness cycle is controlled by the hypothalamus located in the lower portion of the brain.

MEHANDI COLOUR SKIN
Mehandi or henna leaves contain a colouring matter called Lawsone. This is a dye which can bind with the protein in hair and nails. This protein is called keratin. The skin of the palms and soles also contain keratin.

BLOOD PRESERVED
Outside the body, blood coagulates in five to ten minutes. Coagulation involves alteration in one of the plasma protein called fibrinogen into an insoluble protein called fibrin with the help of calcium icons. Coagulation can, therefore, be prevented by removing the calcium icons from blood. For preservation of blood meant for transfusion, sodium citrate is added which removes calcium form blood by forming a soluble complex with it.

BODY TEMPERATURE MAINTAINED
The body has an in built temperature regulating mechanism in the shape of the hypothalamus in the brain. The heat of the blood passing through it and the messages sent from the temperature sensitive nerve endings on the skin feed information to the hypothalamus. Once region of the hypothalamus so sensitive to temperature above normal while another region is sensitive to a fall in body heat. 

When the body gains too much heat it loses the extra heat by sweating. When the environment is cold, the body generates extra heat by stepping up metabolism and muscle activity such as shivering. These changes are brought by path ways controlled by the nerves that are concerned with reflex control of bodily functions. 

AIDS DETECTED
AIDS is caused by a virus called human immunodeficiency virus or HIV. The presence of this virus is detected by screening the blood of a person for special kinds of proteins called antibodies which are produced in the body in response to viral infection A characteristic feature of antibody molecules is that they are highly specific. They bind only to certain regions of the antigen. This is similar to a key fitting only a specific lock. AIDS detection tests are actually based on detecting the binding of the antibodies present in the blood with an artificially synthesized antigen attached to a solid surface.

ALCOHOLIC BEVERAGES INTOXICATE
Alcoholic beverages contain ethyl alcohol which on reaching the brain and slows down the activity of the central nervous system especially those parts of the brain that control a person’s behavior.

BATS FLY IN THE SKY
Bats navigate by making use of sounds that they emit. The way these sounds bounce off from nearby objects and obstacles and return to the bat’s ears enables it to gauge distance and avoid obstacles. Being in the range of 100,000 hertz, these sounds are inaudible to human ears which can hear only unto 20,000 hertz. 

Bats can discriminate between faint echoes of their own sound in the presence of other sounds. Interestingly. Large bats such as the flying fox do not use sound for navigating but rely on vision instead. They fly and feed by day and become disoriented if forced to fly in the dark.

CAMELS SURVIVE IN DESERTS
Camels can survive for days in desert with little food and no water due to their unique physiology. Contrary to the common belief, a camel does not store water serves as a reverse for energy and a source of water when in need. When the fat is utilized by the camels body, hydrogen is released which combines with oxygen to form water.

FIREFLIES GLOW
A firefly emits yellow-green light from special organs located in the lower portion of its abdomen. These organs contain in enzyme called luciferase which acts as a catalyst. When luciferin comes in contact with oxygen from air in presence of lucifrease, it gets oxidized emitting flashes of light. The light produced is, cool.

LIZARDS WALK ON WALLS
The feet of lizards are adapted for walking on rough as well as smoth surface. The lizards have slits on their toes and these functions like suction disks helping them to cling to smooth areas such as a glass pane. The claws present on toes help the lizard to hold on to and walk on a rough surface. in this way a lizard can even walk upside down across a plastered ceiling or on a glass roof without trouble. 

ANIMALS SEE AT NIGHT
Members of the cat family, like the tiger and cat or other nocturnal animals can see in near darkness because of the presence of large number of cells called rod cells in the retina. These cells are sensitive to dim light and help the animal to see in near darkness. Other nocturnal animals like owl and Loris have very large pupils which allow more light to enter the eyes. In addition, the retina of these animals has a layer called ‘tapetum lucidum’. This reflects inwards the light falling on retina and thus help in gathering all the light available in dark surrounding. 

FRUITS RIPEN
Unripe fruits are typically hard, green, and have a sour on astringent taste due to the presence of organic acids such as malic, citric, and tartaric. They have high amounts of large molecule carbohydrates called polysaccharides and low amount of protein. Ethylene gas which triggers the ripening process emanates from fruits and causes rapid and dramatic changes. The green color of the fruits disappears and red, or yellow coloured pigments such as anthocyanins and carotenoids appear giving the fruits distinct colour.

The polysaccharides are broken down to smaller sugars which give the fruits the sweet taste. The fruits also begin to soften. Volatile substances are produced during ripening which gives many fruits their distinct aroma.

PLANTS CAPTURE INSECTS
Certain plants which grow in soil poor in nutrients depend on insects for their nutritional requirement. These carnivores’ plants use a combination of deception and a trapping device to get their meals.

Some insects – eating plants like the venus fl trap (dionaea musipula) have leaves moulded into twin blade traps with teeth like projections on their margins which interlock to trap any insect sitting on the leaf. Leaves of another plant names drosera have glandes all over them which secrete a sticky substance to trap insects. The pitcher plant has fluid filled pitches – shaped structure into which unwary insects fall and drown. All insects eating plants secrete digestive juices to digest their prey.

WATER RISE IN TALL TREES
Cells in the root hairs of plant contain dissolved sugars and salts. Water surrounding the root moves into them to equalize the pressure. This is called osmosis.

The increased water pressure in root hairs forces water up-word, cell by cell through the roots and trunk to reach the leaves. In addition to this, during the growing season, a tree passes tones of water into the atmosphere from its leaves through transpiration.

This creates a partial vacuum that is quickly filled by the water being pushed up from the roots. Water molecules stick together, and as water is lost during transpiration, this cohesion causes a chain reaction that is transmitted all the way down. 

ANNULAR RINGS IN PLANTS FORMED
The annular rings or the altering dark light circles seen in a cross section of a tree are produced because of differences in the rate of growth in different seasons. As it happens , the entire tissue in the trunk of a tree does not divide or grow to increase the girth of the tree. There is a layers of dividing tissue called the cambium sandwiched between the fibrous xylem (the water carrying tissue).

However, the cambium divides at different rates in different seasons. In winter, its growth is slower than in other season, say in spring, when conditions for growth are fairly favorable. Thus, the relatively small numbers of cells produced in winter remain compacted together producing a dark band while the cells produced during the spring season spread out into a boarder light band. These growth patterns are repeated at annual intervals and hence the annular rings thus indicate the age of a tree as well as the changes in climate that might have occurred in the tree’s life time.

LIGHTING OCCURS
Lightening is coused by an electric discharge in thunder clouds. These are clouds that rise to great heights and have strong air currents in them. The ice crystals, water droplets and other particles present in these collide with each other and get electrically charged.

Air usually works as an insulator to prevent these electric charges from escaping. But when the charge build up in the thundercloud crosses a certain level, the insulation effect of air breaks down and causes a massive discharge which we see as flashes of lighting. As the discharges takes place, the surroundings are suddenly expands to produce the sound which we hear as thunder. Lighting can pass from one cloud to another or from a cloud to the ground.

ARTIFICIAL RAIN PRODUCED
Rain occurs when the water vapour in the clouds forms water drops or ice crystals large and heavy enough to fall to the ground. This process is hastened if the cloud has tiny particles of matter for the water vopour to condense on. Artificial rain produced by introducing these particles into the clouds by a process called cloud seeding.

Clouds can be seeded in different ways. The seeding agent can be sprayed into a cloud from an air plane or sent up in a rocket. If the wind is strong enough, it can be dispersed in the form of smoke from the ground. At cloud temperature above 0oC solid carbon dioxide or crystals of silver dioxide is used. Water vapour in the clouds condenses around the seeding agent to form rain drops. If the temperature is below 0oC ice crystals are formed. When the ice crystals pass through air warmer than 0oC, they melt and fall as rain. Artificial rain is best produced from moistureladen clouds. It cannot be produced from a cloudless sky.

WOOLLENS KEEP US WARM
Wool fibres are made of a particular protein (keratin) which is a bad conductor of heat and as a result does not allow heat to escape. This helps us in keeping warm during winter. These fibers are wavy in structure, the property commonly known as crimp. Crimp imparts resilience to the fibres due to which fibres quickly recover from wrinkling and crushing. Because of this wavy structures the fibres do not come perfectly close and a result a large number of air pockets are formed. The air entrapped within these pockets acts as an insulator and traps body heat inside.

CURD FORMED
This bacteria produces produces lactic acid by the fermentation of the milk sugar lactose. The lactic acid so formed by the bacteria action contains positive hydrogen ions which are attracted to the negative particles of the protein casein. As the latter are neutralized, these protein molecules no longer repel one another but coagulate. The optimum temperature at which lactobacillus acts is around 40 c. so milk is usually warmed to this temperature before setting it to curdle. 

Stars Born
Stars take birth out of clouds of hydrogen, helium and dust particles present in galaxies. Due to the turbulent motion in these gas clouds, the dust particles undergo random collisions and condense under the influence of strong gravitational pull.

As the gases and dust particles begin condensing, the temperature inside rises due to increasing pressure. As the condensing mass grows bigger the gravitational pressure at the centre increases further till the intense heat raises the temperature to around 10 millions degree Celsius. At this temperature the hydrogen atoms start colliding so vigorously that they fuse with each other to form helium atoms. In the process some mass is lost. For every 1000 kg of hydrogen used up, 993 kg of helium is formed. The rest is converted into tremendous amount of energy according to the relation ship E=mc², where E is the energy, m the mass and c the velocity of light. This energy is liberated in the form of light and heat and a star is born.

TEMPERATURE OF SUN MEASURED
There are many ways of measuring temperature. The most common is the mercury thermometer in which an expanding Column of mercury in a glass capillary indicates the temperature. But a mercury thermometer cannot be used to measure temperature above 357ºC. for measuring higher temperature such as that in a furnace, instruments called pyrometers are used. But for measuring the sun’s temperature a different technique is used. It makes use of the fact that the colour at which a hot object gives off the maximum energy is directly related to the temperature of the hot body.

This is governed by a law known as Wien’s law. The sun gives off light which is a mixture of several colours. When the spectrum of sunlight is analyzed by using special instruments called bolometer it is found that the maximum intensity falls in the green part of the sun’s spectrum. From this and by using Wien’s law, we obtains a value of 5800 k for the sun’s surface temperature.

SPACESUITS PROTECT
A spacesuit is protective gear that protects an astronaut from the hostile environment of extremely low pressure, low temperature and radiation in space. It is made up of several layers of strong synthetic materials including Teflon and nylon which shield the astronauts from tiny particles called micrometeoroids. The intense solar radiations are reflected by a white plastic layer which has a metallic coating. The interior of the suit is pressurized but for which the astronaut’s blood would boil off in the vacuum of space. A backpack carries water both for drinking as a well as to keep the astronaut cool. Water flows through piper in the astronaut’s suit and carries away body heat.

FIREWORKS DISPLAY COLOURS
Fireworks are produced by metals or metal salts present in the fireworks. Metals have the property of emitting light of a particular colour while they burn. For instance, when sodium or its salts burn, yellow light is given out. Similarly, on burning, finally divided aluminum gives out bright white light. Strontium salts gives out red colour while copper and barium salts produce instance blue and green colours respectively. Fireworks manufactures make use of this property of metals and their salts and use them in various combinations in crackers to produce spectacular fireworks display.

PLANTEST SPHERE SHAPED
All the planets of our solar system were formed out of the same cloud of gas and dust that gave rise to the sun, some five billion years ago. As random Collisions and gravitational forces compacted the gas and dust particles into the various planets the heat generated brought the masses into a molten state. Since surface tension of a liquid tends to make the surface area minimum and the sphere has the minimum surface area for a given volume, all the plants in the molten state become spherical and have retained that shape as they cooled.

SEA WATER SALTY
Sea water is salty because it containts many dissolved salts, mostly sodium chloride with small proportions of potassium, magnesium, calcium and carbonates. The salt comes mainly with river water that flows through rocks eroded by frost and rain. The gradual wearing away of mountains releases rnemicals which are carried down by rivers to the ocean comes mainly with river water that flows through rocks eroded by frost and rain. The gradual wearing away of mountains releases memicals which are carried down by rivers to the ocean as dissolved salts.

Some salt also enters sea water from the rocks beneath the sea bed; the river water carrying salts to the sea does not taste salty because the salts are present in extremely small concentration. Whereas in the case of the oceans, water is continuously evaporating even as more and more salt continues to be added with river water. This, over millions of years has led to the high concentration of dissolved salts in sea water.

LIGHTNING ACCOMPANIED BY THUNDER
Lightning occurs when a massive electrical discharge takes place between two oppositely charged clouds or between a charged cloud and the ground. The charges Develop in thunder clouds due to the friction of water droplets with air as the droplets move up and down with the rising and descending air currents within the cloud. During a bolt of lightening, thousands of amperes of electricity flow through the air in a fraction of a second. This rapidly heats up the air along its path which expands very fast producing shock waves which we hear as thunder. Although the lightning and thunder are produced at the same instant, we hear the thunder later because light travels faster than sound. 

SYNTHETIC FABRICS DRY TO QUICKLY
Synthetic fibers are made artifically. They are solid, smooth and straight in structure unlike a natural fiber like cotton which is hollow. So when synthetic fabrics are soaked in water, only surface of the fibers gets wet as water does not enter the body of the fiber. That is why these fabrics absorb very little amount of water and dry quickly as the water drips away. Such fabrics are also known as, ‘dripdry’.

SOAP FORM LATHER IN HARD WATER
Soaps are salts of fatty acids. Common soaps are water soluble and potassium salts of fatty acids. Soap cannot form lather unless it dissolves in water. Hard water does not dissolve soap because the calcium and magnesium salts present in it produce insoluble substances by reacting with soap. These insoluble substances separate out as scum and reduce the effectiveness of the soap as a Cleansing agent.

FLUORESCENT TUBES CONSUME LESS POWER
In filament lamps a good part of the electrical energy is used up in heating the filament which in turn glows throwing light around. While in fluorescent tubes light is produced by electrical discharge in a glass tube, the inside of which is coated with a fluorescent material. The filaments at the two ends are used only to start the discharge. The discharge passing through vapors of mercury produce UV radiations which exites the fluorescent. Coating producing visible light. Since very little heating is involved in the process and most of the electrical energy is used in producing light, fluorescent tubes consume less power than filament lamps.

ICE MELTS WHEN SUBJECTED TO PRESSURE
One of the laws of fusion is that the melting point of substances which expand on freezing is lowered by the increase of pressure, while it is raised in the case of those which contract on solidification. Ice belongs to the first category of substances, that is, it expands on freezing. Ice has an open structure which collapses when subjected to pressure, producing water which occupies lesser volume. That is why ice when subjected to pressure. 

THERE A DISTURBANCE ON TV WHEN WE OPERATE AN ELECTRICAL SWITCH
When an electrical switch is operated it produces a spark at the contact point. This spark emits electromagnetic radiation. Since radio and TV signals are also electromagnetic in nature, the bursts of electromagnetic radiation produced by a spark is also received by the radio or TV set. This lead to the disturbance which is heard as cracking sounds on the radio and snowy lines on the TV picture.

COLOURED SOAPS PRODUCE WHITE BUBBLES
Foam or lather is nothing but a large collection of small soap bubbles. A sop bubble is, in turn, a very thin film of soap solution enclosing some air. Because of the low surface tension of soap solution, the film can stretch and spread and form innumerable bubbles with a very large total surface area. Because of this, whatever slight tint present in the thin film of the coloured soap solution gets subdued. Although a soap film is more or less transparent, the lather or foam looks white because the light striking this large collection of bubbles gets scattered. That is why all kinds of foam look white.

THINGS BURN
Burning is a chemical process in which the material burns combines with oxygen with the generation of large amounts of heat. As a result the temperature of the burning material rises to several hundred degrees Celsius and it may burst into flames. Therefore, any substance that readily combines with oxygen at a few hundred degrees and produces a lot of heat will burn if ignited. Such substances such as paper, wood, cloth, plastics, rubber etc. are usually rich in carbon and hydrogen. Some volatile liquids such as alcohol, petrol, etc. catch fire easily because they produce highly combustible vapors. 

A DRIED PIECE OF COTTON APPEAR DARKER WHEN WET
Cotton is a natural fibre. When woven or knitted into cloth, the fibres are loosely packed and contain lot of air spaces. When light falls on these fibres it is scattered from the boundaries of the fibres and the colour of the clot appears lighter. But when the fabric is soaked in water, the air pockets of the fibres get filled with water. This reduces the amount of Light Scattered from the fibric. Therefore, more light reflected from the coloured fabric reaches to the eye and the coloured appear deeper. However, synthetic and silk fibres are smooth in structure and leave no air space when woven or knitted. So the colour or silk or synthetic fabric is not altered when the fabric is soaked in water.

WATER AND OIL MIX
A phenomenon called polarity prevents oil and water from mixing. All molecules carry electrical charge which is distributed uniformly or non-uniformly over the length of the molecule. 

In polar compounds, the positive and negative charges are concentrated at the two ends of the molecule. When such substances are mixed together, the positive and negative regions of their molecules attract each other and as a result a clear solution is obtained. Water is a polar substance and mixes freely with other polar substances. Oil molecules, on the other hand, are non-polar. When polar and non-polar substances are mixed together, the mutual attraction of polar molecules separates out the non-plar molecules and the two substances do not mix.

ACID RAIN
Natural rain always contain small amount of dissolved carbondioxide which makes it slightly acidic. But large-scale burning of coal or oil in industries, power plants and vehicles produce large amounts of gases such as sulphur dioxide, nitrogen oxides, etc., which are released into the atmosphere. Under favourable conditions they react with water vapour and oxygen in the atmosphere to produce sulphuric and nitric acids which is eventually come down with rain, snow or fog. The countries wrost affected by acid rains are southern Sweden, Northway, parts of central Europe and Eastern Region of North America.

AIDS
Acquired Immune Deficiency Syndrome (AIDS) is the most devastatign and fatal diseases of the 21st Century it is a viral disease caused by the Human Immune Deficiency Virus (HIV).