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ComBUSTION AND FLAME, , ‘ ), Il: use different kinds of fuel for, various purposes at home, in, industry and for running, , automobiles. Can you name a few fuels, used in our homes? Name a few fuels, used in trade and industry. What fuels, are used for running automobiles? Your, list will contain fuels like cowdung,, wood, coal, charcoal, petrol, diesel,, compressed natural gas (CNG), etc., , You are familiar with the burning of a, candle. What is the difference between the, burning of a candle and the burning of a, fuel like coal? May be you were able to, guess right: candle burns with a flame, whereas coal does not. Similarly, you will, find many other materials burning, without a flame. Let us study the chemical, process of burning and the types of flame, produced during this process., , 6.1 What is Combustion?, , Recall the activity of burning of, magnesium ribbon performed in, Class VII. We learnt that magnesium, burns to form magnesium oxide and, produces heat and light (Fig. 6.1)., , We can perform a similar activity with, a piece of charcoal. Hold the piece with, a pair of tongs and bring it near the, flame of a candle or a Bunsen burner., What do you observe?, , We find that charcoal burns in air., We know that coal, too, burns in air, producing carbon dioxide, heat and, light., , , , , , Fig. 6.1 : Burning of magnesium, , A chemical process in which a, substance reacts with oxygen to give, off heat is called combustion. The, substance that undergoes combustion, is said to be combustible. It is also called, a fuel. The fuel may be solid, liquid or, gas. Sometimes, light is also given off, during combustion, either as a flame or, as a glow., , In the reactions mentioned, above magnesium and charcoal are, combustible substances., , We were told that, food is a fuel for, our body.

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y, , s, , ais proken down by, foro 9 with oxygen and, a peat is prod, feat that in Class VII., , petit 6.1, Coll me materials like straw,, , collect $, “atchsticks. kerosene oil, paper,, vn nails, stone pieces, glass etc., , pader the supervision of your, teacher try to burn each of these, materials one by one. If combustion, takes place mark the material, combustible, otherwise mark it, non-combustible (Table 6.1)., , Table 6.1 : Combustible and Noncombustible Substances, , Tans Combustible, , , , , , , , Noncombustible, Wood, Paper, Iron nails, Kerosene oil, , , , “Stone piece, , , , Straw, , , , Charcoal, , a, , Glass, , a, , , , , , a, , , , § Pe you name some more, hie which are combustible? You, add those to Table 6.1., , Let us investigate conditions under, , , , Activity 6.2, , (Caution : Be careful whi, “burning candle)., , Fix a lighted candle on a table. Pu, , a glass chimney over the candle and, rest it on a few wooden blocks in, such a way that air can enter the, , , , , ae ae, (a) (b) (, Fig. 6.2: Experiment to show that air is, essential for burning, , chimney [Fig. 6.2(a)]. Observe what, happens to the flame. Now remove, the blocks and let the chimney rest, on the table [Fig- 6.2(b)]. Again, observe the flame. Finally, put a, glass plate over the chimney [Fig., 6.2(o)]. Watch the flame again. What, happens in the three cases? Does, the flame flicker off? Does it flicker, ive smoke? Does it burn, unaffected? Can you infer anything, at all about the role played by air in, , the process oO, —— ee —— —, , We find that for combustion. air is, essary. The candle burns freely in, , when air can enter the, from below. In case (b). when, t enter the chimney from, ame flickers and produces, ase (Cc). the flame finally, the air is not, , nec, case (a), chimney, air does no, pelow. the fll, smoke. In ¢, , oes off because, , available., , ia, hich combustion takes places

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In the sun, heat and light are, produced by nuclear reactions. You, will learn about this process in higher, , classes., , , , , , Activity 6.3, ace a piece of burning wood or, al on an iron plate or Tawa., ver it with a glass jar or a, umbler, or a transparent plastic jar., rve what happens. Does, rcoal stop burning after, _ sometime? Can you think of the, , reason why it stops burning?, , , , , , , , You might have heard that when the, clothes of a person catch fire, the person, , is covered wilh a blanket to extingy, fire (Fig. 6.3). Can you guess whys Ish, , , , Fig. 6.3 : Blanket trapped around a person, whose clothes caught fire, Now recall some of your experiences,, Does a matchstick burn by itself?, How does it burn’?, , You must have had an experience of, burning a piece of paper. Does it burn, when a burning matchstick is brought, near it?, , Can you burn a piece of wood by, bringing a lighted matchstick near it?, , Why do you have to use paper or, kerosene oil to start fire in wood or coal?, Have you heard of forest fires’?, , df *, During extreme heat 4, of summer, at some, ‘ z, places dry grass 4, catches fire. From |, the grass, it spreads, to trees, and very 6., , soon the whole forest, is on fire (Fig. 6.4). It, is very difficult to, control such fires., , 66), , , , oy, a Mn,, , Fig. 6.4: Forest fire

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y, , 2 riences tell you that, shes’ pstances catch fire at, , ' jee mperatures?, ore west temperature at which a, 7 i catches fire is called its, sl gemperature:, pio" {ell now Why a matchstick, ou te ., t catch fire on its own at room, jes a ure? Why does the matchstick, eo ymning on rubbing it on the side, ‘ ‘be matchbox?, agyistory of the matchstick is very, ° More than five thousand years, small pieces of pinewood dipped, a nur were used as matches in, ancient Egypt. The modern safety, ich Was developed only about two, pundred years ago., , Amixture of antimony trisulphide,, , otassium chlorate and white, hosphorus with some glue and, starch was applied on the head of a, match made of suitable wood. When, struck against a rough surface, white, phosphorus got ignited due to the heat, of friction. This started the, combustion of the match. However,, white phosphorus proved to be, dangerous both for the workers, involved in the manufacturing of, matches and for the users., , These days the head of the safety, match contains only antimony, trisulphide and potassium chlorate., The rubbing surface has powdered, Glass and a little red phosphorus, a! much less dangerous)., the hike match is struck against, sch ing surface, some red, ie phorus gets converted into white, , oe. This immediately reacts, mate Potassium chlorate in the, , hstick head to produce enough, , eat to igni is i :, and e ignite antimony trisulphide, , ~~, , We find that a combustible substance, cannot catch fire or burn as long as its, temperature is lower than its ignition, temperature. Have you ever seen, cooking oil catching fire when a frying, pan is kept for long on a burning stove?, Kerosene oil and wood do not catch fire, on their own at room temperature. But,, if kerosene oil is heated a little, it will, catch fire. But if wood is heated a little., it would still not catch fire. Does it mean, that ignition temperature of kerosene oil, is lower than that of wood? Does it mean, that we need to take special care in, storing kerosene oil? The following, activity shows that it is essential for a, substance to reach ignition temperature, to burn., , Activity 6.4, , , , What happens to the empty paper, cup? What happens to the paper cup, with water? Does water in this cup, become hot?, , “sestoxawRugeJ———t=C<CSsti‘is™*™S*~*~*~*~*~*™*SCY

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Ifwe continue heating, the cup. we call even boil, water in the papet cup., , Can you think of an, explanation for this, , phenomenon?, The heat supplied to, the paper cup is, , transferred to water by, conduction. So, in the, presence of water. the, ignition temperature of, paper is not reached., Hence, it does not burn., The substances which, have very low ignition, temperature and can, easily catch fire with, a flame are called, inflammable substances. Examples of, inflammable substances are petrol,, alcohol, Liquified Petroleum Gas (LPG), etc. Can you list some more inflammable, , substances?, , 6.2 How Do We Control Fire?, , You must have seen or heard of fire, breaking out in homes, shops and, factories. If you have seen such an, accident, write a short description in, your note book. Also, share the, experience with your classmates, , Find out the telephone number of the, fire service in your area. If a fire, breaks out in your house or in your, neighbourhood, the first thing to do, is to call the fire service, , It is important that all of us, _ know the telephone numbers, Of the fire service., , es, , , , Fig. 6.6: Firemenextinguish the fire by thro, , wing water under pressure, , Does your city/town have a fire, brigade station?, , When a fire brigade arrives. what, does it do? It pours water on the fire, (Fig. 6.6). Water cools the combustible, material so that its temperature is, brought below its ignition, temperature. This prevents the fire, from spreading. Water vapours also, surround the combustible material, helping in cutting off the supply of air, So, the fire is extinguished., , You have learnt that there are thre, essential requirements for producing, fire. Can you list these requirements’, , These are: fuel, air (to supp!, oxygen) and heat (to raise the, temperature of the fuel beyond, ignition temperature). Fire can be, controlled by removing one or more, these requirements. The job of &!, extinguisher is to cut off the supp)”, air, or to bring down the tempera, of the fuel, or both, Notice that theft" |, , , , |