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COMBUSTION AND FLAME W e 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 r...

COMBUSTION AND FLAME W e 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 Fig. 4.1 : Burning of magnesium without a flame. Let us study the chemical process of burning and the types of flame A chemical process in which a produced during this process. substance reacts with oxygen to give off heat is called combustion. The 4.1 What is Combustion? substance that undergoes combustion Recall the activity of burning of is said to be combustible. It is also called magnesium ribbon per for med in a fuel. The fuel may be solid, liquid or Class VII. We learnt that magnesium gas. Sometimes, light is also given off burns to form magnesium oxide and during combustion, either as a flame or produces heat and light (Fig. 4.1). as a glow. We can perform a similar activity with In the reactions mentioned a piece of charcoal. Hold the piece with above magnesium and charcoal are a pair of tongs and bring it near the combustible substances. 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 We were told that food is a fuel for producing carbon dioxide, heat and our body. light. 2024-25  Activity 4.2 (Caution : Be careful while handling Rightly so. In our body burning candle). food is broken down by Fix a lighted candle on a table. Put reaction with oxygen and a glass chimney over the candle and heat is produced. We learnt that in Class VII. rest it on a few wooden blocks in such a way that air can enter the Activity 4.1 Collect some materials like straw, matchsticks, kerosene oil, paper, iron nails, stone pieces, glass etc. Under the supervision of your teacher try to burn each of these materials one by one. If combustion takes place mark the material (a) (b) (c) combustible, otherwise mark it Fig. 4.2: Experiment to show that air is non-combustible (Table 4.1). essential for burning Table 4.1 : Combustible and Non- chimney [Fig. 4.2(a)]. Observe what combustible Substances happens to the flame. Now remove Material Combustible Non- the blocks and let the chimney rest combustible on the table [Fig. 4.2(b)]. Again observe the flame. Finally, put a Wood glass plate over the chimney [Fig. Paper 4.2(c)]. Watch the flame again. What happens in the three cases? Does Iron nails the flame flicker off? Does it flicker Kerosene oil and give smoke? Does it burn Stone piece unaffected? Can you infer anything at all about the role played by air in Straw the process of burning? Charcoal We find that for combustion, air is Matchsticks necessary. The candle burns freely in Glass case (a) when air can enter the chimney from below. In case (b), when Can you name some more air does not enter the chimney from substances which are combustible? You below, the flame flickers and produces can add those to Table 4.1. smoke. In case (c), the flame finally Let us investigate conditions under goes off because the air is not which combustion takes place. available. COMBUSTION AND FLAME 41 2024-25  is covered with a blanket to extinguish fire (Fig. 4.3). Can you guess why? We have read that the sun produces its own heat and light. Is it also some kind of combustion? In the sun, heat and light are produced by nuclear reactions. You will learn about this process in higher classes. Fig. 4.3 : Blanket wrapped around a person whose clothes caught fire Activity 4.3 Now recall some of your experiences. Place a piece of burning wood or Does a matchstick burn by itself? charcoal on an iron plate or Tawa. How does it burn? Cover it with a glass jar or a You must have had an experience of tumbler, or a transparent plastic jar. burning a piece of paper. Does it burn Observe what happens. Does when a burning matchstick is brought charcoal stop burning after near it? sometime? Can you think of the Can you burn a piece of wood by reason why it stops burning? bringing a lighted matchstick near it? Why do you have to use paper or You might have heard that when the kerosene oil to start fire in wood or coal? clothes of a person catch fire, the person Have you heard of forest fires? During extreme heat of summer, at some places dry grass catches fire. From the grass, it spreads to trees, and very soon the whole forest is on fire (Fig. 4.4). It is very difficult to control such fires. Fig. 4.4 : Forest fire 42 SCIENCE 2024-25  Do these experiences tell you that We find that a combustible substance different substances catch fire at cannot catch fire or burn as long as its different temperatures? temperature is lower than its ignition The lowest temperature at which a temperature. Have you ever seen substance catches fire is called its cooking oil catching fire when a frying ignition temperature. pan is kept for long on a burning stove? Can you tell now why a matchstick Kerosene oil and wood do not catch fire does not catch fire on its own at room on their own at room temperature. But, temperature? Why does the matchstick if kerosene oil is heated a little, it will start burning on rubbing it on the side catch fire. But if wood is heated a little, of the matchbox? it would still not catch fire. Does it mean that ignition temperature of kerosene oil The history of the matchstick is very is lower than that of wood? Does it mean old. More than five thousand years that we need to take special care in ago small pieces of pinewood dipped storing kerosene oil? The following in sulphur were used as matches in activity shows that it is essential for a ancient Egypt. The modern safety substance to reach ignition temperature match was developed only about two hundred years ago. to burn. A mixture of antimony trisulphide, potassium chlorate and white Activity 4.4 phosphorus with some glue and starch was applied on the head of a (Caution : Be careful while handling match made of suitable wood. When burning candle). struck against a rough surface, white phosphorus got ignited due to the heat Make two paper cups by folding a of friction. This started the sheet of paper. Pour about 50 mL of combustion of the match. However, water in one of the cups. Heat both white phosphorus proved to be the cups separately with a candle dangerous both for the workers (Fig. 4.5). What do you observe? 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. water The rubbing surface has powdered paper glass and a little red phosphorus cups (which is much less dangerous). When the match is struck against the rubbing surface, some red phosphorus gets converted into white Fig. 4.5 : Heating water in a paper cup phosphorus. This immediately reacts with potassium chlorate in the What happens to the empty paper matchstick head to produce enough cup? What happens to the paper cup heat to ignite antimony trisulphide with water? Does water in this cup and start the combustion. become hot? COMBUSTION AND FLAME 43 2024-25  If we continue heating the cup, we can even boil water in the paper 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 Fig. 4.6: Firemen extinguish the fire by throwing water under pressure a flame are called inflammable substances. Examples of Does your city/town have a fire inflammable substances are petrol, brigade station? alcohol, Liquified Petroleum Gas (LPG) When a fire brigade arrives, what etc. Can you list some more inflammable does it do? It pours water on the fire substances? (Fig. 4.6). Water cools the combustible material so that its temperature is 4.2 How Do We Control Fire? brought below its ignition temperature. This prevents the fire You must have seen or heard of fire from spreading. Water vapours also breaking out in homes, shops and surround the combustible material, factories. If you have seen such an helping in cutting off the supply of air. accident, write a short description in So, the fire is extinguished. your note book. Also, share the You have learnt that there are three experience with your classmates. essential requirements for producing fire. Can you list these requirements? Find out the telephone number of the These are: fuel, air (to supply fire service in your area. If a fire breaks out in your house or in your oxygen) and heat (to raise the neighbourhood, the first thing to do temperature of the fuel beyond the is to call the fire service. ignition temperature). Fire can be controlled by removing one or more of these requirements. The job of a fire It is important that all of us extinguisher is to cut off the supply of know the telephone numbers air, or to bring down the temperature of the fire service. of the fuel, or both. Notice that the fuel 44 SCIENCE 2024-25  The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. Water is also not suitable for fires involving oil and petrol. Do you recall that water is heavier than oil? So, it Fig. 4.7 : Fire extinguisher sinks below the oil, and oil keeps burning on the top. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. CO 2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in most cases it does not harm the electrical equipment. How do we get the supply of carbon dioxide? It can be stored at high pressure as a liquid in cylinders. In what form is the LPG stored in cylinders? When released from the cylinder, CO2 expands enormously in volume and cools down. So, it not only forms a blanket around the fire, it also brings down the temperature of the fuel. That is why it is an excellent fire extinguisher. Another way to get CO 2 is to release a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate. Near the fire, these chemicals give off CO2. in most cases cannot be eliminated. We find that the gas burns rapidly If, for instance, a building catches fire, and produces heat and light. Such the whole building is the fuel. combustion is known as rapid 4.3 Types of Combustion combustion. There are substances like Bring a burning matchstick or a gas phosphorus which burn in air at room lighter near a gas stove in the kitchen. temperature. Turn on the knob of the gas stove. What The type of combustion in which do you observe? a material suddenly bursts into CAUTION : Do not handle the gas flames, without the application of any stove yourself. Ask your parents apparent cause is called spontaneous to help. combustion. COMBUSTION AND FLAME 45 2024-25  Spontaneous combustion of coal dust has resulted in many disastrous fires in coal mines. Spontaneous forest fires are sometimes due to the heat of the sun or due to lightning strike. However, most forest fires are due to the carelessness of human beings. It is important to remember that the Fig. 4.8: Colours of a candle flame and the campfires must be completely flame of a kitchen stove extinguished before leaving a forest after a picnic, or a visit. We generally have fireworks on festival days. When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound. A large amount of gas formed in the reaction is liberated. Such a reaction is called explosion. Explosion can also take place if pressure is applied on the cracker. 4.4 Flame Observe an LPG flame. Can you tell the colour of the flame. What is the colour of a candle flame? Fig. 4.9 : Flames of kerosene lamp, candle and Recall your experience of burning a Bunsen burner magnesium ribbon in Class VII. If you do not have experience of burning the Record your observations and remaining items in Table 4.2 you can mention whether on burning the do that now. material forms a flame or not. Table 4.2 Materials forming Flame on Burning S.No. Material Forms flame Does not form flame 1. Candle 2. Magnesium 3. Camphor 4. Kerosene Stove 5. Charcoal 46 SCIENCE 2024-25 4.5 Structure of a Flame Activity 4.5 Light a candle (Caution : Be careful). Hold a 4-5 cm long thin glass tube with a pair of tongs and introduce its one end in the dark zone of a non-flickering candle flame (Fig. 4.10). Bring a lighted Fig. 4.11 matchstick near the other end of the glass tube. Do you see a flame When the candle flame is steady, caught at this end of the glass tube introduce a clean glass plate/slide into after a while? If so, what is it that the luminous zone of the flame produces a flame? Notice that the wax (Fig. 4.11). Hold it there with a pair of near the heated wick melts quickly. tongs for about 10 seconds. Then remove it. What do you observe? Fig. 4.12 Fig. 4.10 A circular blackish ring is formed on the glass plate/slide. It indicates the The substances which vapourise deposition of unburnt carbon particles during burning, give flames. For present in the luminous zone of the example, kerosene oil and molten wax flame. rise through the wick and are vapourised Hold a thin long copper wire just during burning and form flames. inside the non-luminous zone of flame Charcoal, on the other hand, does not for about 30 seconds (Fig. 4.12). vapourise and so does not produce a Notice that the portion of the copper flame. In Activity 4.5, could the vapours wire just outside the flame gets red hot. of wax coming out of the glass tube be Does it indicate that the non-luminous the cause of the flame produced? zone of the flame has a high COMBUSTION AND FLAME 47 2024-25 temperature? In fact, this part of the 4.6 What is a Fuel? flame is the hottest part (Fig. 4.13). Recall that the sources of heat energy outer zone of for domestic and industrial purposes hottest complete are mainly wood, charcoal, petrol, part combustion (blue) kerosene etc. These substances are moderately called fuels. A good fuel is one which is hot middle zone of readily available. It is cheap. It burns partial combustion easily in air at a moderate rate. It least (yellow) produces a large amount of heat. It does hot not leave behind any undersirable innermost zone of substances. wax candle unburnt wax vapours (black) There is probably no fuel that could be considered as an ideal fuel. We Fig. 4.13 : Different zones of candle flame should look for a fuel which fulfils most of the requirements for a particular use. Goldsmiths blow the outermost zone Fuels differ in their cost. Some fuels of a flame with a metallic blow-pipe for are cheaper than others. melting gold and silver (Fig. 4.14). Why Make a list of fuels familiar to you. do they use the outermost zone of the Group them as solid, liquid and gaseous flame? fuels as in Table 4.3. 4.7 Fuel Efficiency Suppose you were asked to boil a given quantity of water using cow dung, coal and LPG as fuel. Which fuel would you prefer? Give your reason. You may take the help of your parents. Do these three fuels produce the same amount of heat? The amount of heat energy produced on complete combustion of 1 kg of a fuel is Fig. 4.14 : Goldsmith blowing through a called its calorific value. The calorific metallic pipe value of a fuel is expressed in a unit Table 4.3 : Types of Fuels S. No. Solid Fuels Liquid Fuels Gaseous Fuels 1. Coal Kerosene oil Natural gas 2. 3. 48 SCIENCE 2024-25 called kilojoule per kg (kJ/kg). particles. These fine particles are Calorific values of some fuels are given dangerous pollutants causing in Table 4.4. respiratory diseases, such as asthma. 2. Incomplete combustion of these Table 4.4 : Calorific Values of different fuels gives carbon monoxide gas. It is a Fuels very poisonous gas. It is dangerous to Fuel Calorific Value burn coal in a closed room. The carbon (kJ/kg) monoxide gas produced can kill persons sleeping in that room. Cow dung cake 6000-8000 Wood 17000-22000 Oh! So, that is why we are Coal 25000-33000 advised never to sleep in a Petrol 45000 room with burning or smouldering coal fire in it. Kerosene 45000 Diesel 45000 3. Combustion of most fuels releases Methane 50000 carbon dioxide in the environment. CNG 50000 Increased concentration of carbon LPG 55000 dioxide in the air is believed to cause Biogas 35000-40000 global warming. Hydrogen 150000 Global warming is the rise in temperature of the atmosphere of the Burning of Fuels Leads to Harmful earth. This results, among other things, Products in the melting of polar glaciers, which leads to a rise in the sea level, causing The increasing fuel consumption has floods in the coastal areas. Low lying harmful effects on the environment. coastal areas may even be permanently 1. Carbon fuels like wood, coal, submerged under water. petroleum release unburnt carbon 4. Burning of coal and diesel releases For centuries, wood was used as sulphur dioxide gas. It is an extremely domestic and industrial fuel. But now it has been replaced by coal and other suffocating and corrosive gas. Moreover, fuels like LPG. In many rural parts of petrol engines give off gaseous oxides our country, people still use wood as a of nitrogen. Oxides of sulphur and fuel because of its easy availability and nitrogen dissolve in rain water and form low cost. However, burning of wood gives acids. Such rain is called acid rain. It is a lot of smoke which is very harmful for very harmful for crops, buildings human beings. It causes respiratory problem. Also, trees provide us with and soil. useful substances which are lost when The use of diesel and petrol as fuels wood is used as fuel. Moreover cutting in automobiles is being replaced by of trees leads to deforestation which is CNG (Compressed Natural Gas), because quite harmful to the environment, as CNG produces the harmful products in you learnt in Class VII. very small amounts. CNG is a cleaner fuel. COMBUSTION AND FLAME 49 2024-25 KEYWORDS WHAT YOU HAVE LEARNT ACID RAIN Ü The substances which burn in air are called combustible. CALORIFIC VALUE Ü Oxygen (in air) is essential for combustion. COMBUSTION Ü During the process of combustion, heat and light are given out. DEFORESTATION Ü Ignition temperature is the lowest temperature at which a combustible substance catches EXPLOSION fire. Ü Inflammable substances have very low FLAME ignition temperature. FIRE EXTINGUISHER Ü Fire can be controlled by removing one or more requirements essential for producing fire. FUEL Ü Water is commonly used to control fires. Ü Water cannot be used to control fires involving FUEL EFFICIENCY electrical equipment or oils. GLOBAL WARMING Ü There are various types of combustions such as rapid combustion, spontaneous IDEAL FUEL combustion, explosion, etc. Ü There are three different zones of a flame - IGNITION dark zone, luminous zone and non-luminous TEMPERATURE zone. INFLAMMABLE Ü An ideal fuel is cheap, readily available, readily SUBSTANCES combustible and easy to transport. It has high calorific value. It does not produce gases or residues that pollute the environment. Ü Fuels differ in their efficiency and cost. Ü Fuel efficiency is expressed in terms of its calorific value which is expressed in units of kilojoule per kg. Ü Unburnt carbon particles in air are dangerous pollutants causing respiratory problems. Ü Incomplete combustion of a fuel gives poisonous carbon monoxide gas. Ü Increased percentage of carbon dioxide in air has been linked to global warming. Ü Oxides of sulphur and nitrogen produced by the burning of coal, diesel and petrol cause acid rain which is harmful for crops, buildings and soil. 50 SCIENCE 2024-25  Exercises 1. List conditions under which combustion can take place. 2. Fill in the blanks. (a) Burning of wood and coal causes of air. (b) A liquid fuel, used in homes is. (c) Fuel must be heated to its before it starts burning. (d) Fire produced by oil cannot be controlled by. 3. Explain how the use of CNG in automobiles has reduced pollution in our cities. 4. Compare LPG and wood as fuels. 5. Give reasons. (a) Water is not used to control fires involving electrical equipment. (b) LPG is a better domestic fuel than wood. (c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not. 6. Make a labelled diagram of a candle flame. 7. Name the unit in which the calorific value of a fuel is expressed. 8. Explain how CO2 is able to control fires. 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain. 10. Which zone of a flame does a goldsmith use for melting gold and silver and why? 11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel. 12. Can the process of rusting be called combustion? Discuss. 13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time? COMBUSTION AND FLAME 51 2024-25  Extended Learning — Activities and Projects 1. Survey the availability of various fuels in your locality. Find out their cost per kg and prepare a tabular chart showing how many kJ of various fuels you can get for every rupee. 2. Find out the number, type and location of fire extinguishers available in your school, nearby shops and factories. Write a brief report about the preparedness of these establishments to fight fire. 3. Survey 100 houses in your area. Find the percentage of households using LPG, kerosene, wood and cattle dung as fuel. 4. Talk to people who use LPG at home. Find out what precautions they take in using LPG. 5. Make a model of a fire extinguisher. Place a short candle and a slightly taller candle in a small dish filled with baking soda. Place the dish at the bottom of a large bowl. Light both the candles. Then pour vinegar into the dish of baking soda. Take care. Do not pour vinegar on the candles. Observe the foaming reaction. What happens to the candles? Why? In what order? baking soda + vinegar Fig. 4.15 For more information, visit: l www.newton.dep.anl.gov/askasci/chem03/chem03767.htm l http://en.wikipedia.org/wiki/combustion 52 SCIENCE 2024-25