Combustion and Flames PDF

COMBUSTION AND FLAME W e use different kinds of fuel for d various purposes at home, in industry and for running...

COMBUSTION AND FLAME W e use different kinds of fuel for d various purposes at home, in industry and for running e automobiles. Can you name a few fuels h used in our homes? Name a few fuels used in trade and industry. What fuels T s are used for running automobiles? Your l i list will contain fuels like cowdung, R wood, coal, charcoal, petrol, diesel, b compressed natural gas (CNG), etc. E You are familiar with the burning of a u candle. What is the difference between the C burning of a candle and the burning of a p fuel like coal? May be you were able to N re guess right: candle burns with a flame whereas coal does not. Similarly, you will find many other materials burning © e Fig. 6.1 : 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 b produced during this process. substance reacts with oxygen to give off heat is called combustion. The 6.1 What is Combustion? substance that undergoes combustion o is said to be combustible. It is also called t Recall the activity of burning of magnesium ribbon performed in Class a fuel. The fuel may be solid, liquid or t VII. We learnt that magnesium burns to gas. Sometimes, light is also given off form magnesium oxide and produces during combustion, either as a flame or o heat and light (Fig. 6.1). as a glow. We can perform a similar activity with In the reactions mentioned above n a piece of charcoal. Hold the piece with 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.  Activity 6.2 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 d Activity 6.1 e Collect some materials like straw, h matchsticks, kerosene oil, paper, iron nails, stone pieces, glass, etc. T s Under the supervision of your l i teacher try to burn each of these R materials one by one. If combustion b takes place mark the material E combustible, otherwise mark it as (a) (b) (c) u Fig. 6.2: Experiment to show that air is non-combustible (Table 6.1). C essential for burning p Table 6.1 Combustible and non- chimney [Fig. 6.2(a)]. Observe what N re combustible Substances happens to the flame. Now remove Material Combustible Non- the blocks and let the chimney rest on the table [Fig. 6.2(b)]. Again © e combustible observe the flame. Finally, put a Wood glass plate over the chimney [Fig. b Paper 6.2(c)]. Watch the flame again. What happens in the three cases? Does Iron nails the flame flicker off? Does it flicker o Kerosene oil and give smoke? Does it burn t Stone piece unaffected? Can you infer anything t at all about the role played by air in Straw the process of burning? o Charcoal We find that for combustion, air is n 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 6.1. 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 65  is covered with a blanket to extinguish fire (Fig. 6.3). 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 d will learn about this process in higher e classes. Fig. 6.3 : Blanket wrapped around a person whose clothes caught fire h Activity 6.3 Now recall some of your experiences. T i s Place a piece of burning wood or Does a matchstick burn by itself? R l 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 E b tumbler, or a transparent plastic jar. burning a piece of paper. Does it burn Observe what happens. Does when a burning matchstick is brought C u charcoal stop burning after near it? p sometime? Can you think of the Can you burn a piece of wood by N re 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? © e clothes of a person catch fire, the person Have you heard of forest fires? b t o During extreme heat of summer, at some t places dry grasses catch fire. From o grasses, it spreads to trees, and very soon n the whole forest is on fire (Fig. 6.4). It is very difficult to control such fires. Fig. 6.4 : Forest fire 66 SCIENCE  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 d start burning on rubbing it on the side catches fire. But if wood is heated a e of the matchbox? little, it would still not catch fire. Does it mean that ignition temperature of h The history of the matchstick is very kerosene oil is lower than that of wood? old. More than five thousand years T s Does it mean that we need to take i ago small pieces of pinewood dipped special care in storing kerosene oil? The l in sulphur were used as matches in R following activity shows that it is ancient Egypt. The modern safety essential for a substance to reach b match was developed only about two E hundred years ago. ignition temperature to burn. u A mixture of antimony trisulphide, C potassium chlorate and white Activity 6.4 p phosphorus with some glue and N re 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 Make two paper cups by folding a © e phosphorus got ignited due to the heat 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 b white phosphorus proved to be the cups separately with a candle dangerous both for the workers (Fig. 6.5). What do you observe? involved in the manufacturing of o matches and for the users. t These days the head of the safety match contains only antimony t trisulphide and potassium chlorate. water o The rubbing surface has powdered paper glass and a little red phosphorus cups n (which is much less dangerous). When the match is struck against the rubbing surface, some red phosphorus gets converted into white Fig. 6.5 : 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 67  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 d transferred to water by conduction. So, in the e presence of water, the h ignition temperature of paper is not reached. T i s Hence, it does not burn. l The substances which R have very low ignition E b temperature and can easily catch fire with a Fig. 6.6: Firemen extinguish the fire by throwing water under pressure C u flame are called p inflammable substances. Examples of Does your city/town have a fire N re 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 © e substances? (Fig. 6.6). Water cools the combustible material so that its temperature is 6.2 How do We Control Fire? brought below its ignition temperature. b This prevents the fire from spreading. You must have seen or heard of fire Water vapours also surround the breaking out in homes, shops and o combustible material, helping in cutting factories. If you have seen such an t off the supply of air. So, the fire is accident, write a short description in extinguished. your note book. Also, share the t You have learnt that there are three experience with your classmates. essential requirements for producing o fire. Can you list these requirements? Find out the telephone number of the These are: fuel, air (to supply oxygen) n fire service in your area. If a fire breaks out in your house or in your and heat (to raise the temperature of the neighbourhood, the first thing to do fuel beyond the ignition temperature). is to call the fire service. Fire can be controlled by removing one or more of these requirements. The job of a fire extinguisher is to cut off the It is important that all of us supply of air, or to bring down the know the telephone numbers temperature of the fuel, or both. Notice of the fire service. that the fuel in most cases cannot be 68 SCIENCE  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 d also not suitable for fires e involving oil and petrol. Do you recall that water is h heavier than oil? So, it sinks below the oil, and oil s Fig. 6.7 : Fire extinguisher T i keeps burning on top. l For fires involving electrical equipment and inflammable materials like R petrol, carbon dioxide (CO2) is the best extinguisher. CO 2, being heavier than b oxygen, covers the fire like a blanket. Since the contact between the fuel and E oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in u most cases it does not harm the electrical equipment. C How do we get the supply of carbon dioxide? It can be stored at high pressure p as a liquid in cylinders. In what form is the LPG stored in cylinders? When N re 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 © e 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. b eliminated. If, for instance, a building combustion is known as rapid o catches fire, the whole building is the combustion. t fuel. There are substances like phosphorus which burn in air at room 6.3 Types of Combustion t temperature. Bring a burning matchstick or a gas The type of combustion in which a o lighter near a gas stove in the kitchen. material suddenly bursts into flames, Turn on the knob of the gas stove. What without the application of any apparent n do you observe? cause is called spontaneous combustion. CAUTION : Do not handle the gas stove yourself. Ask your parents Spontaneous combustion of coal dust to help. has resulted in many disastrous fires in coal mines. Spontaneous forest We find that the gas burns rapidly fires are sometimes due to the heat and produces heat and light. Such COMBUSTION AND FLAME 69  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 campfires must be completely extinguished before leaving a forest after a picnic, or a visit. Fig. 6.8: Colours of a candle flame and the flame of a kitchen stove d We generally have fireworks on e festival days. When a cracker is ignited, a sudden reaction takes place with the h evolution of heat, light and sound. A T s large amount of gas formed in the i reaction is liberated. Such a reaction is R l called explosion. Explosion can also take place if pressure is applied on the E b cracker. u 6.4 Flame C p Observe an LPG flame. Can you tell the N re colour of the flame. What is the colour of a candle flame? Fig. 6.9 : Flames of kerosene lamp, candle and Bunsen Burner © e Recall your experience of burning a magnesium ribbon in Class VII. If you 6.5 Structure of a Flame do not have experience of burning the Activity 6.6 b remaining items in Table 6.2 you can do that now. Light a candle (Caution : Be careful). o Record your observations and Hold a glass tube with a pair of t mention whether on burning the tongs and introduce its one end in material forms a flame or not. the dark zone of a non-flickering S.No. o t Material Table 6.2 Materials forming Flame on Burning Forms flame Does not form flame n 1. Candle 2. Magnesium 3. Camphor 4. Kerosene Stove 5. Charcoal 70 SCIENCE  candle flame [Fig. 6.10 (a)]. Bring a lighted matchstick near the other end of the glass tube. Do you see a flame? If so, what is it that produces a flame? Notice that the wax near the heated wick melts quickly. Fig. 6.10 (b) e d h A circular blackish ring is formed on the glass plate/slide. It indicates the T i s deposition of unburnt carbon particles l present in the luminous zone of the R flame. b Hold a thin long copper wire just E inside the flame for about 30 seconds u [Fig. 6.10 (c)]. C N re p © e Fig. 6.10 (a) b The substances which vapourise during burning, give flames. For o example, kerosene oil and molten wax t rise through the wick and are vapourised during burning and form flames. t Charcoal, on the other hand, does not o vapourise and so does not produce a Fig. 6.10 (c) flame. In Activity 6.6, could the vapours n of wax coming out of the glass tube be Notice that the portion of the copper the cause of the flame produced? wire just outside the flame gets red hot. When the candle flame is steady, Does it indicate that the non-luminous introduce a clean glass plate/slide into zone of the flame has a high the luminous zone of the flame [Fig. 6.10 temperature? In fact, this part of the (b)]. Hold it there with a pair of tongs flame is the hottest part [Fig. 6.10(d)]. for about 10 seconds. Then remove it. Goldsmiths blow the outermost zone What do you observe? of a flame with a metallic blow-pipe for COMBUSTION AND FLAME 71  are mainly wood, charcoal, petrol, outer zone of hottest complete kerosene, etc. These substances are part combustion (blue) called fuels. A good fuel is one which is readily available. It is cheap. It burns moderately hot middle zone of easily in air at a moderate rate. It partial combustion produces a large amount of heat. It does least (yellow) not leave behind any undersirable hot substances. d innermost zone of There is probably no fuel that could wax candle unburnt wax e be considered as an ideal fuel. We vapours (black) should look for a fuel which fulfils most h Fig. 6.10 (d): Different Zones of Candle flame of the requirements for a particular use. Fuels differ in their cost. Some fuels T s melting gold and silver (Fig. 6.11). Why i are cheaper than others. do they use the outermost zone of the R l Make a list of fuels familiar to you. flame? Group them as solid, liquid and gaseous E b fuels as in Table 6.3. u 6.7 Fuel Efficiency C N re p 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 © e the help of your parents. Do these three fuels produce the same amount of heat? b The amount of heat energy produced on Fig. 6.11 : Goldsmith blowing through a complete combustion of 1 kg of a fuel is metallic pipe o called its calorific value. The calorific t value of a fuel is expressed in a unit 6.6 What is a Fuel? called kilojoule per kg (kJ/kg). t Recall that the sources of heat energy Calorific values of some fuels are given for domestic and industrial purposes in Table 6.4. S. No. no Solid Fuels Table 6.3 Types of Fuels Liquid Fuels Gaseous fuels 1. Coal Kerosene oil Natural gas 2. 3. 72 SCIENCE  Table 6.4 : Calorific Values of 2. Incomplete combustion of these fuels Different Fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous Fuel Calorific Value to burn coal in a closed room. The (kJ/kg) carbon monoxide gas produced can Cow dung cake 6000-8000 kill persons sleeping in that room. Wood 17000-22000 Coal 25000-33000 d Oh! So, that is why we are Petrol 45000 advised never to sleep in a e Kerosene 45000 room with burning or Diesel 45000 smouldering coal fire in it. h Methane 50000 T s CNG 50000 3. Combustion of most fuels releases i carbon dioxide in the environment. LPG 55000 R l Increased concentration of carbon Biogas 35000-40000 dioxide in the air is believed to cause b Hydrogen 150000 E global warming. u Burning of Fuels Leads to Harmful Global warming is the rise in C Products temperature of the atmosphere of the p The increasing fuel consumption has earth. This results, among other N re harmful effects on the environment. things, in the melting of polar glaciers, 1. Carbon fuels like wood, coal, which leads to a rise in the sea level, causing floods in the coastal areas. © e petroleum release unburnt carbon Low lying coastal areas may even be particles. These fine particles are permanently submerged under water. dangerous pollutants causing b respiratory diseases, such as asthma. 4. Burning of coal and diesel releases For centuries, wood was used as sulphur dioxide gas. It is an o domestic and industrial fuel. But now extremely suffocating and corrosive t it has been replaced by coal and other gas. Moreover, petrol engines give off fuels like LPG. In many rural parts of t our country, people still use wood as gaseous oxides of nitrogen. Oxides a fuel because of its easy availability of sulphur and nitrogen dissolve in o and low cost. However, burning of rain water and form acids. Such rain wood gives a lot of smoke which is n is called acid rain. It is very harmful very harmful for human beings. It for crops, buildings and soil. You have causes respiratory problem. Also, already learnt about it in Class VII. trees provide us with useful substances which are lost when wood The use of diesel and petrol as fuels is used as fuel. Moreover cutting of in automobiles is being replaced by trees leads to deforestation which CNG (Compressed Natural Gas), because is quite harmful to the environment, CNG produces the harmful products in as you learnt in Class VII. very small amounts. CNG is a cleaner fuel. COMBUSTION AND FLAME 73 KEYWORDS WHAT YOU HAVE LEARNT ACID RAIN
The substances which burn in air are called combustible. CALORIFIC VALUE
Oxygen (in air) is essential for combustion. COMBUSTION
During the process of combustion, heat and light are given out. DEFORESTATION
Ignition temperature is the lowest temperature d at which a combustible substance catches EXPLOSION fire. e
Inflammable substances have very low FLAME ignition temperature. h FIRE EXTINGUISHER
Fire can be controlled by removing one or more T s requirements essential for producing fire. i FUEL
l Water is commonly used to control fires. R
Water cannot be used to control fires involving FUEL EFFICIENCY b electrical equipments or oils. E GLOBAL WARMING
There are various types of combustions such u as rapid combustion, spontaneous C IDEAL FUEL p combustion, explosion, etc.
There are three different zones of a flame - N re IGNITION dark zone, luminous zone and non-luminous TEMPERATURE zone. © e INFLAMMABLE
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 b residues that pollute the environment.
Fuels differ in their efficiency and cost. o
Fuel efficiency is expressed in terms of its t calorific value which is expressed in units of kilojoule per kg. t
Unburnt carbon particles in air are dangerous o pollutants causing respiratory problems.
Incomplete combustion of a fuel gives n poisonous carbon monoxide gas.
Increased percentage of carbon dioxide in air has been linked to global warming.
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. 74 SCIENCE  Exercises 1. List conditions under which combustion can take place. 2. Fillin 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 (d) Fire produced by oil cannot be controlled by. e 3. Explain how the use of CNG in automobiles has reduced pollution in our cities. h 4. Compare LPG and wood as fuels. T i s 5. Give reasons: l (a) Water is not used to control fires involving electrical equipment. R (b) LPG is a better domestic fuel than wood. b (c) Paper by itself catches fire easily whereas a piece of paper wrapped E around an aluminium pipe does not. C u 6. Make a labelled diagram of a candle flame. p 7. Name the unit in which the calorific value of a fuel is expressed. N re 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. © e 10. Which zone of a flame does a goldsmith use for melting gold and silver and why? b 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. o 12. Can the process of rusting be called combustion? Discuss. t 13. Abida and Ramesh were doing an experiment in which water was to be t 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 o flame. Whose water will get heated in a shorter time? n COMBUSTION AND FLAME 75  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. d 3. Survey 100 houses in your area. Find the percentage of households e using LPG, kerosene, wood and cattledung as fuel. 4. Talk to people who use LPG at home. Find out what precautions h they take in using LPG. T s 5. Make a model of a fire extinguisher. Place a short candle and a i slightly taller candle in a small dish filled with baking soda. Place R l the dish at the bottom of a large bowl. Light both the candles. Then b pour vinegar into the dish of baking soda. Take care. Do not pour E vinegar on the candles. Observe the foaming reaction. What happens u to the candles? Why? In what order? C N re p © e baking soda + b vinegar Fig. 6.12 o For more information, visit: t  www.newton.dep.anl.gov/askasci/chem03/chem03767.htm  http://www.einstrumentsgroup.com/gas_analyzers/combustion/ t what-is-combustion.php o  http://library.kcc.hawaii.edu/external/chemistry/ everyday_combustion.html n  http://en.wikipedia.org/wiki/combustion  http://wwwchem.csustan.edu/consumer/fuels/heats%20.htm 76 SCIENCE