Heat should be supplied to raise the temperature of the fuel beyond its ignition temperature.<\/li>\n<\/ul>\nQuestion 2.
\nFill in the blanks:
\na. Burning of wood and coal causes …………… of air.
\nb. A liquid fuel, used in homes is ……………
\nc. Fuel must be heated to its …………… before it starts burning.
\nd. Fire produced by oil cannot be controlled by ……………
\nAnswer:
\n(a) pollution,
\n(b) kerosene,
\n(c) ignition temperature,
\n(d) water<\/p>\n
Question 3.
\nExplain how the use of CNG in automobiles has reduced pollution in our cities.
\nAnswer:
\nThe use of diesel and petrol as fuels in automobiles is a major cause of air pollution today. During combustion, these fuels release a large amount of unburnt carbon particles which are highly poisonous and harmful for living beings and the environment. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma. Incomplete combustion of these fuels gives off carbon monoxide gas. It is a very poisonous gas.<\/p>\n
Combustion of most fuels releases carbon dioxide in the environment. Increased emission of carbon dioxide in the air is one of the major causes for global warming. The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel. It has high fuel efficiency. Hence use of CNG in automobiles has reduced pollution in our cities to a noticeable extent.<\/p>\n
Question 4.
\nCompare LPG and wood as fuels.
\nAnswer:
\nWood has traditionally been used as a kitchen fuel and is still predominantly being used in rural areas. Burning of wood releases many air pollutants which can result in respiratory problems. Moreover, incomplete oxidation during burning of wood releases carbon monoxide gas which is a poisonous gas. LPG is a much better fuel because it bums without giving any smoke. It produces lesser amount of air pollutants. Complete oxidation during burning of LPG does not lead to the formation of carbon monoxide gas.<\/p>\n
<\/p>\n
Question 5.
\nGive reasons:
\na. Water is not used to control fires involving electrical equipment.
\nb. LPG is a better domestic fuel than wood.
\nc. Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
\nAnswer:
\na. Pure water is a bad conductor of electricity but normal water contains many salts and hence is a good conductor of electricity. Trying to put off fires involving electrical equipment with water can result in electric shock. Due to this, water is not used to control fires involving electrical equipment.<\/p>\n
b. LPG is a better domestic fuel than wood because of several reasons. Unlike wood, LPG bums without smoke. This makes the life of the cook more comfortable and they do not have to worry about blackening of pots and pans. Moreover, use of LPG as domestic fuel also rules out the chances of getting respiratory disorders which may happen when someone uses wood as kitchen fuel. Storage and transportation of LPG is easier as compared to that of wood,<\/p>\n
c. The ignition temperature of paper is lower as compared to that of aluminium. When paper is wrapped around an aluminium pipe, the ignition temperature increases. That is why paper itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.<\/p>\n
Question 6.
\nMake a labelled diagram of a candle flame.
\nAnswer:
\n<\/p>\n
Question 7.
\nName the unit in which the calorific value of a fuel is expressed.
\nAnswer:
\nKilojoule per kilogram (kJ\/kg).<\/p>\n
Question 8.
\nExplain how CO2<\/sub> is able to control fires.
\nAnswer:
\nCarbon dioxide is heavier than oxygen. Due to this, carbon dioxide forms a blanket around the burning material and makes a barrier between the burning material and oxygen. As oxygen supply is cut off, it helps in extinguishing the fire.<\/p>\nQuestion 9.
\nIt is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
\nAnswer:
\nGreen leaves contain moisture and as a result, the ignition temperature of green leaves is much higher than that of dry leaves. Due to this, it is difficult to bum a heap of green leaves but dry leaves catch fire easily.<\/p>\n
Question 10.
\nWhich zone of a flame does a goldsmith use for melting gold and silver and why?
\nAnswer:
\nThe outermost zone of a flame is the hottest zone. Due to this, goldsmith uses the outermost zone of a flame for melting gold and silver.<\/p>\n
<\/p>\n
Question 11.
\nIn 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.
\nAnswer:
\nTotal mass of the fuel = 4.5 kg
\nHeat produced by burning the given mass of fuel = 180,000 kJ.
\nWe know that calorific value of fuel = \\(\\frac{\\text { Heat produced in } \\mathrm{kJ}}{\\text { Total mass burnt }}\\)
\n= \\(\\frac{180,000 \\mathrm{~kJ}}{4.5 \\mathrm{~kg}}\\) = 40,000 kJ\/kg
\nHence, the calorific value of the given fuel is 40,000 kJ\/kg.<\/p>\n
Question 12.
\nCan the process of rusting be called combustion? Discuss.
\nAnswer:
\nAlthough the process of rusting also involves oxidation, yet it cannot be termed as combustion. The reason for this is that combustion is defined as a process in which oxidation is accompanied by heat, and heat is not produced during rusting.<\/p>\n
Question 13.
\nAbida 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?
\nAnswer:
\nSince Abida has kept the beaker in the luminous zone of the flame, the beaker will take more time to get heated as this zone is moderately hot. On the other hand, Ramesh has kept the beaker in the non-luminous zone of the flame so his beaker will be heated in shorter time as this is the hottest zone of the flame.<\/p>\n
NCERT Learning Activities and Projects<\/span><\/p>\nQuestion 1.
\nSurvey 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.
\nHint:
\nDo it yourself.<\/p>\n
Question 2.
\nFind 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.
\nHint:
\nBasically there are two types of fire extinguishers. They are carbon dioxide fire extinguishers and water fire extinguisher.
\nOthers may include water and foam, dry chemical, wet chemical, clean agent, water mist, etc.
\nCarbon dioxide fire extinguisher is used when there are fires caused by oil, petrol
\nor by any electric appliance. Water fire extinguisher is used to fight fires caused by wood or paper.<\/p>\n
<\/p>\n
Question 3.
\nSurvey 100 houses in your area. Find the percentage of households using LPG, kerosene, wood and cattle dung as fuel.
\nHint:
\nDo it yourself.<\/p>\n
Question 4.
\nTalk to people who use LPG at home. Find out what precautions they take in using LPG.
\nHint:
\nThe following precautions must be taken while using LPG cylinders:
\na. Before igniting the gas, we should make sure that there is no foul smell of the leaking gas. If there is a foul smell of gas, then the doors and windows should be opened at once to allow the gas to escape. The gas cylinder, rubber tubing and gas stove should then be properly checked to detect the cause of gas leakage. Preferably, a mechanic should be called to fix the defect. The gas should be used only after the defect has been rectified. Never use a leaking gas cylinder.
\nb. We should not use any open flames, like a kerosene stove or an electric heater, near the gas cylinder.
\nc. The rubber pipe connecting the gas cylinder to the gas stove should be checked regularly for any wear and tear.
\nd. The gas cylinder should be handled with care to avoid any damage to the cylinder valve.
\ne. The cylinder valve should be closed when the gas stove is not in use.
\nf. While lighting the gas stove, we should first open the cylinder (regulator) valve and then turn the knob of the gas stove.<\/p>\n
Question 5.
\nMake 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?
\n
\nHint:
\nWhen vinegar is combined with baking soda, the two react and produce carbon dioxide gas. The carbon dioxide gas is heavier than the surrounding air so it sinks into the bottom of the bowl. As the reaction continues, more and more carbon dioxide gas is produced which begins to fill up the bowl slowly. When the level of carbon dioxide has risen to the level of the flame, the flame will be extinguished due to lack of oxygen.<\/p>\n
Activity 1<\/span><\/p>\nObjective: To show that air is necessary for combustion.
\nMaterials Required: A lighted candle, glass chimney, a glass plate and wooden blocks.
\nProcedure:<\/p>\n
\n- Fix a lighted candle on a table.<\/li>\n
- Put a glass chimney over the candle and rest it on a few wooden blocks in such a way that air can enter the chimney. Observe what happens to the flame.<\/li>\n
- Now remove the blocks and let the chimney rest on the table. Again observe the flame.<\/li>\n
- Finally, put a glass plate over the chimney. Watch the flame again.<\/li>\n<\/ul>\n
\nObservations:<\/p>\n
\n- When we bum the candle under the chimney rested on the wooden blocks, the candle bums in the presence of air.<\/li>\n
- When we remove the blocks, the candle flame flickers and produces smoke in the absence of air.<\/li>\n
- When we put a glass plate on it, in the complete absence of air, the flame finally goes off.<\/li>\n<\/ul>\n
Conclusion: In the absence of air, the candle flame goes off. Hence, air is necessary for combustion.<\/p>\n
<\/p>\n
Activity 2<\/span><\/p>\nObjective: To show that a substance does not catch fire at a temperature below its ignition temperature.
\nMaterials Required: Candle (two), sheet of paper and water.
\nProcedure:<\/p>\n
\n- Take a sheet of paper and make two paper cups (100 mL) by folding the paper.<\/li>\n
- Place some water into one of the cups so as to fill it up to one-third of its height.<\/li>\n
- Hold both the cups separately over burning candle and observe.<\/li>\n<\/ul>\n
\nObservations:<\/p>\n
\n- When the cup is empty, it catches fire easily.<\/li>\n
- When the cup having water in it is heated, it does not catch fire and the water becomes hot and starts boiling.<\/li>\n<\/ul>\n
Conclusion: When we heat a paper bowl containing water, the water present in it absorbs the heat coming from the burning source and thus prevents the paper from reaching its ignition point. Hence, it does not bum.<\/p>\n
Contents of a Modern Matchstick:
\nMatchsticks are in use since ages. Modem matchsticks are made up of a mixture of antimony trisulphide and potassium chlorate with some glue and starch applied on the head of the matchstick. The mbbing surface has powdered glass and some red phosphorus. On striking match stick against the rough surface, red phosphorus gets converted into white phosphorus and it reacts with potassium chlorate to ignite antimony trisulphate, due to which combustion takes place.
\n<\/p>\n
Types of Combustion: There are following three types of combustion:
\ni. Rapid Combustion: When combustion happens at a faster rate, it is called rapid combustion. Petrol and gas show rapid combustion. For example, bring a burning matchstick or gas lighter near a gas stove in the kitchen. Turn on the l knob of the gas stove. We find that the gas burns rapidly and produces heat and light.<\/p>\n
ii. Spontaneous Combustion: When combustion starts on its own without an apparent cause, it is called spontaneous combustion. For example, many disastrous fires in coal mines result due to this kind of combustion. The heat rays coming from the sun or a lightning strike might be responsible for this kind of combustion.<\/p>\n
iii. Explosion: When combustion reaction is so sudden that it releases a large amount of heat, light and sound, it is called explosion. Fire-crackers explode because of this type of combustion.
\n<\/p>\n
Flame: Flame is the visible and gaseous part of fire. When vapour of a substance undergoes combustion, it results in the formation of flame. Only those substances which vapourise on burning produce flame. If a substance does not get vapourised, it does not produce flame on burning.<\/p>\n
Three Distinct Zones of a Candle Flame<\/p>\n
\n- Outer Zone: This zone is blue in colour. It is the hottest part of the flame because compLete combustion takes place in this zone.<\/li>\n
- Middle Zone: This zone is yellow or orange in colour. This zone is moderately hot because partial combustion takes place in this zone. It contains unburnt carbon particles.<\/li>\n
- Inner Zone: This zone is dark in colour. This is the least hot part of the flame because no combustion takes place in this zone. Here, we can find the unburnt wax vapours of a candle.<\/li>\n<\/ol>\n
<\/p>\n
<\/p>\n
Activity 3<\/span><\/p>\nObjective: To show the presence of unburnt carbon particles in the middle zone of the candle flame.
\nMaterials Required: A wax candle, matchbox, glass slide and a pair of tongs.
\nProcedure:<\/p>\n
\n- Light the candle and allow the flame to become steady.<\/li>\n
- Introduce a clean glass slide into the luminous region of the flame holding it with a pair of tongs.<\/li>\n
- Remove the glass slide after 3-4 minutes.<\/li>\n<\/ul>\n
\nObservation: A circular grey ring forms on the glass.
\nConclusion: The circular dark ring is formed on the slide by the deposition of unburnt carbon particles present in the luminous (middle) zone of the candle flame.<\/p>\n
Fuel: The substance that undergoes combustion is called a fuel. The fuel can be in the form of solid, liquid or gas.<\/p>\n