Ch 8: Combustion and Flame
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Define combustion
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Combustion is a chemical process in which a substance reacts with oxygen giving out heat and light.
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What are the conditions necessary for combustion?
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1. Presence of a combustible substance 2. Presence of a supporter of combustion. 3. Gaining of ignition temperature of the combustible substance
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Define combustible substances
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Substances which catch fire and burn are called combustible substances. Eg-paper, wood, coal
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Define noncombustible substances
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Noncombustible substances are those that do not burn in air. Eg-water, carbon dioxide, glass
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Define supporter of combustion
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The substance that helps in the process of combustion is called the *supporter of combustion*. Oxygen is a supporter of combustion.
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Define ignition temperature.
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Ignition temperature of a substance is the minimum temperature at which the substance catches fire.
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Define hydrocarbons
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compounds of hydrogen and carbon.
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What is liquid hydrogen used as?
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Liquid hydrogen is used as a rocket propeller as it releases a large amount of energy on burning.
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What are the types of combustion?
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Based upon the rate at which it takes place, combustion can be classified into four types-spontaneous, explosion, rapid or fast and slow combustion.
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Define spontaneous combustion
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In such a combustion, a substance with low ignition temperature catches fire on its own. eg-sodium and phosphorous
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Define explosion
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A reaction in which substance burns with the creation of heat, light and a large amount of gas is called explosion. eg-fire cracker
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Define rapid combustion
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Combustion in which the substance burns rapidly and the whole substance burns completely eg-petrol, kerosene and LPG
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Define slow combustion
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Combustion which takes place when some substances that have moderate ignition temperature burn slowly. eg- wood, coal and candle
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define fuels
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Substances that undergo combustion and produce energy in the form of heat. Ex-wood, coal, petroleum, kerosene, paraffin wax and natural gas
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Define complete combustion
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When the combustible material burns completely in the presence of sufficient amount of oxygen, it is said to be a complete combustion. Carbon forms carbon dioxide
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Define incomplete combustion
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Incomplete combustion takes place in an insufficient supply of oxygen. Carbon forms carbon monoxide.
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Define calorific value
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The quantity of heat produced by a unit mass of a fuel on complete combustion. It is expressed in kilojoules/gram (kJ/g).
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What are the qualities of a good fuel?
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A good fuel should 1. have a high calorific value 2. have a definite ignition temperature (well above the room temperature and not too high) 3. be least pollution 4. be easy and safe to handle 5. have low content of noncombustible substances and burn completely or leave no residue 6. be easy to transport 7. require less space for storage 8. be cheap and should be easily available
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Why is hydrogen not used as a domestic fuel?
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Hydrogen has the highest calorific value but still is not used as a domestic fuel as it is highly combustible and explodes. Safe storage of hydrogen is also difficult.
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What is the calorific value of: wood coal kerosene petrol methane hydrogen
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wood: 17 coal: 25-33 kerosene: 48 petrol: 50 methane: 55 hydrogen: 150
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What are the disadvantages of solid fuels like coal and wood?
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1. These fuels require a large storage space and a big means of transport to carry. 2. They have a low calorific value and take longer to burn completely. 3. Some varieties of coal do not burn completely and release a lot of smoke. 4. Burning of coal also leaves lot of residue. 5. Acid rains, global warming and pollution are some of the harmful effects of burning these fuels.
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What are the advantages of liquid fuels and gaseous fuels?
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1. They can be easily stored in containers and are easily transportable. 2. They leave no residue due to complete combustion and are less polluting.
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What is flame?
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The region of burning of the fuel where combustion takes place is called flame.
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What gives flame?
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The substances which vaporise during burning gives flames.
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What are the zones of a flame?
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1. blue zone 2. Innermost/darker zone 3. Luminous zone 4. Outer Non-luminous zone
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What is the difference between the zones of flame?
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The difference in the zones is mainly due to the differences in the range of combustion taking place in a particular region.
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Explain dark zone
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Dark zone is the innermost zone where no combustion takes place. It is surrounded by the vapours of the combustible substance. This makes it look dark and it is the coldest zone.
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Explain luminous zone
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This zone is the zone of partial or incomplete combustion. Here the supply of oxygen reduces and the carbon particles leave the flame as smoke and soot. There is a deposition of unburnt carbon particles in this zone.
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Explain non luminous zone
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The thin outermost zone is faintly bluish in colour. This zone is also called the zone of combustion. In a good supply of oxygen, complete combustion of the substance takes place to produce carbon dioxide gas. This zone is non luminous but the hottest zone of the flame.
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Explain blue zone
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At the bottom is the blue zone. It is due to the complete combustion of carbon particles.
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What is matchstick made of?
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The tip of a matchstick is dipped into a paste made up of antimony trisulphide (combustible substance), white phosphorus (kindling material), potassium chlorate (oxidising agent) and glass powder.
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Why is sodium kept under kerosene oil?
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Since, sodium has a very low ignition temperature and if it is left in the open, it combines with the atmospheric oxygen and burn. Also, sodium cannot be kept in water, as it is highly reactive and chemically combines with water releasing hydrogen.
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Paper is a combustible substance but not a fuel. Justify
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Paper is a combustible substance but not a fuel because fuels are substances that produce light and heat on burning and release energy but paper only burns and release no energy.
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Compare LPG and wood as domestic fuels
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*LPG* 1. LPG has a low ignition temperature, so it burns easily. 2. Has a high calorific value. 3. Can be stored easily *Wood* 1. Wood has a high ignition temperature, so it takes time to burn. 2. Has a low calorific value 3. Requires a lot of space
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It's difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
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It's difficult to burn a heap of green leaves because the green leaves contain high amount of water vapour/moisture and it takes time to evaporise the water vapour hence its difficult to burn green leaves. Whereas dry leaves have a low ignition temperature and they catch fire easily.
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Can the process of rusting be called combustion?
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No, the process of rusting can't be called combustion because rusting doesn't produce heat and light, it only produces reddish flaky substance. Rusting is simply called oxidation reaction.
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Why is carbon dioxide used to extinguish electrical fire?
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We can't extinguish electrical fire by water as water is a good conductor of electricity and if used it might give shocks. That is why carbon dioxide is used to extinguish electrical fires as carbon dioxide acts a blanket around the fire, hence extinguishing it since it cuts off the supply oxygen.
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Can fire caused by petrol be extinguished with water. Give reason.
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No, fire caused by petrol can't be extinguished with water because oil is lighter than water and it will float on its surface. Hence fire caused by petrol can't be extinguished with water because it is less dense than water.
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Can you burn a candle inside a closed jar? Give two reasons.
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No, we can't burn a candle inside a closed jar. The two reasons for it are: 1. Supporter of combustion (oxygen) is absent. 2.
