1. Combustion-
ü Combustion is a chemical process in which a substance reacts with oxygen and releases heat in the process.
ü Combustible substances are those substances that undergo combustion readily.
ü It means that these substances releases heat and sometimes light when they react with oxygen.
ü Flammable substances are substances which have low ignition temperature and catch fire easily.
ü They burn with a flame. For example, petrol, LPG etc.
ü Conditions necessary for the combustion to take place are-
n Combustion requires fuel.
n Combustion only takes place when there is a fuel which produces heat and light when it catches fire. For example, petrol and diesel.
n Combustion requires air.
Note: Combustion requires air - A candle keeps burning in open air. But when we cover it with a glass or a jar, it only burns until the oxygen inside it is consumed. Once the oxygen is exhausted, the flame of the candle flickers off.
Combustion requires heat- The substance must reach its ignition temperature to catch fire. Ignition temperature is the lowest temperature at which a substance catches fire or starts burning.
Table 01- Substances and their burning temperature
2. Types Of Combustion-
ü Explosion- When a material bursts suddenly to produce heat, light and sound on the application of heat or pressure, it is called an explosion. E.g. Crackers and fireworks which release a large amount of gas too.
ü Spontaneous Combustion- Combustion in which a material bursts into flames suddenly without applying heat. E.g. Phosphorus which burns at room temperature. Spontaneous combustion of coal dust often causes accidental fires in coal mines. Heat from the sun or lighting may also cause spontaneous forest fires.
ü Rapid Combustion- Combustion in which a gas burns quickly producing heat and light in the process. E.g. LPG
3. Control Of Fire-
ü We can control the fire by- removing the fuel; cutting off the air supply (or oxygen supply); cutting off heat or lowering the temperature of the fuel.
ü Fire Extinguishers- Fire extinguishers are devices used to put out fires. They either cut off the air supply to fire or cool off the fuel or both.
n The three main types of fire extinguishers are-
n Dry Powder Based Fire Extinguisher- This type of fire extinguishers contain a mixture of baking soda (sodium bicarbonate) and sand. When you throw it over the fire, the baking soda decomposes by its heat to produce carbon dioxide. Since CO2 is heavier than air, it descends to envelop the burning flame and cuts off its contact with air (and the oxygen supply).
2NaHCO3 → Na2CO3 + H2O + CO2
n Soda-acid Based Fire Extinguisher- This fire extinguisher is a metallic cylinder which contains the sodium bicarbonate (NaHCO3) solution. At the bottom of the cylinder, the concentrated sulphuric acid (H2SO4) is placed in a thin sealed glass tube. A fixed wire gauze surrounds this tube.
n Foam Based Fire Extinguisher- Like the soda-acid fire extinguisher, it uses sodium bicarbonate solution. However, in this case, a substance called Siporin or Turkey Red Oil is added to the solution to produce foam along with the gas from the nozzle. Since this foam is lighter than oil, it floats on the surface of the oil and cuts off its air supply. Hence, it is very effective in putting out oil fires.
Q. When should we use or not use water to extinguish the fire?
Ans.- When wood, paper and clothes are on fire, we can use water to extinguish them. Water lowers the temperature of burning material below ignition temperature and thus, the fire stops burning. We should not use water when electrical equipment is on fire as water may conduct electricity and give a shock to people dousing the fire. Also, it should not be used when oil or petrol catches fire as water is lighter than oil and petrol and sinks down. Oil and petrol keep floating on the top and keep burning.
Q. What should we do when electrical equipment or inflammable materials (like petrol) catch fire?
Ans.- Carbon Dioxide is the best fire extinguisher in such cases. CO2 is heavier than oxygen and hence, covers the burning material like a blanket and cuts off its oxygen supply. Also, it does not harm the electrical equipment. CO2 can be stored as a liquid in cylinders at high pressure. When it is released, it immediately expands, cools down, and envelopes the fire - bringing down the temperature of the fuel. One can also pour dry chemicals like sodium bicarbonate (or baking soda) or potassium carbonate on the fire as they release CO2 near a fire.
4. Flame-
ü Flame is a hot glowing body of ignited gas which is produced when something is on fire.
ü Some materials burn with a flame and some do not.
Material | Forms flame | Does not form a flame |
Candle | Forms flame | |
Magnesium | | Does not form a flame |
Camphor | Forms flame | |
Kerosene Stove | Forms flame | |
Charcoal | Forms flame | |
Table 02- Materials forming flame on burning
Figure 01- Structure of a flame
ü Outermost zone is blue in color and is the hottest part of the flame. This is also the zone where complete combustion takes place.
ü Middle zone is yellow in color and is moderately hot. In this zone, partial combustion takes place.
ü Innermost zone is black in color and least hot. Here, we can find the un-burnt wax vapors of a candle.
5. Fuel-
ü Fuels are substances that give us heat which we use for domestic and industrial purposes, such as wood, kerosene, and petrol.
ü A good fuel is which- has proper ignition temperature, does not produce undesirable or poisonous substances and cause pollution, does not leave behind much ash, is cheap, is readily available, produces a large amount of heat or have high calorific value, has a moderate rate of consumption, is easily controllable, is easy to handle and transport, and has low moisture content.
6. Fuel Efficiency-
ü Fuel efficiency of a fuel depends on its calorific value.
ü The calorific value of a fuel depends on the amount of heat produced by complete combustion of 1 kg of the fuel. The unit used to measure the calorific value of a fuel is kilojoule per kg (kJ/kg).
Table 03- Fuels We Use
7. Some Important Points-
ü Acid Rain- When the pollutants like sulphur dioxide and nitrogen oxides dissolve in rainwater, it forms an acid. The rain of that acid is called acid rain.
ü Calorific Value- The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. It is expressed in a unit called kilojoule per kg (kJ/kg).
ü Combustion- A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
ü Deforestation- It is the process of cutting of trees on a large scale.
ü Explosion- The process of combustion in which a large number of gases are evolved with the production of a tremendous amount of heat, light and sound, is called an explosion. The flame is produced when the combustible substance vaporizes during burning.
ü Fire Extinguisher- Fire extinguisher is used to control the fire. The job of a fire extinguisher is to cut off the supply of air or to bring down the temperature of the fuel, or both.
ü Fuels- A fuel is a substance, which may be burnt to produce considerable heat without the formation of undesirable products.
ü Fuel Efficiency- Fuel efficiency is expressed in terms of its calorific value which is the amount of heat energy produced on complete combustion of 1 kg of fuel.
ü Global Warming- It is the rise in temperature of the atmosphere of the earth due to the combustion of fuels.
ü Ideal Fuel- The fuel, which fulfils all the requirement for a particular use is called an ideal fuel.
ü Ignition Temperature- The lowest temperature at which a substance catches fire is called its ignition temperature.
ü Inflammable Substance- The substances, which have very low ignition temperature and can easily catch fire with a flame are called Inflammable substances.
NCERT PRACTICE QUESTIONS
1. List conditions under which combustion can take place.
Ans.- Combustion can take place in the presence of:
(a) a combustible substance. (b) oxygen, that is, the supporter of combustion. (c) attainment of ignition temperature of the substance.
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 ______.
Ans.- (a) pollution (b) LPG (c) ignition temperature (d) water
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans.- The use of CNG in automobiles has reduced pollution in our cities as it is a quality fuel and has some benefits- (a) It gives out less carbon dioxide gas, carbon monoxide gas, sulphur dioxide and nitrogen dioxide, which is beneficial as they play crucial role in global warming and acid rain. (b) It leaves behind no residue after its combustion.
LPG | Wood |
(i) It does not cause pollution on combustion. | (i) It pollutes air on its combustion. |
(ii) No smoke is produced. | (ii) It produces smoke. |
(iii) It is a liquid fuel. | (iii) It is a solid fuel. |
(iv) It has more calorific value (55000 kJ/kg). | (iv) It has less calorific value (17000 kJ/kg). |
(v) It can be easily transported, as it is stored in cylinders. | (v) It can’t be transported easily like LPG 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.
Ans.- (a) Since water is a good conductor of electricity, it may result in electric shocks to the person trying to extinguish fire.
(b) LPG is better domestic fuel than wood because it does not produce gases, nor does it leave any residue behind. Moreover, it has more calorific value than wood.
(c) As its ignition temperature is low, the paper by itself catches fire easily. But a piece of paper wrapped around an aluminium pipe does not catch fire easily, as the heat being given gets absorbed by the aluminium pipe and the piece of paper does not get its ignition temperature.
6. Make a labelled diagram of a candle flame.
7. Name the unit in which the calorific value of a fuel is expressed.
Ans.- The unit in which the calorific value of a fuel is expressed is kilojoules per kilogram (kJ/kg).
8. Explain how CO2 is able to control fires.
Ans.- As CO2 is heavier than oxygen, it forms a blanket around fire, because of which the supply of air is stopped. Men over, it brings down the temperature of the burning substance. In these ways, it plays a significant role in controlling fire.
9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans.-The green leaves hold some amount of water, so its ignition temperature gets increased and it does not burn easily. On the other hand, dry leaves are waterless, so they catch fire easily.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans.A goldsmith uses the outermost zone of a flame, which is non-luminous, to melt gold and silver as it is the hottest zone of the flame, having more temperature.
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.
Ans.- Calorific value of a fuel = Heat Produced Amount to fuel
= 1800004.5 kJ/kg
= 40,000 kJ/kg.
12. Can the process of rusting be called combustion? Discuss.
Ans.The process of rusting emits heat during the formation of its oxide. So we can call the process of rusting as slow combustion.
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?
Ans.- The water which was put by Ramesh will get heated in a shorter time; because he had put it nearer to the hottest zone of the flame.
Junior Science Topic - Combustion And Flame. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 06)
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