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NCERT Class 8 Science Chapter 6 Combustion and Flame
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Combustion and Flame
Chapter: 6
NCERT TEXT BOOK EXERCISES
Q. 1. List the conditions under which combustion can take place.
Ans. Conditions for the combustion:
1. The presence of a combustible substance.
2. The presence of a supporter of combustion.
3. Initial heating to bring the combustible substance to its ignition temperature.
Q. 2. Fill in the blanks:
(a) Burning of wood and coal causes _________of air.
Ans. Pollution.
(b) A liquid fuel, used in homes is __________.
Ans. Kerosene.
(c) Fuel must be heated to its ___________ before it starts burning.
Ans. Ignition temperature.
(d) Fire produced by oil cannot be controlled by __________.
Ans. Water.
Q. 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans. The use of CNG in automobiles has reduced air pollution in our cities in following ways:
(i) It produces less amount of carbon monoxide gas.
(ii) It produces less amount of carbon dioxide gas.
(iii) It produces less amount of sulphur dioxide and nitrogen dioxide which cause acid rain.
(iv) No residue remains after combustion.
Q. 4. Compare LPG and wood as fuels.
Ans.
| LPG | Wood |
| 1. It has more calorific value i.e., 55000 kJ/kg. | 1. It has less calorific value i.e., 17000 to 22000. |
| 2. It is smoke free fuel. | 2. It gives out a lot of smoke which is quite dangerous. |
| 3. It is transport | 3. It is difficult to transport wood |
| 4. It is easily stored in cylinders. | 4. It is difficult to store as it needs space to store. |
| 5. It does not cause any environmental problems. | 5. It is cut so lead to deforestation thus gives rise to many natural and environmental problems. |
Q. 5. Give reasons:
(a) Water is not used to control fires involving electrical equipment.
Ans. This is because of the following reason ordinary water conducts electricity to some extent. So, when water is thrown over the burning electrical appliance (or burning electric wires), it can give electric shock to the persons involved in fire-fighting. Thus, water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
Ans. LPG has more calorific value and produces no pollution, So, it is 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. The ignition temperature of paper is less, so it catches fire easily. It does not catch fire when wrapped around aluminium pipe because aluminium absorbs the heat, so paper does not attain its ignition temperature.
Q. 6. Make a labeled diagram of candle flame.
Ans.
Q. 7. Name the unit in which the calorific value of a fuel is expressed.
Ans. Kilo joules per kg (kJ/kg)
Q. 8. Explain how CO is able to control fires.
Ans. (i) CO2 forms a blanket around fire due to which supply of air is stopped.
(ii) CO2 also brings down the temperature of the fuel.
Q. 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans. The green leaves contain some water dew with which the ignition temperature of leaves increase and they do not catch fire easily while dry leaves have no water, so they catch fire easily.
Q. 10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. A goldsmith uses the outer zone (non. luminous zone) of candle flame to melt gold and silver because it is the hottest zone and has more temperature.
Q. 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. Total mass of fuel = 4.5 kg
Total heat produced = 180,000 kJ
Heat produced by burning 1 kg of fuel
= 180,000 kJ/4.5 kg
= 40,000 kJ/kg
Hence, calorific value of fuel
= 40,000 kJ/kg.
Q. 12. Can the process of rusting be called combustion. Discuss.
Ans. The process of rusting cannot be called combustion because in this process no heat and light is produced. Due to this reason, rusting is not a case of combustion.
Q. 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 heated by Ramesh will get heated in a shorter time because he kept beaker nearer the hottest zone of the flame.
