Exercise
(1) List conditions under which combustion can take place.
Ans- Combustion can take place in the presence of
(i) Fuel
(ii) Oxygen
(iii) Heat to raise the temperature beyond ignition temperature.
(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 can not be controlled by _____.
Ans-
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is LPG.
(c) Fuel must be heated to its ignition temperature before it starts burning.
(d) Fire produced by oil can not be controlled by water.
(3) Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans– CNG is a cleaner fuel. It produced harmful products in very small amounts.
Therefore, the use of CNG in automobiles has reduced pollution in our cities.
(4)Compare LPG and wood as fuels.
Ans-
Comparison of LPG and wood as fuels are tabulated in the table below:-
| SN | Wood | Fuel |
| 1. | It is found in solid state. | It is found in liquid state. |
| 2. | We get it from forests. | We get it by refining petroleum. |
| 3. | Wood is used at home in small pieces. | It is used in cylinders at home.
|
| 4. | Burning of wood produces carbon -di oxide which is harmful for humans. | It is less harmful. |
| 5. | It has high ignition temperature. | It has low ignition temperature. |
| 6. | It has low calorific values about 17000 – 22000 kj/kg. | It has high calorific values about 55000 kj/kg. |
(5) Give reasons.
(a) Water is not used to control fire 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) Water is the most effective extinguisher but it does not work when electrical goods are on fire. Water may conduct electricity and harm fire brigade persons trying to extinguish fire.
(b) LPG is better fuel than wood because it does not produce smoke and carbon unburnt particles which are not good for our respiratory system.
(c) Paper catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not catches fire. It happens because paper transferred the heat to aluminium and igneous temperature of paper does not reach. Thus, paper does not burn.
(6) Make a labeled diagram of a candle flame.
(7) Name the unit in which the calorific value of a fuel is expressed.
Ans– The calorific value of a fuel is expressed in a unit called kilojoule per kg (kj/kg).
(8) Explain how CO2 is able to control fires.
Ans– CO2 gas is the best option for the fire by electrical equipment. Being of heavier than the oxygen, carbon –di – oxide covers the fire and cut the supply of oxygen which is very necessary for fire. Therefore, fire gets controlled. We can get Co2 immediately from the fire extinguisher cylinder in which gas is stored at high pressure in liquid form. It spreads and cools down immediately after it is released. Therefore, it is the best to use fire extinguisher because it cut off the oxygen and cools the temperature of fuel.
(9) It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans- Dry leaves have low ignition temperature than the green leaves which have moisture in it. Thus, green leaves take time to burn. Therefore, it is difficult to burn a heap of green leaves but dry leaves catch fire easily.
(10) Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans- Goldsmith blows the outer most zone of flame to melt gold and silver because combustion of unburnt fuel takes place (full combustion) and outermost part has highest 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-
Weight of the fuel = 4.5 kg
Heat produced = 180,000 kj
Calorific value =
Calorific value =
= 40,000 kj/kg
Hence, the calorific value of the fuel is 40,000 kj/kg.
(12) Can the process of rusting be called combustion? Discuss.
Ans– No, rusting is not combustion because heat and light is produced in the process of combustion. In both processes oxygen is necessary but both are different process.
(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- Ramesh’s beaker will get heated in the shorter time because outer zone of the flame is the hottest zone of the flame. Thus, it will be hot easily and early than Abida ‘s beaker because she is heating on dark zone which is the coolest zone of the flame.
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