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Combustion And Flame - Class 8 : NCERT Exercise Questions


Q.1 List conditions under which combustion can take place.
Sol. The conditions necessary for combustion to take place are as follows:
(i) Fuel or Combustible substance
(ii) Air (With presence of Oxygen in it)
(iii) Temperature above the Ignition temperature

Q.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 __________.
Sol. (a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is Liquified Petroleum Gas (LPG).
(c) Fuel must be heated to its Ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by water.

Q.3 Explain how the use of CNG in automobiles has reduced pollution in our cities.
Sol. CNG produces the harmful products in very small amounts. And it is a cleaner fuel. Because of these reasons the use of CNG in automobiles has reduced pollution in our cities.

Q.4 Compare LPG and wood as fuels.
Sol. The comparison between LPG and wood is as follows:

Parameters

LPG Wood

Pollution

Less

More

Residue

No residue

Ash as residue

Fuel efficiency More

Less

Calorific Value

55000 kJ/kg

17000 to 22000 kJ/kg

Storage & Transport Facilities Good

Bad

 Q.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.
Sol. (a) Water is not used to control fires involving electrical equipment because it may conduct electricity and harm those trying to douse the fire.
(b) LPG is a better domestic fuel than wood because burning of wood gives a lot of smoke which is harmful for human beings which causes respiratory problems. Also, trees provide us with useful substances which are lost when wood is used as fuel. On the other hand, LPG burns easily in air at a moderate rate and it produces a large amount of heat. It does not leave behind any undesirable substances.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not because wrapping paper around an aluminium pipe increases its Ignition temperature.

Q.6 Make a labelled diagram of a candle flame.
Sol. 

Q.7 Name the unit in which the calorific value of a fuel is expressed.
Sol. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Q.8 Explain how CO2 is able to control fires.
Sol. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled.

Q.9 It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Sol. The ignition temperature of dry leaves is lower than that of green leaves, hence, dry leaves catch fire easily.

Q.10 Which zone of a flame does a goldsmith use for melting gold and silver and why?
Sol. Goldsmith use outermost zone of flame for melting gold and silver because it is the hottest part of the flame.

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.
Sol. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
Calorific Value = Heat produced/Mass of fuel
                         = 180,000/4.5
                         = 40,000 kJ/kg

Q.12 Can the process of rusting be called combustion? Discuss.
Sol. No, the process of rusting cannot be called combustion because combustion is a fast process while rusting is a slow process. Moreover, combustion needs ignition temperature to take place while rusting can take place at room temperature.

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?
Sol. Ramesh’s water will get heated in a shorter time as the outermost part of the flame is the hottest one.



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