CLASS 8 SCIENCE CHAPTER-6 COMBUSTION AND FLAME

Question 1. List conditions under which combustion can take place.
Solution :
The burning of a substance in the presence of oxygen is defined as combustion.
The conditions under which combustion can take place are
*The presence of fuel plays a significant role.
*The presence of air or oxygen.
*Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)

Question 2. Fill the blank with the appropriate answer.
1.The burning of wood and coal causes a serious problem leading to _________
2.The liquid fuel used for cooking in homes is called as _______
3.Before the substance starts burning, the fuel must be heated to its_____
4.The fire which is produced by oil cannot be controlled by ____

Solution :
1.Pollution
2.LPG(Liquefied petroleum gas)
3.Ignition temperature
4.Water

Question 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Solution : The use of CNG in automobiles has reduced pollution in our cities because CNG does not produce any poisonous gas on burning. That is why pollution in our cities is reduced.

Question 4. Compare LPG and wood as fuels.
Solution : LPG burns easily and produces more heat in comparison to wood. Besides, it is a clean fuel, it does not produce fume and ashes as wood do. LPG can be stored and transported easily and conveniently.

Wood:-
     1.It is considered as a traditional fuel used for both domestic and industrial purposes.
     2.Wood produces a lot of smoke which pollutes the atmosphere as well as cause respiratory diseases.
     3.The usage of wood to a large extent causes deforestation.
     4.The calorific value of wood ranges between 17000 to 22000 kJ/kg
     5.However, wood may be used as a furnace, stove or fireplace in indoors while it is used for a campfire, furnace at outdoors.

LPG:-
     1.The usage LPG (Liquefied petroleum gas) has replaced wood.
     2.It doesn’t release smoke and other pollutants
     3.It is a cleaner fuel
     4.The fuel efficiency of LPG is more than that of wood.
     5.The calorific value of LPG is 55000 kJ/kg
     6.Hence LPG is mostly preferred choice

Question 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.
Solution :
(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.

Question 6.  Make a labelled diagram of a candle flame.

Solution :

ncert solution

Question 7. How do we express the unit of calorific value of a fuel?
Solution :
Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg
kJ-kilo joules
kg- kilogram

Question 8. Why is it difficult to burn a heap of green leaves, whereas dry leaves catch fire easily?
Solution : A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore it is difficult to catch fire.
But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore it catches fire easily.

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

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

Question 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.
Solution : 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

Question 12. Can the process of rusting be called combustion? Discuss.
Solution : 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.

Question 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?
Solution : 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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