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NCERT Answers of Class 8 Science: Chapter 6, Combustion and Flame


Class 8 Cbse Ncert Science

Chapter 6, Combustion and Flame

Terms & Definitions
Calorific Value The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its Calorific value, expressed in terms of kJ/kg.  
Ignition Temperature It is the lowest temperature at which a combustible substance can easily catch fire. Inflammable substances have very low ignition temperature.

NCERT Answers of Textbook Exercise Questions (Important Only)

Question.1: List the conditions under which combustion can take place.
Answer: Following conditions are necessary for combustion -
     (i)      The substance has to be combustible
    (ii)      It should have low ignition temperature
  (iii)      There should be adequate supply of air.

Question.2: Fill in the blanks -
      a)     Burning of wood and coal causes ______ of air.
      b)     A liquid fuel, used in home is _______.
      c)     Fuel must be heated to its ________ before it starts burning.
      d)     Fire produced by oil can not be controlled by _________.
Answer: (a) pollution (b) kerosene (c) ignition temperature (d) water.

Question.3: Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer: CNG has replaced patrol and diesel as fuel in automobiles because these produce a lot of un-burnt carbon particles and emit carbon dioxide and nitrogen oxides and sulphur dioxide. These all gases are poisonous gases and cause various environmental hazards. But CNG is safe because it produces these materials in very small amounts thus reducing pollution in cities. Hence it becomes necessary to implement CNG in place of other fuels in automobiles to reduce the pollution. 

Question.4: Compare LPG and wood fuels.
·     It has more calorific value about 55000 kJ/kg
·     It is smokeless fuel.
·     Easy to transport.
·     Easily stored in cylinders.
·     Does not cause any environmental problem.
·     Have very low ignition temperature.
·     It has less calorific value about 17000 to 22000 kJ/kg
·     Gives out lot of smoke which is quite dangerous.
·     Difficult to transport.
·     Needs a lot of space to store.
·     To get it we need to trees to cut leading to deforestation which turn give rise to many natural and environmental problems.
·     Ignition temperature is not low.

Question.5: Give reasons:
a.     Water is not used to control the fire involving electrical equipment.
b.     LPG is better domestic fuel than wood.
c.      Paper by itself catches fire whereas a piece of paper wrapped around an aluminium pipe does not.
     a) Water is a conductor of electricity, so it can easily conduct electric current and cause danger of electric shocks or short-circuits. Therefore water cannot be used to control the fire involving electrical equipment.
     b) There are a number of reasons because of which we can say LPG is definitely a better domestic fuel than wood. The reasons are - [Refer to the answer of Q.4 above].
     c) Paper by itself catches fire easily because it has low ignition temperature but when wrapped around an aluminium pipe its temperature is lowered due to aluminium metal absorbing the heat supplied to paper. So it does not catch fire.   

Question.6: Make a labeled diagram of a candle flame.
Answer: Already given in Chapter 6, NCERT Science textbook.

Question.9: It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer: Green leaves have moisture in them, which absorbs heat from them and raises their ignition temperature which does not allow them to catch fire easily. On the other hand dry leaves have low ignition temperature and so, burn easily.

Question.10: Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer: A goldsmith uses the outer part of the candle flame for melting gold and silver because in this zone the temperature is the highest which helps to melt these metals easily.

Question.11: In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180000 kJ. Calculate the calorific value of the fuel.
Answer: Heat produced after burning 4.5 kg fuel = 180000 kJ
Or, Heat produced after burning 1 kg fuel = (180000 ÷ 4.5) kJ = 40000kJ
So, Calorific value of fuel = 40000 kJ/kg

(Please be in touch of this site CBSE Guide NCERT Solutions : Soon we will be posting additional important CBSE Sample Questions on this Chapter, Combustion and Flame) 

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