NCERT CLASS 8 SCIENCE CHAPTER 6 COMBUSTION AND FLAME

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  NCERT CLASS 8 SCIENCE CHAPTER 6
  COMBUSTION AND FLAME 

QUESTION NO 1.  

   List conditions under which combustion can take place.

ANSWER -

   Followings are the conditions under which combustion can take place -
  1.  There should be presence of Oxygen.
  2.  There should be a presence of inflammable substance.
  3.  Requirement of the ignition Temperature. 

QUESTION NO 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 cannot be controlled by ----------------------.

ANSWER 

(a) pollution
(b)kerosene
(c) ignition temperature
(d) water

QUESTION NO 3

Explain how the use of CNG in automobiles has reduced pollution in our cities.

ANSWER

    CNG is a cleaner fuel compared to petrol and diesel. It produces lesser amount of harmful gases while burning. Because of this advantage over the other fuels it is now being used in automobiles in many cities. Use of CNG has helped in reducing pollution in our cities.

QUESTION NO 4

    Compare LPG and wood as a fuel.

ANSWER

WOOD                              LPG

Wood has traditionally been          LPC  fuel burns without
used as fuel and is still being         giving smoke. While burning
used in rural areas. Burning         it produces lesser amount of air  
of wood creates air pollutants      pollutants. Complete oxidation 
which results respiratory              during burning of LPG does not 
problems. In addition                      lead to carbon monoxide production.
incomplete oxidation during         Hence it is safe compare to wood as 
burning of wood creates carbon   a fuel.
monoxide which is a poisonous
gas.

QUESTION NO 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 a aluminium pipe does not.

ANSWER 

(a ) Water is a good conductor of electricity. If water will be used to douse th nue fire of electrical equipment can result in electrical shock. By using of water in electrical equipment it may cause to damage the equipments also. Hence water must not be used to douse the fire in electrical equipments.

(b) LPG is a better domestic fuel than wood because of having several reasons. Unlike wood,LPG burns without smoke. Burning without smoke curb the respiratory disease caused by fuels. whereas wood burns with smoke and harmful gas so there is chance of respiratory diseases due to continuous use. Since it gives a clear flame the pots and pans does not become black. Storage and transportation of LPG is easier compared to wood. 

(c) kg)The ignition temperature of the paper is less so it catches fire easily. It does not catch fire when wrapped around aluminium pipe because aluminium absorb the heat,so paper goes not attain ignition temperature.

QUESTION NO 6

    Make a labelled diagram of a candle flame.



QUESTION NO 7

  Name the unit in which the calorific value of a fuel is expressed.

ANSWER

  Kilojoules per kg  ( kJ/kg)

QUESTION 8

   Explain how CO2   is able to control fire.

ANSWER 

  1.    CO2   forms a blanket  around fire due to which supply of air is cut off and fire can be controlled.
  2.     CO2  brings down the temperature of the fuel so  it helps in control the fire. 

QUESTION NO 9

     It is difficult to burn  a heap of green leaves but dry leaves catch fire easily. Explain.

ANSWER     

    The green leaves contain some water because of this the ignition temperature of leaves increase. Hence they do not catch fire easily while dry leaves have no water and their ignition temperature is low so it catch fire easily.

QUESTION NO 10      

          Which zone of a flame does a goldsmith use for melting gold and silver and why?           

ANSWER 

    A goldsmith uses the outer zone of the flame to melt gold and silver because it is hottest zone and has more temperature.

QUESTION NO 11

   In an experiment 4.5 kg of fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

ANSWER

   According to the definition calorific value is the amount of heat energy produced on complete combustion of 1 kg of fuel.

Total mass of the fuel = 4.5 kg
Total heat produced    =  180,00 kJ
Heat produced by burning 1 kg of fuel =  180,000/4.5 kg
                                                                      =   40,000 kJ/ kg

So calorific value of fuel   =  40,000 kJ /kg.

QUESTION NO 12

     Can the process of rusting be called combustion? Discuss.

ANSWER

    The process of rusting there is  no heat and light is produced. where as in the process of combustion heat and light produced. Hence the process of rusting cannot be called combustion.

QUESTION NO 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.

ANSWER 

   The outermost zone of the flame is the hottest and has more heat. Ramesh kept the beaker in the outermost part of the flame so his beaker will be heated in shorter time.



    

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