NCERT solution for class 8 //Science //Chapter 6 -- Combustion and flame

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NCERT solution for class 8  //Science //Chapter 6 --  Combustion and flame


Q1. List conditions under which combustion can take place 

Answer - Combustion can takes place under following conditions 

  •  Presence of goods fuel 
  • Presence of more oxygen in air 
  • Ignition temperature 

Q2. Fill in the blanks.

  a) Burning of wood and coal causes  pollution of air pollution 

  b) A liquid fuel used in homes is LPG  ( Petroleum gas) 

  c) Fuel must be heated to its ignition temperature before it starts burning 

   d) Fire produced by oil cannot be controlled by 

Water

Q3. Explain how the use of CNG in automobiles has reduced pollution in our cities .

Answer - CNG is a cleaner fuel it does not causes any harmful product but fossil fuel like a petroleum causes the incomplete carbon particles along with carbon monoxide gas and it increases the air pollution. It May cause the respiratory diseases therefore the use of CNG reduce the pollution in our cities

Q4. Compare LPG and wood as fuels 

Answer - wood 

  • It is a traditional fuel use for both domestic as well as industrial purpose 
  • Its ignition temperature is less than LPG and it may cause the lot of smoke which cause air pollution 
  • More uses of wood for fuel causes deforestation 
  • The calorific value of wood is between 17,000 to 22000 kJ/ kg

LPG 

  • LPG is a clean fuel as compared to wood 
  • It has more ignition temperature and wood 
  • It does not cause any smoke and not release any harmful product in atmosphere 
  • Calorific value of LPG is 55000 kJ/ kg

Q5. 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 . 

Answer -  

a) Water is good conductor of electricity so if we use it to control the fire on electric equipment  it may damage the electric equipment and also harmful for the person who operating it 

b) calorific value of LPG is high and it does create pollution also not prepared any residue therefore it is a good domestic fuel 

c) Ignition temperature of paper is less so it catches fire easily but when it around the aluminium pipe the heat absorbed by aluminium pipe and paper does not reach upto ignition temperature so  it does not catches fire easily

Q6. Make a labelled diagram of a candle flame ?

Answer- 


Q7. Name the unit in which the calorific value of a fuel is expressed? 

Answer - the calorific value of a fuel is expressed in kilo joule per kilogram ( kj/ kg)

Q8. Explain how CO2 is able to control fires ? 

Answer - CO2 is non combustible gas and it is heavier than oxygen it covers the fire like a blanket and it break down the contact between the oxygen and fuel . So the intensity of the fire decreases due to decrease of O2 in atmosphere

Q9 . It is difficult to burn a heap of green Leaves but dry leaves catch fire easily explain ? 

Answer - Green Leaves contain moisture and these moisture does not allowed to  them to catch fire easily but dry leaves have no moisture in them therefore it catches fire easily

Q10 . Which zone of the flame does a goldsmith use for melting gold and silver and why ? 

Answer - Goldsmith generally use the outer part of the flame because the outermost part is the hottest part of the flame and it undergoes complete combustion process

Q11. In an experiment 4.5 kg of a fuel was completely burnt the heat produced was measured to be 18O,000 kg calculate the calorific value of the fuel .

Answer - 

The calorific value of fuel is the amount of heat produced by the complete combustion of 1kg of fuel

Therefore , 

Heat produced by 4.5 kg of fuel = 180000 kJ 

Heat produced by 1 kg of fuel= 180000 /4.5 x 1 

            = 40000kJ/ kg

Calorific value of the fuel is 40000 kJ/ kg

Q12. Can the process of rusting be called combustion discuss ? 

Answer - Yes the rusting process may be called combustion but it is the slow combustion because combustion is nothing but the substance react with oxygen and give out the energy in the form of light and heat and during the process of rusting of iron the heat is produced so it is exothermic reaction

Q13. Abida and Rameshwar 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 - Ramesh water will heat up early and in a shorter time because the outermost zone of the flame is the hottest zone while the yellow zone is  less hot

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