CLASS 8 Chapter 6 Combustion and Flame

Question 1. List conditions under which combustion can take place.

Solution 1. Combustion can take place under the following conditions.

  1. In presence of oxygen
  2. In presence of a combustible substance
  • Acquisition of ignition temperature

Question 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 ______

Solution 2. (a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is LPG

(c) Fuel must be heated to its ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by water.

 

Question 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Solution 3. The use of CNG in automobiles has reduced pollution in our cities because of the following benefits of CNG:

  1. After the combustion of CNG no residue is left behind.
  2. Very less amounts of harmful gases such as carbon dioxide, carbon monoxide, Sulphur dioxide, and nitrogen dioxide are produced by using CNG.

Question 4. Compare LPG and wood as fuels.

Solution 4.

LPG Wood
·       It does not pollute the environment after combustion. ·       It pollutes the environment after combustion.
·       It does not produce smoke. ·       It produces smoke.
·       It is in liquid state. ·       It is in solid state
·       The calorific value is high (55000KJ/kg) ·       The calorific value is less (17000KJ/Kg)
·       Transportation of this fuel is easy. ·       Transportation of this fuel is not easy.

 

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 5
. (a) water is a good conductor of electricity and it may harm a person trying to extinguish a fire.

(b) LPG is a better domestic fuel than wood because it does not pollute the environment and it does not leave any residue after combustion and the calorific is also high in LPG than wood.

(c) the paper catches fire because of low ignition temperature but when the paper is wrapped around an aluminium pipe this pipe absorbs the heat and the paper does not get its ignition temperature which is why it does not catch fire.

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

Solution 6.

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Question 7. Name the unit in which the calorific value of a fuel is expressed.

Solution 7. The unit of the calorific value of the fuel is KJ/Kg

Question 8. Explain how CO2 is able to control fire.

Solution 8. CO2 is able to control fire because of the following characteristics of CO2:

  1. It stops the supply of air by forming a blanket around the fire.
  2. It is heavier than oxygen.
  3. It cools the temperature of a burning substance.

Question 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Solution 9.  The ignition temperature of the heap of green leaves is high because of the small amount of water present in it which is why it is difficult to burn the heap of green leaves. On the other hand, dry leaves have low ignition temperatures hence they easily catch fire.

Question 10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Solution 10. A goldsmith uses the hottest zone of the flame which is the outermost zone of the flame to melt gold and silver.

 

Question 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.

Solution 11. Calorific value of a fuel = Heat Produced/Amount of fuel
= 1800004.5 kJ/kg
= 40,000 kJ/kg.

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

Solution 12. The process of rusting can be called slow combustion because this process emits heat during oxide formation.

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 13. The water in the beaker that is put on the outermost part of the flame by Ramesh will get heated in a shorter time because this part is the hottest zone of the flame.