Combustion and Flame | NCERT Solution | Class VIII

Q1 List conditions under which combustion can take place.
Ans. Combustion is a process of reaction of a substance with oxygen. There are certain conditions required for combustion to take place. They are:
(i) A fuel Image result for images of combustion
(ii) Air (for oxygen supply)
(iii) Ignition temperature

Q2 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 _________ .
Ans.
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel used in homes is liquefied petroleum gas (LPG) .
(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.
Ans. Combustion of fuels like petroleum causes formation of unburnt carbon particles along with carbon monoxide gas. These harmful pollutants enter the air and cause respiratory diseases. Compressed Natural Gas (CNG) produces these harmful products in very less quantity. It is a comparatively cleaner fuel. Therefore, the use of CNG has reduced pollution in our cities.

Q4 Compare LPG and wood as fuels.
Ans.
Wood

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

      LPG

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

Q5 Give reasons.
(a) Water is not used to control fires involving electrical equipment.
Ans:

  • Water is a good conductor of electricity.
  • If added to an electrical fire, the water would just spread the electricity further.
  • The person dousing the fire might get an electric shock

(b) LPG is a better domestic fuel than wood.
Ans:

  • LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
  • Wood on the other hand releases lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
  • Hence 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.
Ans:

  • The paper by itself catches fire easily because of its low ignition temperature
  • The piece of paper wrapped around an aluminum pipe doesn’t catch fire because aluminum is a good conductor of electricity.
  • While the paper wrapped around an aluminum pipe results in an increase in ignition temperature. So there is a transfer of heat from paper to the aluminum pipe. Hence it doesn’t catch fire.

Q6 Make a labelled diagram of a candle flame.
Ans.
Image result for images of combustion

Q7 Name the unit in which the calorific value of a fuel is expressed
Ans. The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).
kJ-kilojoules
kg- kilogram

Q8 Explain how CO2 is able to control fires
Ans. CO2 is a non-combustible gas and extinguishes fire in two ways:
(i) Since it is heavier than oxygen, it covers the fire like a blanket and cuts off the contact between oxygen and fuel.
(ii) In cylinders, CO2 is kept in liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.

Q9 It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain
Ans. A heap of green leaves have a lot of moisture in them.  hence its ignition temperature is high. Therefore it does not catch fire easily.
However, dry leaves have no moisture in them, hence its ignition temperature is low. Therefore it catches fire easily.

Q10 Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. Goldsmiths use the outermost part/zone of the flame to melt gold and silver. This is because the outermost zone of the flame undergoes complete combustion and is the hottest part of the flame.

Q11 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.
Ans.

NCERT Solutions for Class 8 Science-chpater-6-sol-1

Q12 Can the process of rusting be called combustion? Discuss.
Ans. Combustion is a chemical process in which a substance reacts with oxygen and gives out energy during the process in the form of either heat or light or both. Rusting of iron is an exothermic process as heat is released during rusting. Hence, it is a kind of slow combustion.

Q13 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 water in the Ramesh's beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone, while the yellow zone (in which Abida had kept the beaker) is less hot.

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