NCERT Solutions Class 8 Science Combustion and Flame

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NCERT Solutions for Class 8 Science for chapter 6 Combustion and Flame

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

Answer.

Necessary conditions for combustion are:

(1) Fuel: Fuel is the raw material for combustion.

(2) Air: Combustion takes place only in the presence of air or oxygen.

(3) Heat: Heat is required to raise the temperature of a fuel so that it attains its ignition temperature. Ignition temperature is the minimum temperature at which a substance catches fire and starts burning.

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______

Answer.

(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.

Answer.

Combustion of fuels like petrol and diesel produces un- burnt particles along with carbon monoxide gas. These pollutants cause many respiratory problems. Oxides of sulphur and nitrogen are also released due to combustion of petrol and diesel. Oxides of sulphur and nitrogen cause acid rain. CNG produces the harmful products in very small amount. CNG is a cleaner fuel.

Question - 4

Compare LPG and wood as fuels.

Answer.

Comparison of LPG and wood as fuels

LPG as fuels

Wood as Fuels

Burning of LPG does not produce smoke and other pollutants. It is a cleaner fuel.

Burning of wood produces a lot of smoke and other pollutants which may cause respiratory problems.

 

LPG is a liquefied form of petroleum.

Wood is obtained from trees. Therefore, use of wood as fuels may lead to deforestation.

Calorific value of LPG is very high (55000 kJ/kg).

Calorific value of wood is low (17000 -22000 kJ/kg).


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.

Answer.

(a) Water cannot be used to put off fire caused by the electrical equipments because it may conduct electricity and harm those trying to douse the fire.

(b) Burning of wood produces a lot of smoke and un- burnt particles which cause respiratory problems. LPG does not produce smoke and its calorific value is much higher as compared to wood. Therefore, LPG is a better domestic fuel than wood.

(c) For burning, combustible substance must attain its ignition temperature. Piece of paper wrapped around an aluminium pipe does not catch fire on heating because it is unable to reach its ignition temperature due to transfer of heat to aluminium pipe which is a good conductor of heat.


Question - 6

Make a labelled diagram of a candle flame.

Answer.

image

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

Answer.

Calorific value of a fuel is expressed in Kilo Joule per kg (kJ/kg).


Question - 8 - Explain how CO2 is able to control fires.

Answer.

  • Carbon dioxide is the best extinguisher for fires involving electrical equipments and inflammable materials like petrol because CO2 is heavier than oxygen and it covers the fire like a blanket.
  • It controls fire by cutting off the supply of oxygen.


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

Answer.

A heap of leaves contains a lot of water. Due to presence of water, ignition temperature of green leaves is not reached easily. Hence, it is difficult to burn a heap of green leaves. Dry leaves do not contain water. Therefore, ignition temperatures of dry leaves are easily attained and catch fire easily.


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

Answer.

Goldsmith uses outermost zone of the flame for melting gold and silver. This is because the outermost zone of the flame is hottest part of the flame. In this zone complete combustion takes place.

Question - 11 - 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 flame.

Answer.

Calorific value of the fuel is the amount of heat energy produced on complete combustion of 1 kg of fuel.

Amount of heat produced by 4.5 kg of fuel = 180,000 kJ Therefore, heat produced by 1 kg of fuel = 180,000/4.5 = 40,000 kJ/kg

Hence, calorific value of given fuel is 40,000 kJ/kg.


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

Answer.

Combustion is the chemical process in which a substance reacts with oxygen and gives out energy in the form of either heat or light or both. It is a very fast process. On the other hand, rusting is a slow process which involves the conversion of iron into rust in presence of air (oxygen) and water. In both the processes, oxygen is common reactant.

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?

Answer.

The water in the Ramesh’s beaker will require shorter time for heating. This is because Ramesh kept beaker in the outermost part of the flame which is hottest zone of the candle flame. The water in Abida’s beaker will take comparatively longer to heat up as the yellow zone is less hot when compared to the outermost part of the flame.

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