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Chapter 6 Combustion and Flame Solutions

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

Answer - 1 : -

There are three essential conditions of combustion:

  1. Presence of a combustible substance.
  2. Presence of oxygen, i.e., supporter of combustion.
  3. Attainment of ignition temperature.

Question - 2 : -
Fill in the blanks:
(1) Burning of wood and coal causes………….of air.
(2) A liquid fuel, used in homes is…………..
(3) Fuel must be heated to its………… before it starts burning.
(4) Fire produced by oil cannot be controlled by……………..

Answer - 2 : -

  1. pollution
  2. Kerosene
  3. ignition temperature
  4. water.

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

Answer - 3 : -

CNG is cheap, readily available and highly combustible. It has high calorific value. It does not produce gases or leaves residues when used in automobiles, so it reduces the pollution in our cities.
The use of CNG in automobiles has reduced pollution in our cities because CNG produces the harmful products in very small amounts and it is a cleaner fuel.

Question - 4 : -
Compare LPG and wood as fuels.

Answer - 4 : -

LPG

Wood

1. No residue after burning.

1. Leaves a lot of ash on burning.

2. It can be easily stored in cylinders for convenient transportation.

2. Cannot be transported easily like LPG.

3. Burns easily.

3. Catches fire with more difficulty.

4. Low ignition temperature.

4. High ignition temperature.

5. No smoke on burning.

5. Burns with smoke.

Question - 5 : -
Give reasons:
(1) Water is not used to control fires involving electrical equipment.
(2) LPG is a better domestic fuel than wood.
(3) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer - 5 : -

  1. Water is not used to control the fire involving electric equipment because it is a good conductor of electricity. It conducts electricity resulting in electric shock to the user.
  2. LPG is better domestic fuel than wood because it neither produces gases nor leaves residues that pollute the environment.
  3. The paper by itself catches fire easily because its ignition temperature is low, while a piece of paper wrapped around an aluminium pipe does not catch fire, because the heat given is absorbed by aluminium paper and the paper does not attain its ignition temperature.

Question - 6 : -
Name the unit in which the calorific value of a fuel is expressed.

Answer - 6 : -

The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Question - 7 : -
Explain how CO2 is able to control fires.

Answer - 7 : -

CO2, being heavier than oxygen, covers the fire like blanket and also brings down the temperature of fuel. Since the contact between the fuel and oxygen is cut off, the fire comes under control.

Question - 8 : -
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer - 8 : -

To bum a heap of green leaves is difficult, because its ignition temperature is high, but dry leaves catch fire easily as its ignition temperature is very low.

Question - 9 : -
Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer - 9 : -

A goldsmith uses the outermost zone of a flame for melting gold and silver because it is the hottest zone of the flame (temperature ≈ 800°C) and is non-luminous in nature.

Question - 10 : -
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.

Answer - 10 : -

The calorific value of the fuel:
Calorific value = heat produced/amount of fuel = 1,80,000 kJ/4.5 kg = 40,000 kJ/kg

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