Q-1 List conditions under which combustion can take place.
Ans.The burning of a substance in the presence of oxygen is defined as combustion.
The conditions under which combustion can take place are---
1) The presence of air or oxygen.
2) The presence of fuel plays a significant role.
3) Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest
temperature.
Que-2 Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans. CNG plays an important role in reducing pollution among automobiles for the following reasons---
1) CNG is comparatively a cleaner fuel.
2) The CNG can be an alternative for diesel, petrol and propane/LPG.
3) It usually contains a few undesirable gases than the other fuels mentioned above.
The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon
monoxide, which leads to respiratory diseases.
Que-3 Compare LPG and wood as fuels.
Ans.
Wood
1) It is considered as a traditional fuel used for both domestic and industrial purposes.
2) Wood produces a lot of smoke which pollutes the atmosphere and causes respiratory diseases.
3) The usage of wood to a large extent causes deforestation.
4) The calorific value of wood ranges between 17000 and 22000 kJ/kg
However, wood may be used as a furnace, stove or fireplace indoors while it is used for a campfire,
furnace outdoors.
LPG
The usage LPG (Liquefied petroleum gas) has replaced wood.
1) It doesn’t release smoke and other pollutants.
2) It is a cleaner fuel.
3) The fuel efficiency of LPG is more than that of wood.
4) The calorific value of LPG is 55000 kJ/kg.
Hence, LPG is the most preferred choice.
Q- 4 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.
Ans.a) 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, being a cleaner fuel than wood, doesn’t release smoke and other pollutants.
*Wood, on the other hand, releases a 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) The paper by itself catches fire easily because of its low ignition temperature.
The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good
conductor of electricity.
While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So,
there is a transfer of heat from paper to the aluminium pipe. Hence, it doesn’t catch fire.
Q-5 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).
Q-6. Explain how CO2 is able to control fires.
Ans. CO₂ is heavier than air and forms a blanket around the fire, cutting off the supply of oxygen and
thereby stopping the combustion process.
Q-7. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans. Green leaves contain moisture, which absorbs heat and makes them difficult to ignite. In contrast,
dry leaves have no moisture and easily catch fire as they burn faster.
Q-8. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. A goldsmith uses the outermost (non-luminous) zone of the flame because it is the hottest zone
and ensures complete combustion, making it ideal for melting gold and silver.
