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Science class 8 ch- 6 COMBUSTION AND FLAME Important notes //Ak learning //Ak Raj ,Part -1

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CH-6
    COMBUSTION AND FLAME
  • Introduction
We all need fire to carry out almost all of the chores in our day to day life. We light candles in case of power cut to get heat and light. We also burn wood to conduct bonfire. In all of these cases the substances undergoes burning to give off heat and light. Liquefied petroleum gas, compressed natural gas, petrol, diesel, coal, kerosene etc. are some of the substances that undergoes combustion to give off heat and light. We will discuss the phenomenon of combustion and its related terms below.
On the occasion of Holi, people ignite Bonfires. In it dry woods are ignited and they start burning giving off heat and light. In this process as soon as the woods are ignited the wood catches fire and burns with the help of atmospheric oxygen. This kind of chemical phenomena in which a substance reacts  with atmospheric oxygen to give off heat and light is termed as combustion.

Combustion can be classified into three types namely- Rapid combustion, spontaneous combustion and Explosion.
RAPID COMBUSTION:-
  • The type of combustion reaction that occurs with a great speed producing an enormous heat & light in a short time is known as Rapid combustion.
  • For instance, when we bring a matchstick near the gas stove it rapidly catches  fire.
  • Similarly, as soon as we strike the matchstick against the match box it rapidly catches fire.
SPONTANEOUS COMBUSTION:-
  • The type of combustion reaction that occurs on its own, without the submission of heat is known as Spontaneous combustion.
  • For instance, white phosphorus burst into flames spontaneously at the room temperature without the submission of heat.
  • Forest fire, spontaneous combustion of coal dust, etc. are some other examples of this kind of combustion.
EXPLOSION:-
  • The type of combustion reaction that occurs with the evolution of tremendous amount of heat, light, gases and sound is known as Explosion.
  • For instance, when we burn a cracker it bursts with the liberation of tremendous amount of heat, light, gases and sound. It is explosion.
FUELS:-
  • The substance undergoing combustion are termed as combustible substances or fuels. In the above example of Bonfire, wood is used as a fuel. In earlier days people used to cook food from the heat liberated by the burning of coal which was used as a fuel. In modern we cook food using LPG cylinders which is fuel and undergoes burning liberating tremendous heat to cook food.
  • Fuel can be available in three forms.
Solid fuels:- wood, charcoal, coal, etc.
Liquid fuels:- petrol, diesel, kerosene, etc
Gaseous fuels:- compressed natural gas, liquefied petroleum gas, biogas, hydrogen etc.

IDEAL FUEL:-
  • An ideal Fuel is one which possesses the following characteristics:
  • It should be readily available.
  • It should be inexpensive.
  • It should burn easily in air at a judicious rate.
  • It should liberate large amount of heat.
  • It should leave behind any undesirable residues.
  • It should be easy enough to store, transport, and handle.
  • It should have high calorific value.
  • It should have ignition temperature above the room temperature.
  • It should produce less hazardous products on burning.
  • But in reality there is no such fuel that can be regarded as an ideal fuel. We generally opt for the fuel that is capable of meeting maximum of our
  • requirements.

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