Combustion : Science Cbse / Ncert Revision Notes
The chemical process in which a substance reacts with oxygen to give off heat is called combustion.
A substance that undergoes combustion is called a combustible substance or a fuel. The type of combustion differs depending on the type of fuel used. Liquefied Petroleum Gas or LPG burns very quickly producing a lot of heat. This is called rapid combustion.
When white phosphorous is left out in the open at room temperature for some time, it burns all by itself. This is called spontaneous combustion. Forest fires and fires in coal mines are because of spontaneous combustion. The burning of crackers produces a large amount of heat, light and sound because of chemical reaction. This type of combustion is called explosion.
Without oxygen, even a combustible substance will not burn. Temperature is an important condition for combustion to occur. The lowest temperature at which a substance catches fire is called its ignition temperature. Kerosene is a fuel that has a low ignition temperature.
A low ignition temperature means the substance will catch fire quickly and is highly inflammable. Some inflammable substances are petrol, LPG, ether and alcohol.
A matchstick is made of a mixture of antimony trisulphide and potassium chlorate mixed with a little bit of red phosphorous. A lighter depends on lighter fluid, which is highly inflammable as well. It is made out of naphtha or liquid butane. Air, heat and fuel are needed for a fire to be created.
Fire can be easily stopped by stopping the supply of either air or heat. Water is not a good extinguisher for electrical fires. If electrical wiring is on fire, pouring water on it will conduct the electricity through the water and may cause the person dousing the fire to be electrocuted.
As water is heavier than petrol, it is not useful for extinguishing oil or petrol fires. For electrical and oil fires, it is best to use carbon dioxide as an extinguisher. Carbon dioxide is heavier than oxygen, so it covers the flame like a blanket, cutting off contact between the fuel and oxygen. Powder of sodium or potassium bicarbonate can also be used to get carbon dioxide.
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The first automatic fire extinguisher was patented in England by a celebrated chemist called French C. Hopffer. The modern fire extinguisher was invented by British Captain George William Manby.