Chapter: Combustion And Flame
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The chemical process in which a substance reacts with oxygen to give off heat is called combustion. 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.
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.
A matchstick is made of a mixture of antimony trisulphide and potassium chlorate mixed with a little bit of red phosphorous. Fire can be easily stopped by stopping the supply of either air or heat.
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.
Kerosene oil and molten wax are substances that give a flame while burning. A luminous flame is a bright yellow flame that gives off light. A luminous flame undergoes incomplete combustion as it does not get the oxygen that it requires.
A non-luminous flame is colourless and is much hotter. A non-luminous flame undergoes complete combustion as it draws much more oxygen and gets much hotter.
Fuel is any material that is burned to obtain energy that can be used to heat or move another object. Fuel releases energy through a chemical reaction known as combustion. Wood was the first fuel that was used 2 million years ago by homo erectus, the predecessor of human beings.
In rural areas, cow dung and wood are still used as fuel because these are very cheap and easily available. However, burning wood produces a lot of smoke, which is very harmful to humans, since it causes respiratory problems.
Unburned carbon particles released when carbon fuels like wood, coal and petroleum burn, cause pollution and respiratory diseases such as asthma.
Incomplete combustion of carbon fuels causes the release of carbon monoxide – a very harmful gas. Combustion of fuels causes the release of carbon dioxide, which leads to global warming.
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