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trisulphide and start combustion.

How can we control fire?
To control fire, any of the three conditions that are necessary for burning can be
removed. Thus, a fire can be extinguished in three ways.

1. By removing the fuel (combustible substance).
Suppose fire starts in a room, all the combustible substances like furniture clothes,
etc, should be removed so that the fire may not spread and the fire can be
controlled.

2. By removing the heat (or by lowering the temperature)

Water is used to remove the heat or to lower the temperature of a burning
substance and to make it too cool to burn further. It is the most common fire
extinguisher. Water cools the combustible substance below its ignition temperature,
and it stops burning. The fire produced by burning oil and petrol cannot be
extinguished by using water, as water is heavier than oil and petrol. So, when water
is thrown over oil and petrol it settles down. The oil floats on water and continues to
burn.
The fire caused by an electrical short circuit cannot be extinguished by using water,
as ordinary water conducts electricity to some extent, and it can give an electric
shock to the person involved in fire-fighting
3. By cutting off the air supply.
Many fires can be extinguished by cutting off the air supply to the
burning substances. The air supply can be cut off
by covering the burning substance with carbon dioxide, sand, a
blanket or a damp cloth etc. Page3 Carbon dioxide is heavier than
oxygen. It can be stored as a liquid at high pressure. When released
it spreads over the fire and extinguishes it. Another way to obtain
carbon dioxide is to release a lot of dry powder of chemicals like
sodium bicarbonate or potassium bicarbonate over the fire, the heat
of the fire decomposes the chemical to release carbon dioxide gas
and the fire can be extinguished. We can use fire extinguishers to
extinguish the fire. Most commonly used fire extinguisher other than
discussed above is soda acid fire extinguisher. Soda acid fire extinguisher contains sodium
bicarbonate and sulphuric acid. When combined it releases carbon dioxide gas water and sodium
sulphate. The mixture of carbon dioxide and water can be sprayed over the fire to extinguish the
fire. It cuts off the supply of air as carbon dioxide is heavier than oxygen and water cools the
combustible substance. The reaction can be written as follows.
Sodium bicarbonate + Sulphuric acid Sodium sulphate + Water +Carbon dioxide
NaHCO2 + H2SO4 → Na2SO 4 + H2O + CO2

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