Combustion is a chemical process in which a substance reacts rapidly with oxygen and produces heat and light. The process is commonly associated with the formation of a flame. Understanding combustion is crucial in comprehending various natural and human-induced processes.

1. COMBUSTION
The process of burning of substances in air or oxygen with the evolution of heat and light is known as combustion.
COMBUSTIBLE SUBSTANCES
Substances that burn in air or oxygen to produce heat and light are called combustible substances.
INFLAMMABLE SUBSTANCES
Substances that have a very low ignition temperature and hence can catch fire easily are called inflammable
substances.
COMBUSTION COMBUSTIBLE SUBSTANCES INFLAMMABLE SUBSTANCES
IGNITION TEMPERATURE
The lowest temperature at which a combustible substance starts burning in the presence of air is called its ignition
temperature. Different substances have different ignition temperatures.
The conditions necessary for combustion are:
1. Presence of a combustible substance
2. Presence of supporter of combustion
3. Heating the substance to its ignition temperature
In the given figure the paper cup with water in it does not burn because it loses its heat to the water by conduction and
hence its temperature does not rise above its ignition temperature even when held above the candle flame.
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2. 1. RAPID COMBUSTION
The type of combustion in which a substance burns rapidly to produce heat and light is called rapid
combustion. For e.g. Burning of paper, LPG etc.
2. SPONTANEOUS COMBUSTION
The type of combustion in which a material bursts into flame without the application of heat is called
spontaneous combustion.
For e.g. Sodium is stored in kerosene and phosphorous is stored in water to prevent spontaneous combustion
as they burst into flame when they come in contact with oxygen at room temperature due to low ignition
temperature.
3. EXPLOSION
The combustion reaction that occurs suddenly with the evolution of a large amount of gas is called an
explosion. For e.g. Firecrackers
Fire can be extinguished by one of the following methods:
1. By removing the combustible substance
2. By cooling the substance below its ignition temperature
3. Cutting off the air supply
There are three parts in a soda-acid type fire extinguisher:
1. Container – It is cylindrical in shape and contains sodium bicarbonate
 
3
NaHCO solution.
2. Bottle – It is small in size and contains concentrated sulphuric acid
 
2 4 .
H SO
3. Knob – Bottle is attached to it.
WORKING
 When the knob is struck, the bottle breaks and  
2 4
H SO reacts with 3
NaHCO liberating large amount of 2
CO
gas with great force.
 2
CO forms a blanket around the fire cutting off the air supply, due to which the fire gets extinguished.
CHEMICAL REACTION
3 2 4 2 4 2 2
2 2 2
Sulphuric Sodium Water Carbon
Sodium
Acid Sulphate Dioxide
Bicarbonate
NaHCO H SO Na SO H O CO
 
  
Effective for fires caused by solids and liquids
Effective for fires caused by liquids and faulty electric wires
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3. A fuel is a substance that can be used to produce heat at a reasonable cost. Fuels contain potential energy that is
locked within the chemical bonds of their constituent molecules. When they are burnt, a large part of this energy is
released as heat. This forms the basis of their use as fuels.
Calorific value of a fuel can be defined as the amount of heat liberated when one kilogram of the fuel is completely
burnt in sufficient supply of oxygen.
This S.I. unit of calorific value is joule per kilogram (J/Kg), though commonly the unit kilojoule per kilogram (kJ/Kg) is
used.
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