Combustion is a chemical process in which a substance reacts with oxygen to give off heat. The substance that undergoes combustion is said to be combustible. It is also called fuel. The fuel may be solid, liquid or gas. Oxygen (in the air) is essential for combustion. During the process of combustion, heat and light are given out. The lowest temperature at which a substance catches fire is called its ignition temperature. The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Ex: petrol, alcohol, Liquified Petroleum Gas (LPG) etc

Types of Combustion

Types of Combustion
Spontaneous combustion of coal dust has resulted in many disastrous fires in coal mines. Spontaneous forest fires are sometimes due to the heat of the sun or due to lightning strike.


It is a zone or burning vapour. The substances which vapourise during burning give flames. Example: Kerosene oil and molten wax.
Parts of a Flame
There are three different zones of a flame – dark zone, luminous zone and non-luminous zone.


An ideal fuel is cheap, readily available, readily combustible and easy to transport. It has a high calorific value. It does not produce gases or residues that pollute the environment. Fuels differ in their efficiency and cost.

Fuel Efficiency

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Types of Fuel

Effects of Burning of Fuels

(i) Carbon fuels like wood, coal petroleum release unburnt carbon particles. These are dangerous pollutants causing respiratory diseases, such as asthma. (ii) Incomplete combustion of carbon fuels gives carbon monoxide which is a poisonous gas. (iii) Increased concentration of carbon dioxide in the air is believed to cause global warming. (iv) Oxides of Sulphur and nitrogen dissolve in rainwater and form acids. Such rain is called acid rain. It is very harmful to crops, buildings and soil. (v)Unburnt carbon particles in the air are dangerous pollutants causing respiratory problems. (vi)Incomplete combustion of fuel gives poisonous carbon monoxide gas. (vii) an Increased percentage of carbon dioxide in the air has been linked to global warming.

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