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These are the substances that form source of energy. They exists as Solids, Liquids, and Gases. A good fuel is one which is readily available. It is cheap. It burns easily in air at a moderate rate. It produces a large amount of heat. It does not leave behind any undesirable substances.

Example : Wood, charcoal, petrol, Kerosene, etc.


Coal is a Solid Fuel

Diesel is a Liquid Fuel

Car using C.N.G

Fuel Efficiency

Fuel efficiency is measured as the amount of heat produced by burning 1 Kg of fuel known as the calorific value of the fuel .It is expressed in a unit called kilojoules per Kg.


Calorific Value

Consider the calorific values of the fuels as given in the table and classify them as solid, liquid and gaseous fuels and arrange the fuels in the decreasing order of their fuel efficiency.


 Calorific Values of Different Fuels Fuel Calorific Value


Cow dung cake 6000-8000
Wood 17000-22000
Coal 25000-33000
Petrol 45000
Kerosene 45000
Diesel 45000
Methane 50000
CNG 50000
LPG 55000
Biogas 35000-40000
Hydrogen 150000

Classify the fuels as solids, liquids and gases.

S.No Solids Liquids Gases


Global Warming

Global warming is an undesirable phenomenon caused as the result of burning of Carbon fuels like wood, coal, petroleum release carbon dioxide gas responsible for trapping infrared radiation from sun and not allowing it to escape from the surface of the earth, the presence of excess heat energy radiation that is infra red radiation increases surface temperature of the earth resulting in global warming. The impact of global warming is the melting of polar glaciers, which leads to a rise in the sea level,Causing floods in the coastal areas.

Coastal Areas

Melting of Glacier


Acid Rain

Acid rain is formed when acidic gases such as sulphur dioxide and oxides of nitrogen liberated as pollutants from industries dissolve in rain water.


Acid Rain Formation


Acid rain is very harmful to plants and old monuments.



Trees being destroyed by acid rain

Acid rain destroys the beauty of monuments like Taj Mahal

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