A combustible substance which on burning produces a large amount of heat and light is called a fuel.
Coal, LPG, Petrol, Kerosene, wood etc.

Classification of Fuels
Fuels are classified on the basis of physical state in which they occur. So fuels are classified as solid, liquid and gaseous fuels.
(i) Solid fuels : Combustible substances which are solid at room temperature are called solid fuels. Solid fuels contain mainly carbon both as free and combined carbon. In rural areas, Firewood, Agricultural wastes, Animal-dung cakes are the major source of energy.
    Examples : Some solid fuels are :
    (a) Coal         (b) Coke        (c) Wood        (d) Charcoal            
    (e) Animal-dung cakes            (f) Agricultural wastes

(ii) Liquid fuels : Volatile liquids which produce combustible vapour are called liquid fuels. Kerosene is the most commonly used liquid fuel.
    Examples : Some common liquid fuels are :
    (a) Petrol        (b) Diesel        (c) Kerosene        (d) Alcohol
    Petrol, diesel and kerosene are mixtures of hydrocarbons.

(iii) Gaseous fuels : Combustible gases or mixtures of combustible gases are called gaseous fuels.
    Examples : Some commonly used gaseous fuels are :
    (a) Natural gas                (b) Liquified petroleum gas (LPG)        
    (c) Biogas (or Gobar gas)            (d) Coal gas            
    (e) Water gas                (f) Producer gas
    (g) Hydrogen gas                (h) Compressed Natural (CNG)
    Petroleum gas is obtained as a by-product during the fractional distillation of petroleum.

    An ideal fuel should have the following characteristics :
    (i)  It should be fairly cheap and easily available.
    (ii) It should burn at moderate rate.
    (iii) It should not produce any poisonous and irritating fumes during burning.
    (iv) It should leave no residue (ash) after burning.
    (v)  It should produce large amount of heat per unit mass i.e., it should have high calorific value.
    (vi)  It should be safe and convenient from the storage and transportation point of views.
    (vii)  Its ignition temperature should be above room temperature. So that it is safe to use such as a fuel.

    (i) Cooking and Heating : The most common use of fuels is for cooking and heating. The commonly used domestic fuels are wood, dry cattle dung, coal, charcoal, kerosene (in rural ar

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