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Explain the terms - flame,fuel,ideal fuel,calorific value.
Flame  :A region of burning gases is called a flame .
For example when a match stick catches fire ,the heat released due to the burning of chemicals on the match head ,partly decomposes the wood to form wood gas .The wood gas then catches fire and produces a flame .

If we observe a lighted candle we can observe three zones there .
(i) Blue zone  :  Near the base of flame ,the fresh air rapidly mixes with wax vapor formed from the molten wax.A  part of wax vapor almost completely burns and give rise to blue flame is seen on LPG gas burner .
(ii) Dark inner zone   :  This zone consists of unburnt wax vapour given off by the molten wax .It is the coldest part of the flame .It is slightly dark because a part of wax vapor decomposes on heating to form carbon particles .
(iii) Luminous zone  :  In this zone a partial combustion of wax vapor takes place with the liberation of a lot of heat energy The heat energy so liberated partly decomposes the wax vapor into carbon particles The carbon particles in-turn get very hot and emit yellowish light .The luminous zone of a candle flame is hotter than the dark inner zone ,but not as hot as the non-luminous zone .
(iv)Non luminous zone  : It is a zone of complete combustion of wax vapour and carbon particles .The air from the sides of the flame mixes with unburnt wax vapour and carbon particles from the luminous zone and completely burns them to form carbondioxide gas and water vapour.It is the hottest part of the flame ,which is hardly visible .

Fuel   : Any substance which is easily available and burns in air at a moderate rate, producing a large amount of heat energy ,without leaving behind any undesirable residue is called a fuel .

Characteristics of a good fuel
(i). It should be cheap and readily available
(ii) It should be easy to store and have a large calorific value .
(iii) It should not produce any hazard during transportation
    and produce a very small amount of residues ,such as ash .
(iv) It should have low ignition temperature .
(v) It should burnt at a slow rate and its combustion should be controllable .

Classification of fuels
Solid fuels  :  The fuels which occur in a solid state at room temperature are called solid fuels .
Example  : wood ,coke

Liquid fuels :  The fuels which occur in a liquid state at room temperature are called liquid fuels
Example  :  Petrol,furnace oil

Gaseous fuels   :   The fuels which occur in a gaseous state at room temperature are called gaseous fuels .

Example  :  water gas  ,gobar gas

Calorific value of a fuel  :  The amount of heat energy produced on completely burning one kilogram  of a fuel in pure oxygen is called the calorific value of a fuel  .It is expressed in  kJ /kg.
Ideal fuel   :  A fuel which is cheap ,easily available ,easily stored ,has highest calorific value and does not produce any harmful residues /gases on combustion is called an ideal fuel .


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