A combustible substance is the one which can burn. It does not necessarily mean that it would produce large amount of heat without leaving any residue. Thus all fuels are combustible but not all combustible substances are fuels.

A good fuel produces large amounts of energy and does not leave behind any undesirable substances. Also, it is cheap and is readily available.

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Fuel may be defined as the substance which has high calorific value and least ash content on burning. Many of the substance does no fulfill this criteria therefore cannot be considered as fuel.

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All fuels are combustiblesubstances, but al combustible substances are not fuel . Fuel means a substance which has high calorific value , produces large amount of heat and light on burning ,which leaves very less or no residue and which is cheaply available.

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