Fire is a chemical reaction resulting from the combination of a fuel with atmospheric oxygen. Once initiated, this reaction is self-sustaining, generates high temperatures and produces heat, light, gases and particulate matter.
The visible region of the flame is where this chemical process takes place, i.e. fire is essentially a phase of the gaseous phenomenon: For combustion to take place, solid and liquid fuels must become gases.
By boiling a liquid, this transformation is achieved; the solids reach it through a process called pyrolysis, which generates volatile gases.
The flame itself is a plasma of very hot atoms that emit energy in the form of light while its electrons, which have reached higher energy levels by absolving heat, fall into lower energy states.
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