The flame (from Latin French) is the visible gas part of the flame. This is due to a high-level exothermic reaction in a thin region. A very hot flame is so hot that the density of the ionized gas component is enough to be considered a plasma.

The color and temperature of the flame depends on the type of fuel involved in the combustion, for example, when a lighter is fixed to a candle. The applied heat causes the fuel molecules in the candle wax to evaporate. Then, in this state, they can easily react with the oxygen in the air, so as to release enough heat in the subsequent exothermic reaction, so as to make more fuel evaporate, thus maintaining a stable flame. The high temperature of the flame causes the vaporized fuel molecules to decompose, forming various incomplete combustion products and free radicals, which then react with each other and with the oxidants involved in the reaction. Enough energy in the flame will excite the electrons in some transient reaction intermediates (such as methylene radicals (CH) and diatomic carbon (C2)). When these substances release extra energy, they will emit visible light (see the spectrum below) to explain which specific radicals will produce which specific colors). As the temperature of the flame increases (if the flame contains small particles of unburned carbon or other substances), the average energy of electromagnetic radiation emitted by the flame increases (see blackbody).

Besides oxygen gas, other oxidants can also be used to produce flame. In this process, gaseous hydrogen chloride (HCl) is released as a combustion product. Another of many possible chemical combinations is hydrazine and nitrogen tetroxide, which are high toothed and are commonly used in rocket engines. Fluoropolymers can be used to provide fluorine as an oxidant for metal fuels, such as metals. In the magnesium / Teflon / fluororubber composition.

The chemical kinetics of flame is very complex, usually involving a large number of chemical reactions and intermediate species, most of which are free radicals. For example, the well-known chemical kinetics scheme gri mech uses 53 kinds and 325 basic reactions to describe the combustion of biogas.

There are different ways to distribute the required combustion components to the flame. In a diffusion flame, oxygen and fuel diffuse each other. The fire happened where they met. In premixed flame, oxygen and fuel are premixed, which leads to different types of flame. The candle flame (diffusion flame) works by evaporation of the fuel, which rises in the hot laminar flow, then mixes with the surrounding oxygen and burns.

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