Fire involves a chemical reaction between fuel and atmospheric oxygen. Once initiated it is self-sustaining, generates high temperatures and release a combination of heat, light, noxious gases and particulate matter.
The visible flame is the region in which this chemical process occurs and so flame is essentially a gas phase phenomenon. For flaming combustion to occur, solid and liquid fuels must be converted into gaseous form.
For liquid fuels this is achieved by evaporative boiling. For solid fuels, the solid is chemically decomposed through the process of paralysis to generate volatile gases.
A flame is a region containing very hot atoms. At high enough temperatures all atoms will emit energy in the form of light as their electrons, which have been prompted to higher energy levels by absorbing heat energy, fall to lower energy states. Because this light is emitted in discrete quanta according to the relationship E= hf (where E=energy, h=Planck’s constant and f= frequency), flame colour is related to the magnitude of the energy quantum which is transformed to light.
This can most easily be seen with a Bunsen burner. A Bunsen burner that has a choked air supply burns cool, the light emissions from carbon atoms are relatively low in energy and appear more red or orange.
However, when the Bunsen is allowed air so that combustion is complete, the flame is hotter and the light emitted is of a higher energy and frequency and appears blue.
The luminescence of a flame is only of the story. The structure of the flame region is important to understand too. The flame area in a normal combustion environment, such as an open-air bonfire, is structured by convention currents which form as hotter, lighter air rises and allows cooler fresh air to replace it.
It is this channeling effect and movement of air that shapes the dancing flames. It is interesting that in space, in zero gravity, the hotter and cooler air cannot move by convection, so flames take on weird shapes and may be stifled by their own combustion products.
