Flame

A flame (from Latin flamma) is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction made in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density, they are then considered plasma.^[vague]

Color and temperature of a flame are dependent on the type of fuel involved in the combustion. For example, when a lighter is held to a candle, the applied heat causes the fuel molecules in the candle wax to vaporize. In this state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame. The high temperature of the flame causes the vaporized fuel molecules to decompose, forming various incomplete combustion products and free radicals, and these products then react with each other and with the oxidizer involved in the reaction of the following flame (fire).

One may investigate different parts of a candle flame with the aid of a cold metal spoon: the higher parts of the flame produce water vapor deposition, the result of combustion, the yellow parts in the middle produce soot, and the area near the candle wick produces unburned wax. Goldsmiths use higher parts of a flame with a metallic blow-pipe for melting gold and silver. Sufficient energy in the flame will excite the electrons in some of the transient reaction intermediates such as the methylidyne radical (CH) and diatomic carbon (C₂), which results in the emission of visible light as these substances release their excess energy (see spectrum below for an explanation of which specific radical species produce which specific colors). As the combustion temperature of a flame increases (if the flame contains small particles of unburnt carbon or other material), so does the average energy of the electromagnetic radiation given off by the flame (see Black body).

Other oxidizers besides oxygen can be used to produce a flame. Hydrogen burning in chlorine produces a flame and in the process emits gaseous hydrogen chloride (HCl) as the combustion product. Another of many possible chemical combinations is hydrazine and nitrogen tetroxide which is hypergolic and commonly used in rocket engines. Fluoropolymers can be used to supply fluorine as an oxidizer of metallic fuels, e.g. in the magnesium/teflon/viton composition.

The chemical kinetics occurring in the flame are very complex and typically involve a large number of chemical reactions and intermediate species, most of them radicals. For instance, a well-known chemical kinetics scheme, GRI-Mech, uses 53 species and 325 elementary reactions to describe combustion of biogas.

There are different methods of distributing the required components of combustion to a flame. In a diffusion flame, oxygen and fuel diffuse into each other; the flame occurs where they meet. In a premixed flame, the oxygen and fuel are premixed beforehand, which results in a different type of flame. Candle flames (a diffusion flame) operate through evaporation of the fuel which rises in a laminar flow of hot gas which then mixes with surrounding oxygen and combusts.

Flame color depends on several factors, the most important typically being black-body radiation and spectral band emission, with both spectral line emission and spectral line absorption playing smaller roles. In the most common type of flame, hydrocarbon flames, the most important factor determining color is oxygen supply and the extent of fuel-oxygen pre-mixing, which determines the rate of combustion and thus the temperature and reaction paths, thereby producing different color hues.

In a laboratory under normal gravity conditions and with a closed air inlet, a Bunsen burner burns with yellow flame (also called a safety flame) with a peak temperature of about 2,000 K (3,100 °F). The yellow arises from incandescence of very fine soot particles that are produced in the flame. Also, carbon monoxide is produced, and the flame tends to take oxygen from the surfaces it touches. When the air inlet is opened, less soot and carbon monoxide are produced. When enough air is supplied, no soot or carbon monoxide is produced and the flame becomes blue. (Most of this blue had previously been obscured by the bright yellow emissions.) The spectrum of a premixed (complete combustion) butane flame on the right shows that the blue color arises specifically due to emission of excited molecular radicals in the flame, which emit most of their light well below ≈565 nanometers in the blue and green regions of the visible spectrum.