A flame is the product of a highly exothermic reaction (for example ,combustion) a self-sustaining oxidation reaction, or nuclear fusion in the sun. In other words, a flame could be said to be the visible part of a fire.

The color and temperature of the flame are dependent on the type of fuel involved in the combustion. For example: when a lighter is held to a candle. This applied heat causes the fuel molecules to evaporate, in this state they can then react with oxygen, giving off enough heat in the exothermic reaction to sustain a consistent flame. The resulting increases in temperature tear apart some of the fuel molecules, forming various incomplete combustion products and free radicals. Sufficient energy in the flame will excite the electrons in these products, which results in the emission of visible light. As the combustion temperature increases, so does the energy of the electromagnetic radiation given off by the flame. This is why the hottest visible flame is in the blue/violet region of the visible spectrum.

Other oxidizers besides oxygen can be used. Hydrogen burning in chlorine produces a flame as well, producing gaseous hydrogen chloride (HCl). Another possible combination is hydrazine and nitrogen tetroxide.

A flame could also be the result of other types of reaction. For example the fusion of Hydrogen isotopes, to produce Helium nuclei, in the sun results in the yellow colour of the sun. The energy from this reaction is enough to ionise the gases on the surface to produce a plasma and also to excite the electron's to a higher energy level and back leading to the emmision of photons.

Flames, or at least portions of them, are often plasma or an ionized gas, but in more general terms a flame is a region of exothermic chemical reaction of high enough temperature to emit visible light. A central region enclosed by such a reaction is often also considered part of the flame.

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; where they meet the flame occurs. In a premixed flame, the oxygen and fuel are premixed beforehand, which results in a different type of flame. Candle flames operate through evaporation of the fuel.

Flame color

Flame color depends on three components, blackbody radiation, spectral line emission, and to a lesser degree spectral line absorption. Depending on oxygen supply, which determines the rate of combustion, temperature and reaction paths, different color hues can be observed in flames. Recent discoveries by the National Aeronautics and Space Administration (NASA) of the United States also have found that gravity plays a role. Spiral flames in microgravity, National Aeronautics and Space Administration, 2000. Pictured on the right is a Bunsen burner burning mainly methane.

In a laboratory under normal gravity conditions and with a closed oxygen valve, a Bunsen burner burns with yellow flame (also called a safety flame) at 1,000°C. With increasing oxygen supply less blackbody-radiating soot is produced, and the combustion reaction creates enough energy to ionize gas molecules in the flame, leading to a blue appearance.

Flame temperatures of common items include a blowlamp at 1,300°C, a candle at 1,400°C, or a much hotter oxyacetylene combustion at 3,000°C.

Generally speaking, the coolest part of the flame will be red, transitioning to orange, yellow, and white as the temperature increases as a result of changes in blackbody radiation. For a given flame's region, the closer to white on this scale, the hotter that section of the flame is. A blue-colored flame emerges when the amount of soot decreases and the blue emissions from molecules become dominant.

The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a flame (such as in a candle in normal gravity conditions), making it yellow. In microgravity or zero gravity, such as an outer space environment, convection no longer occurs and the flame becomes spherical, with a tendency to become bluer and more efficient. There are several possible explanations for this difference, of which the most likely is the hypothesis that the temperature is sufficiently evenly distributed that soot is not formed and complete combustion occurs. CFM-1 experiment results, National Aeronautics and Space Administration, April 2005. Experiments by NASA in microgravity reveal that diffusion flames in microgravity allow more soot to be completely oxidized after they are produced than do diffusion flames on Earth, because of a series of mechanisms that behave differently in microgravity when compared to normal gravity conditions. LSP-1 experiment results, National Aeronautics and Space Administration, April 2005. Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer. SOFBAL-2 experiment results, National Aeronautics and Space Administration, April 2005. These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.

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