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This question already has an answer here:

What is fire? Is it a wave or is it matter?

Where does fire come from?

Does everything burn with fire? (for example: water and some metals don't burn).

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marked as duplicate by Emilio Pisanty, Jim, ACuriousMind, John Rennie, Qmechanic Apr 2 '15 at 16:18

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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I'll let the far greater minds than me at Minute Physics give the bulk of the answer. Turns out that the way you perceive fire is the result of numerous chemical and physical processes. I'll discuss and expand on just two major ones that were mentioned in the video I linked:

  1. Atomic Transitions- The distinct colors that you see in a flame are the result of electrons changing energy levels within at atom. When an excited electron drops to a lower energy level, a photon is released, in order to conserve energy. The frequency of the photon released depends on the exact transition the electron makes. In fact, the energy levels of a particular element are somewhat unique, and so the exact colors emitted by electrons changing energy levels are somewhat unique to the element. In chemistry class you may have done a flame ion lab at some point to see that different metals produce flames of different colors, as a result of the different transitions taking place, unique to each element. In the second video I just linked, you can see a variety of colors such as green, blue, red, or orange depending on the element.
  2. Thermal Radiation- However, the red/orange colors you typically see from most combustion reactions in real life are actually the result of a different process. As mentioned in the Minute Physics video, impure flames usually burn reddish orange because the soot and ash particles around the flame are so hot that they emit visible light. This is a different process than the atomic transitions I mentioned. All objects, when heated, emit radiation in the form of electromagnetic waves and the temperature of the object determines the exact wavelengths emitted. You are actually emitting "light" as we speak, but you are so cool that the radiation you emit is in the infrared range, and therefore not visible.
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Fire is a chemical reaction ,some times hot enough to turn the gas around it into plasma which you can basically think of as a hot ionized gas. You can show that fire can form plasma by putting negatively charged plate on the left of a flame and a positively charged plate on the right of the flame. It will split the flame because the plasma is ionized.

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    $\begingroup$ ''Fire is caused by chemical reactions'' ,no fire IS a chemical reaction,and a chemical reaction cannot be plasma ,plasma is a state. $\endgroup$ – Paul Apr 2 '15 at 14:08
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    $\begingroup$ Some fires heat the gas enough to form plasma, some do not. $\endgroup$ – Jim Apr 2 '15 at 14:12
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    $\begingroup$ In everyday flames the concentration of ions and free electrons is extremely small. It's really stretching the definition of plasma to call a flame a plasma. $\endgroup$ – John Rennie Apr 2 '15 at 14:48
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    $\begingroup$ @Paul - So are you talking about a chemical reaction taking place in the region of air where we see the flame? Or is the chemical reaction you're talking about happening only in the burning object at the base of the flame, with the appearance of the flame caused by atoms with excited electrons drifting up away from the burning substance, then transitioning to a lower energy state and giving off photons, as Sean's answer talks about? $\endgroup$ – Hypnosifl Apr 2 '15 at 14:51
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    $\begingroup$ @Paul no fire is the flame, burning is the reaction. $\endgroup$ – JamesRyan Apr 2 '15 at 14:53
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Fire is a chemical reaction which results in the release of energy in the form of light and heat.

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Fire is a chemical process, usually a self-sustaining oxydation. Things "burn" at elevated temperatures. That burning (oxydation) is exothermic. A sustained fire happens when burning something produces enough heat to cause oxidation of nearby fuel to occur, thereby causing it to burn and produce more heat, which causes more fuel to burn, etc.

Most fire is actually a reaction between gasses. The heat causes a combustible gas to be released, which reacts with atmospheric oxygen, which produces more heat, which causes more combustible gas to be released, etc. A candle flame is a good example of this. The heat causes the wax to vaporize, which allows it to react with oxygen. The wick and the liquid wax on the wick and immediately below it don't actually burn. Enough of the heat from the burning immediately above the wick makes it back onto the wick to vaporize more wax.

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  • $\begingroup$ The first paragraph is not true in general. Most fire is actually a chain reaction in which radicals trigger the production of more radicals. A spark plug is not there to heat the fuel, it's to seed it with radicals. The heat feedback can be relevant for vaporising fuel as you say, but it's mostly not what causes the flame itself. (Though I believe it can be under high pressure.) $\endgroup$ – Nathaniel Apr 2 '15 at 14:47
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Do you mean the flame specifically? The flames are easier to explain as they are a more concrete thing.

Flames are gases that are hot enough to emit light in the visible spectrum via thermal radiation. Some proportion of those gases are in the plasma state, but the parts that aren't ionised are also hot enough to emit light in the visible spectrum.

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