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I wanted to know that if FIRE is Matter or Energy. I know that both are inter-convertible. but that doesn't mean there is interconversion taking place just like nuclear fusion or fission. If it is matter, then its has to occupy space and have mass. I really doubt that fire has mass. If it is energy, then i don't get it. Please enlighten me from my ignorance.

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Richard Feynman has answered a similar question asked by young rabis: myspace.com/richard_feynman/blog/332429588 The question was: "Is electricity fire?" His answer may be used for your question, too. So read the chapter of his book carefully to catch the answer which may be more important than the detailed question. ;-) –  Luboš Motl May 11 '11 at 9:01
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Just to point out that something cannot be energy. It can have energy. –  MBN Jun 19 '11 at 2:01
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fission, fusion, and ordinary combustion ('fire') are all processes. –  JustJeff Jun 19 '11 at 23:23
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-1: "Fire" is hot air where the electrons are separated from the nuclei. You can read about this anywhere. –  Ron Maimon Sep 8 '11 at 14:15

6 Answers 6

up vote 18 down vote accepted

Fire is neither. Fire is a process involving both. Fire is the energetic combination of various substances with oxygen to release light and heat. In a gas fire, such as might be found on a stove or in a heater, a light hydrocarbon such propane is broken down into components of hydrogen and carbon which unite with oxygen from the atmosphere to form water and carbon dioxide. In an ordinary wood fire, heat causes the wood to break down and release hydrocarbons which then burn as propane would.

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Reaction with oxygen is a somewhat narrow definition. Reactions with chlorine or fluorine are called "fire" too. Or think of pyrotechnical mixtures, where some solid component "oxidizes" the fuel. –  Georg May 11 '11 at 9:35
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@Georg: Well, I don't want to make it broad enough to cover any redox reaction either... –  wnoise May 11 '11 at 9:45
    
+1 just on the basis of naming it for what it is, a process. –  JustJeff Jun 19 '11 at 23:21
    
What people call "fire" is not combustion, but the yellow plasma. –  Ron Maimon Sep 8 '11 at 14:14
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@Ron Maimon: "Fire" to me means the whole thing, which is only adequately captured as a process. The yellow plasma alone, I'd call "a flame". –  wnoise Nov 29 '11 at 22:48

I suggest that looking for an explanation for 'fire' may be the wrong approach. Science is a process where we try to explain what we perceive. In doing so, we may have to sacrifice (or at least temporarily suspend) a common perspective.

My presumption is that you are probably mostly interested in the visual manifestation of fire - the dancing flames. Explaining the existence of light and its color requires only correctly identifying the sources that emit the light (and showing how they got there). Those sources are known to be from excited electron and molecule de-excitement, plus some blackbody radiation from the larger particles, but this depends on the type of reaction. An ordinary camp fire or a candle, to the extent of my understanding, is a mix of the different sources.

The location of the flame above the reaction is due to convection, powered by the lower density in the hot gas (pushing it up). You also know that the flame follows the wind, which further substantiates the fact that the observed color is, in fact, afterglow kind of effects that come a short time after the main reaction. The dancing of the flame is the same type of movement you will see in any gas with different temperatures or subject to a heat source, which is a great breeding ground for turbulence.

Fire can not be contained and kept indefinitely. It is only there as long as the reactants are present. I mean, you have to have both the "fuel" and oxygen for the case of common fires. One claim that could be made is that fire is a gas that is actively emitting those colors. This may fit some people's definition for fire, but not others. It should again be stressed that a gas meeting this criteria must be actively loosing energy, meaning that it can not exist without constant energy input. Others may focus on the reaction itself, saying that fire is defined by the reaction, so your car uses fire. A favorite term of mine regarding nuclear power plants is the "nuclear fire", indicating that self-sustaining reactions in general are fire-like to some people.

This is how I slice it: I feel comfortable saying that fire is matter emitting energy in a self-sustaining reaction, or the energy that comes from such reactions, or the reaction itself. I see that giving answers of "matter releasing energy", "an energy flow", and "a process". I hope this helps clarify why physicists will often answer "neither" to the question.

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welcome to the site.

In your profile you do not state your background , but from your question I assume that physics is not your strong point.

In this Wiki article fire is described in physics terms, so I will not repeat it here. Wnoise is correct that fire is a process, so your question is more appropriate to "flames".

From the article you can see that flames are gases in various temperatures while molecular interactions increase the kinetic energy of the molecules and are also releasing radiation which goes into heat and light.

A flame has both matter and energy.

Matter is the molecules of the gas, Oxygen, Carbon, which combine and release radiation by combining, which radiation heats the whole gas/air mixture.

Energy in the flame is in the form of heat, i.e. kinetic energy of the molecules,plus in the form of electromagnetic radiation, photons, both infrared and visible.

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""while chemical reactions are releasing radiation which is transformed to heat and light"" Please rethink that part. –  Georg May 11 '11 at 9:20

It's a bit like asking if Concrete is Matter or Energy.

Fire is composed of several states of energy and matter, like sublimated matter turning into gases; or photons released on oxidization of matter; or sound is released via turbulence from convection; plasma occurs whenever the temperature ionizes atomic bonds and Sooty.

There are several papers on the subject e.g. "Yule Log Science" http://www.sciencentral.com/articles/view.php3?type=article&article_id=218392881

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since fire is a combinations of various gases, chemical reactions,light and heat. matter can be defined as any thing which occupy space and have mass or weight. if we see toward fire then we can say that fire is a matter because fire can occupy space and a mixture of gases which must have a mass.

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My first mental picture of this question is like 'what is the flame dancing there', for which I believe is the atoms of the burning material carried by the lifting air(or by the wind, in the presence of... the wind). Most, if not all of the answers above seem to me have explained why there should be radiation when something is burning. But if somehow, either the material is too 'solid' or the convection is not so strong, the atoms refuse to leave the body, there will be no flame then.

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