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Is fire matter or energy?

What is the basic form of fire? physics defines every entity by a basic form either solid or liquid or as a gas,

example: water is liquid, ice as solid, water vapor: gas

so what is fire? when I queried is 'Plasma a fire?' I landed in below link, where it is said that fire isn't plasma, but not defined which basic form it belongs to!

the link: Is fire plasma?

What I understand is, it's a form of energy caused by reaction of gases(?) so it's basically a gas! is it true?

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Rookie Programmer Aravind
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4 Answers4

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Fire is a reaction between molecules in gases. It may look as if a piece of wood is burning, but actually the burning happens in gases given off by the wood as it is heated.

Burning wood, paper etc is a complicated business, so let's take a relatively simple system like burning the gas in your cooker (assuming you use a gas and not electric oven). Actually even reacting gas (methane) with oxygen is a multistep reaction, but basically a methane molecule and oxygen molecules collide, react, and the reaction products split apart with more speed than they started with. The extra speed of the molecules comes from the energy liberated in the reaction.

So the products of combustion are gas molecules (mainly H$_2$O and CO$_2$) moving at high speeds, and in a gas the speed of the molecules is related to the temperature. High speeds mean high temperatures. In other words a flame is just a hot gas.

But when we think of fire we think of the glowing flames. The glow comes from two sources. By far the most common source of the glow is when the molecules containing carbon do not fully burn but leave behind tiny particles of carbon i.e. soot. The particles are heated by the hot gas and they glow just as anything glows when it gets very hot. The yellow/red colour of flames is due to these glowing particles of soot.

The other source of the light happens because the energy (i.e. the temperature) of the combustion products is not all the same. At any temperature the energy of gas molecules is distributed according to the Maxwell-Boltzmann distribution and a small fraction of the gas molecules can have exceedingly high energies. High enough in fact to cause ionisation of other gas molecules, and light emission as they recombine.

It's this ionisation that leads some people to talk about a flame as a plasma. However you need to bear in mind that only a tiny tiny fraction of the combustion products are ionised, so it isn't a plasma in the sense that the Sun contains plasma.

John Rennie
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The basic form of fire is what we perceive which is simply heat and visible light (as we can't see IR). It's not itself a matter... But, instead it's a natural process. Perhaps, it will not completely convert matter into energy because some will be left as the products of the flame. Fire is really a chemistry kinda thing involving rapid combustion (an oxidation - probably exothermic) of flammable materials to produce heat and visible light observed and felt as Flame.

Mostly, fire from your match stick, candle, cigarette, etc. won't be hot enough to form a plasma because all the above are partially ionized. But, there's a possibility for it to become a plasma when it reaches the completely ionizing threshold. The characteristics of fire (or flame) observed depends on the nature of fuel or combustible. The mechanism of fire generally propagates as a chain reaction starting when the temperature of the combustible goes above the threshold ignition temperature.

You're quite right. Hot gas may come as a by-product or partial products of the chemical reaction. You could find out more already asked here...

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Waffle's Crazy Peanut
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Fire usually consists of something that burns (often solid - wood or coal, say) and the flames accompanying the burning. The flames consist of hot reacting gases, which emit light in the frequencies from the emission spectrum of the hot molecules and radicals, giving rise to colors depending on the chemical composition. (For more details and references see the Wikipedia article http://en.wikipedia.org/wiki/Flame)

The emission spectrum is created from excited electrons typically returning to their ground state; the frequencly is determined by the energy difference released, and is specific for each chemical substance. This is being used in flame emission spectroscopy to determine the chemical composition of simple substances. See http://en.wikipedia.org/wiki/Flame_emission_spectroscopy

Arnold Neumaier
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The part of fire that you can see is light, which can't exactly be described as matter in and of itself, so it doesn't have a fully defined material state. It's akin to asking whether a radio wave is a solid, liquid or gas - it doesn't fit that model at all.

However, if we are to relax the semantics a little, the matter that emits the light is a hot gas. You might also find some plasma in there, depending on how hot the burning material is.

Polynomial
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