Fire

[Cutter]

I got to thinking the other day, just what the heck is fire? Is it a solid, liquid, gas, plasma? None of the above? Any one care to enlighten me? :)

[Godless Wonder]

I'll take a stab.

It's oxygen, rapidly combining with stuff, and in so doing, releasing energy, in the form of heat, and light. In addition, often various (hot) gasses are produced, e.g. CO, CO2, etc. The hot gasses, being less dense than the surrounding air, rise upwards.

Perhaps the light emitted in the flames is merely oxygen continuing to combine with various gasses, etc., as they rise upwards. Or perhaps it's merely (or partly) "black body radiation" emanating from the superheated particles of (what will be, when it cools down a bit) smoke. But now, I'm speculating.

Here's what the straight dope (http://www.straightdope.com/columns/021122.html), the first hit that google gives for "what is fire", has to say.

Unfortunately the link to a picture of zero-gravity fire (a candle flame, described a spherical and blue) doesn't appear to work.

[liquid]

Do you mean what is fire, or what are flames? They aren't the same question!

[Godot]

What I recall from physics class is that fire acts much like a fluid does. Whether that is because it is a fluid or simply mimics one, I don't know.

[tommyc]

Originally posted by liquid
Do you mean what is fire, or what are flames? They aren't the same question!
I'm assuming Cutter was asking about flames, but I'd be interested to hear your definition of both.

[Sven]

Originally posted by Godless Wonder
Perhaps the light emitted in the flames is merely oxygen continuing to combine with various gasses, etc., as they rise upwards. Or perhaps it's merely (or partly) "black body radiation" emanating from the superheated particles of (what will be, when it cools down a bit) smoke. But now, I'm speculating.

As far as I remember from my basic chemistry education, the yellowish light of camp fires/candles/etc. emerges from excited (by the heat) carbon atoms. The blueish part is IMHO black body radiation.

As an aside: not only organic material can catch fire, but for example also metals. Kind of ultra-fast corrosion :D

[liquid]

To copy and paste....

Fire is:

*A rapid, persistent chemical change that releases heat and light and is accompanied by flame, especially the exothermic oxidation of a combustible substance.*

Flame is:

*The zone of burning gases and fine suspended matter associated with rapid combustion; a hot, glowing mass of burning gas or vapor.*

I'd go one step further to note that the phenonemon we describe as flame is actually a few different phenomena that produce the visual effect we see. But basically flame comes down to gas & plasma.

[einstein314emc2]

The color you see from flames is from the exicited electrons in the atoms. As the fall back into their normal energy state, they emit radiation as light. Actually, the atoms of your body are also emiting radiation, bur since they are much cooler, it is emitted as infrared. Its the same reason hot stars are blue, and cool stars are red.

[liquid]

If you want to get really pedantic, almost all the visible light you see from your body is a result of electron excitation and radiation emission as well...

[WinAce]

Originally posted by Godless Wonder
Unfortunately the link to a picture of zero-gravity fire (a candle flame, described a spherical and blue) doesn't appear to work.

http://images.search.yahoo.com/search/images?p=microgravity+candle&n=30&fl=0&fr=fp-tab-web-t

[dr_zukov]

New Scientist's Last word articles are the best source for all the really important questions.

For example: Do fish fart? Why is snot green? Does beheading hurt? ...

And What is fire made of? (http://www.newscientist.com/lastword/article.jsp?id=lw426)

[Igor Trip]

Fire is the release of energy from atoms as they combine or recombine from less stable to more stable forms.

The energy is stored in the outer electrons, which can occupy various orbits around the atom. The higher the orbit, the greater the energy.

Take the helium atom. The two electrons are well balanced and will occupy the lowest possible orbit. The hydrogen atom on the other hand with just one electron is unbalanced (imagine a hammer thrower), so the electron cannot drop to the lowest level. Instead it occupies a higher level where the electron has more energy.

When the hydrogen atom combines with another atom (by hitting it at high speed),
it becomes more electrically stable (which is the reason it combines) and the electron can drop to a lower level releasing the excess energy as heat (velocity) and light.

The released energy is absorbed by other atoms which can then themselves combine and release more energy in a chain reaction.

This also explains why you cannot use water as a fuel despite it being hydrogen and oxygen. There’s no energy left to release.

[phil]

fire is a chemical reaction. To be more specific it is an oxidation reaction. The reaction is usually not considered fire unless it is rapid and produces plasma (flames).

If you have ever seen and old book you might note that its pages are yellow, and really old pages are fragile. This is because this oxidation reaction happens with paper at room temperature, but it acts very slowly. Only when a certain temperature is reached is when 'fire' breaks out.

*laugh* I was just thinking. From the time a house is built, it literally is burning down.

The flame of fire is quite interesting in that it has a positive charge. Take a VanDeGraff generator and hold up a burning match or birthday candle up to the terminal. What happens at the postive terminal? the negative?

I can actually put out fire with one terminal of my generator. Try it to see which one does it.

Could this be a new firefighting method?

"Here's the extinguisher cable John. Just be careful of the 500,000 volt spray of ions at the end." :)

-phil