[sapta]

i was thinking about the question about burning of water and got stuck at this one.WHAT IS FIRE?

[Donaldson Tan]

fire is the sensual (sense of touch and sight) manifestation of heat and light energy from combustion/oxidation reaction.

[hmx9123]

Fire is the fourth state of matter that we don't think about often: plasma.  Fire is a low-temperature plasma.  Kinda cool.

[Donaldson Tan]

i thought plasma is fluid matrix of ions and electrons. it doesnt agree with free radical chemistry of combustion.

[hmx9123]

Hmm.  I was told by my HS Physics teacher (who incidentally was a PhD who used to teach at University of Moscow--figure that one out) that it was a low temperature plasma.  However, he may have been incorrect, or simply dumbing things down for us to understand.  We need a P-Chemist to answer this.  Maybe JDurg will read this eventually.  Anyway, here's the definition of a plasma from the 'net:

an electrically neutral ionized gas in an electric
        discharge; distinctly different from solids and liquids
        and normal gases
(physical chemistry) the gaseous state of hot ionized
        material consisting of ions and electrons and present in
        the stars and fusion reactors:  sometimes regarded as a
        fourth state of matter distinct from normal gasses

Hmm, here's an interesting site:

http://www.straightdope.com/columns/021122.html

The strange thing is, a good flame (not this crap you get when a log burns, but like an oxygen-methane torch used for glassblowing) is ionizing.  It's got an oxidizing and a reducing part of the flame where electron transfer can occur.  Perhaps this article isn't talking about welding type torches.  This is quite an interesting phenomenon and definitely warrants further discussion.

[Donaldson Tan]

hmx: i thought you just got your PhD. Isn't it a p-chem PhD?

[jdurg]

I really wouldn't think of a fire as plasma because of one simple thing;  Can a fire conduct electricity?  A plasma has always been explained to me as a gas made completely of ions, therefore it is kind of like a gas but has far different physical properties.  (Hence why it's the 4th state of matter).  As it is a mass of free-flowing ions, it can therefore conduct electricity.  I do not believe that fire is able to hold an electric current.

[Donaldson Tan]

if fire is indeed a low temperature plasma, then perhaps it's plausible to generate energy by having heat resistant electrodes to tap energy from the flowing electrons..

[hmx9123]

No, I didn't just get my PhD.  I just passed my qualifying exams.  There's a big difference.  And it's not in P-Chem.  I'd have quit long ago if that was the case.  I hated P-Chem, and really have no love for it even now.  I guess it's a necessary evil, what with the quantum mechanics and all... ha!

[xiankai]

if fire is indeed a low temperature plasma, then perhaps it's plausible to generate energy by having heat resistant electrodes to tap energy from the flowing electrons..

i think we can already get heat energy from fire

i believe the ionising affect is due to electrons gaining energy and thus leaving the atom, but thats my pure speculation

[Dude]

I would definitely not classify fire as a plasma.  Fire is basically as geodome and jdurg indicated, energy in the form of light and usually heat emanating from a free-radical type oxidation (heat is optional because there are "magician" cases of room temperature fire from alcohol that can be touched).  Radicals do not necessarily imply ionic charge or the conductance of electricity.  There is probably a fine line between fire from plasma at high temperature, however, distinguishing the chemical analytical methods ICP (inductively coupled plasma- which only results from RF coil excitation and flame) from AA (atomic absorption-strictly flame based excitation) should explain some of it.  I can't remember the details of the methods anymore.