Sunday, April 3, 2011

What is Fire?

Fire is a chemical reaction.  Fire is quantum mechanics.  Fire is awesome.  As a very lay scientist I'm going to do my best to describe what is happening in this reaction.

Starting ingredients for fire:  fuel, oxidizer (like oxygen), and heat.

First we need to heat the ingredients to a sufficient energy to start the cascade--think spark, etc.

This heat is used to break the bonds of molecules.  With gasoline or wood the large hydrocarbon molecules are broken up into their subsequent parts.
Counter to what you might think it takes energy to break bonds.  Making bonds produces energy.  This is where things get exciting.  The gas produced by the chemical reaction then reacts with an oxidizer (like oxygen) to produce new bonds.  The act of producing these new bonds creates an enormous amount of heat.

This heat does several things.  First, it perpetuates the reaction.  The heat causes more bonds to be broken, which causes more bonds to be made (since there is more raw material then), which causes more bonds to be broken, to be made, to be broken...

Heat has another by product, though--molecules collide more often and with greater energy.  Since atoms aren't just balls of stuff, they also have charges (like the negative charge for electrons and positive for protons) they don't just hit with each other--they react with each other.

I'll fully admit this analogy may be incorrect since I found pitifully little on it online, but I almost understand these interactions to be like sloshing.  One atom/molecule bombards another atom/molecule causing their negative charges to repel each other.  At some point these interactions cause an electron to jump up an orbital.  In my mind I think of it as being on the opposite side of the atom as the bumping of negative on negative happened (This may be completely wrong and probably is. hehe).  This excitation of electrons to an outer orbital is  only temporary and the electron eventually relaxes back into its ground state and as it does something mysterious happens--a photon is thrown off with a certain amount of energy/frequency (based on how high it fell from and what kind of orbital it was/is in).  That's how light is produced in fire.  And, no, this makes no sense whatsoever.  We don't know why going down an orbital produces a photon.  It just does, okay!

If you think about it, fire producing light isn't as weird as you initially think...Or, actually it is just as weird as it ever was, but it's much more common than we generally appreciate.  Take for example--infrared.   As you well know from spy shows we are constantly emitting infrared electro-magnetic radiation.  Let me remind you what that means: we're emitting photons of energy just below the visible spectrum all the time--and so is just about everything else above absolute zero.  Ever have someone say you look like there's a glow about you?  That you look radiant?  You are!  Literally!  Heat up something and the photons emitted start to have more and more energy until they finally start to reach an energy level that we can see in the visible spectrum.  Fire produces light for the same exact reason that incandescent light bulbs do, lava, heated metal, your toaster/oven does--heat (the bombardment of atoms and the subsequent excitation of electrons and then relaxation which produces a photon).  The heated substance, however, in this case is soot and gas.

You're like Dr. Manhattan, but redder--infraredder.

In summary:

The making of bonds produces heat.

The heat produces light by atoms colliding causing electrons to become excited and then return to a ground state emitting a photon.

Other neatness:  

The first fossil record of forest fires is from 420 million years ago towards the end of the Silurian period.  The things that were missing prior to that were plants on land (470 mya) and then for oxygen levels to be above 13%.

Fire goes up because the heated gas is less dense than the surrounding air.  What would happen to a candle flame if there was little to no gravity??

Pictures from here, here, here, here, here, here, here, here, and here.

Works Sighted [sic]:


  1. Great website, there is a lot of information here, also I love the photos
    Professor Willstrad of new hartford community college

  2. Thanks so much! While I write for myself--to learn and grow--it gives great pleasure to my soul to know that other people read this blog on occasion!


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