Tuesday, December 14, 2010

Why Fire is Cool - entry #1 - What are Flames Made Of?

If I were to start a "why fire is cool" list, it would probably reach a length to rival my "ultimate band name" list.  For the sake of brevity however, I will just share the 4 best entries on the fire list, beginning today with entry #1, a blazing introduction into the nature of fire.

Maybe you've had the experience of sitting around a campfire and being unable to peel your eyes away from the smoldering coals.  When this happens to me I have often found myself wondering what the hell IS fire?

Hot coals for staring at.
A question mark made of fire.

I'll start with the easy things that you may already know:

1 - The logs I throw on the fire are the fuel, made mainly of molecules like cellulose, which contain carbon, hydrogen, and oxygen atoms.
2 - The fuel is reacting with the oxygen in the air, and turning into carbon dioxide and water, like so:
     C6H10O5 + 6 O2  6 CO2 + 5 H2O + energy
3 - As shown above, reactions like this release energy, which we can use for various things:

The recent launch of the SpaceX Falcon 9 rocket.

But what makes up the flame?  You can't put a flame in a bottle like you can with the fuel or the oxygen, so what the hell is it made of?  To answer that question, let's take a look at a simple flame we're all familiar with, that of a candle:

A lit candle, duh.

The candle flame clearly has two main sections, a blue section towards the bottom and an orange section towards the top.  WHAT IS THAT STUFF?

To explain the source of a flame's light, we'll follow the wax.  The wax is melted by the energy from the fire and the molten wax is pulled up the wick via something called capillary action.  That's why the wick is there, to help pull the wax molecules toward the flame!

As the molten wax flows up the wick, the wax molecules get hot enough to become a gas and jump off the wick and into the flame.  After they do this, the energy from the flame breaks apart the chemical bonds that were holding the atoms of the wax molecules together.  This part is different from combustion; the wax molecules haven't combined with oxygen molecules yet, they have simply been blasted to little bits.  The little wax molecule bits have electrons that used to be happy in their big wax molecule bonds, but now these electrons are not bonded to anything, and are known as free radicals.  It's the movement of these free radical electrons in the little wax molecule pieces that gives rise to the blue flame color.  In your everyday life, this is probably the only time you ever see these types of molecules and electrons.

These wax molecule bits with their blue-color-producing electrons are extremely unstable.  From this point free radicals have two choices about how to become more stable.

1 - They can combine with oxygen to form CO2 & H2O.
2 - If they can't find oxygen, they will clump back together with one another, bonding together in all sorts of crazy ways to make all sorts of crazy new molecules.  These crazy new molecules are called soot.

Soot molecules being collected from a candle flame.

These really hot molecules are what make the orange of the flame.  They are orange because they are so hot from all the energy being released around them.  This causes their atoms and electrons to move around in such a way that they produce a glow known as blackbody radiation.  This is the same type of light-producing mechanism found on a red-hot electric stove.

An electric burner glowing red hot due to blackbody radiation.

Overall in a flame, the atoms from the wax molecules combine with the oxygen atoms in air and turn into carbon dioxide, water, and some soot--but most importantly this process releases energy .  It's this energy that continues the combustion reaction of new molecules--blowing new wax molecules to bits and making them shine blue, and heating the crap out of the soot molecules and making them glow red-hot.  So while its the wax molecule bits and the soot molecules that actually release the light of the flame, its the energy that is released from the combustion reaction that makes it all happen.  In the end I guess the answer to my question was quite simple, flames are made of energy.

Related Posts:
FirePost #2: "How Charcoal Changed the World"
FirePost #3: "Ash Ash Baby"
FirePost #4: "Ancient Energy Unleasher"


  1. Brilliant. So the orange part of the flame is just soot buring? Weird.

  2. @Kaber, actually it is not soot burning, it is soot glowing red-hot right before it burns

  3. why is a stove flame blue? is there a free radical situation there too?

    p.s. in doing a series on fire you must include your burning shoes.

    1. Less soot produced, cleaner fuel source.

  4. @Sarita, yes, natural gas from your stove is similar in structure to wax, the carbon chains are just shorter. So, you end up with the same types of free radical molecule bits in the flames.

    Excellent suggestion with the shoes, those pictures exist somewhere, I will dig them out when I'm home for xmas.

  5. Wow, so we've never really seen anything burn, we just see the preamble to the actual reaction? Is most of the energy released heat or light or something else?

  6. @Andrew. Well, without the actual burning, there would be no energy to keep everything going, so it's kind of hard to dis-entangle things.

    Heat is released in the form of infrared radiation, which is just like visible light only of a longer wavelength. So all the energy is released as some form of light, but some is visible, some is not.

    As for the distribution, most of the energy released is released as blackbody radiation (the orange part), so assuming you can neglect any other type of energy release (this may be a big assumption, i'm not sure), according to this blackbody radiation graph:
    And knowing the temperature of a candle is around 1000 degrees C, the vast majority of the energy is released as infrared radiation, or heat.

  7. wow! great post. i'm looking forward to the rest of the series, especially the part where you explain why or why not fire is considered a living thing. keep it coming...

  8. What is also cool to think about is that the flame is technically a "plasma".

  9. @Mitch A. I've seen general reference to typical candle flames being a plasma, but have found no specific mentions of exactly what the ionized species are, aside from them being the ionized alkyl fragments. It seems the biggest question is whether a candle flame is hot enough to ionize air. The most obvious thing present would be nitrogen, though I'm not exactly sure how to take nitrogen's ionization energy and the flame temperature to see whether it is indeed ionized... just doing a back-of-the-envelope calculation using the boltzmann equation it comes out that only an insignificant fraction of nitrogen molecules would be ionized, though I'm not sure if I'm doing that right. Thoughts???

  10. I think you're right that the ionized species are organic fragments from the fuel (wax, wood, etc).

  11. I've been a fire captain 31 years and thought I knew my enemy well until I read this excellent article. Outstanding. Unfortunately with some science, the more you know, the more you realize what you don't know. That is the paradox...answers simply lead to more questions and we become students for life!