Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What fire is?

  1. Jun 27, 2003 #1
    This may be a stuppid question. I don't realy care though. I realy am haveing hard time understanding what fire is? The flame of light. There are certain things I know, like it needs fuel and oxygen and heat to start. Once it is started it will just live on its own until something runs out. What is it? Just the result of matter turning into energy or what. I know that differant colours mean differant temperatures etc. or differant types of elements. I understand the light is a frequency spectrum of radiation. Maybe this is all there is to it? Another thing, I have heard that fire does not burn the oxygen it only needs it to exist for some reason, why is that or is that true?
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. Jun 27, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Fire is a chemical reaction. Specifically, it is oxidation- that is, oxygen combining with other chemicals. Oxygen has such a strong tendency to combine with other chemicals it is remarkable (and fortunate) that there is so much uncombined oxygen in the air!

    Oxidation is an "exothermic" reaction- that means that it gives off energy. A lot of energy tends to excite electrons which then give off electro-magnetic energy- light. Specific amounts of energy (heat) tend to give off specific frequencies (colors) of light: the hotter the more different frequencies so the whiter the light.

    By the way, this NOT "matter turning into energy" in the e= mc2 sense. It is the energy binding the atoms in the compounds that is released.
  4. Jun 27, 2003 #3


    User Avatar
    Science Advisor

    Side Note:

    It is also interesting to note that any oxidation process can be called a "fire". So when you see a piece of metal that is rusting (oxygem bonding with metal) it can be said that the metal is "on fire". During the process of rusting, metal gives off EM radiation, but at such low energies that none of this EM is in the visible range. Nevertheless, heat is produced like with any other fire, just nto very much of it.

    I would not recomand pointing at passing vehicles and yelling at the top of your voice, "your car is on fire!" just because you have spotted a bit of rust, but the statement would be technically true.
  5. Jun 27, 2003 #4
    i must try that sometime lurch. i can imagine the number of worried lookind drivers in my area, hehe but nevertheless good points made here, and youve left me with nothing technical to explain.

  6. Jun 27, 2003 #5


    User Avatar

    Just to add to what everybody else has said and to make sure I have the concepts right myself (and please correct me if I'm wrong). Like HOI already stated molecular oxygen(g) meets w/ carbon, oxidation occurs, and yields carbon monoxide, carbon dioxide, etc,. These are the molecules that are mostly concentrated w/n the (candle) flame. Upon the absence of either oxygen or carbon the flame will die off b/c the rxn will no longer occur.

    Some force acting on it maintains an oblong shape, in the case of a candle light, w/ a definite border which does not just gradually taper off. Since the molecules are in a gaseous state, it's kinetic energy is too high for dipole-dipole interactions to take place among polar CO/CO2* molecules. I believe that it is the surrounding air pressure that is responsible for 'holding in' the flame trying to collapse the flame inward meanwhile the high energy contents w/n the flame push outwards. Is that right or am I going wrong somewhere?
  7. Jun 27, 2003 #6


    User Avatar
    Science Advisor

    The force that makes candle-flames oblong is bouyancy. The hot air of the flame attempts to rise, and as the amount of energy available decreases, the flame tapers as it progresses upward away from it's starting point. This process also keeps a fresh supply of O2 flowing into the base of the flame. In this way, one can almost see the tapered shape of the candle's flame as a timeline diagram of the consumption of that energy released by the chemical reaction happening at the base.

    A candle on the Space Shuttle burns as a nearly perfect sphere. This ball of flame pulses as Oxygen is used up and the flame dies down, heat to drive the expansion of air stops, air rushes in to fill the vacuum thus created, and the influx of fresh air re-ignites the flame. Looks really cool!
    Last edited: Jun 27, 2003
  8. Jun 29, 2003 #7
    By the way, this NOT "matter turning into energy" in the e= mc2 sense. It is the energy binding the atoms in the compounds that is released.

    I was wondering if you could elaborate on this a little more please. Thanks.
  9. Jun 29, 2003 #8
    Energy is from bond breaking and bond forming

    In oxidation(or other kind of reactions), in order to trigger a reaction, energy is given to reactants to break the bonds (the minimum amount of energy needed is called the activation energy) After energy is given to reactants, they become unstable and tend to form bonds. During bond forming, energy is given out. If the energy given out during bond forming is greater than the energy needed to break bonds, then the process exothermic.

    Take oxidation of methane as an example.
    CH4(methane) + 2O2 ----> CO2 + 2H2O

    Energy is needed to break the 4(C-H) bonds in methane and the double bond in oxygen. Energy is released while forming carbon dioxide and water. Energy released when forming CO2 and water is larger than energy given to bread bonds in methane and oxygen, therefore it is an exothermic process, and energy is given out.
  10. Jun 29, 2003 #9
  11. Jun 30, 2003 #10


    User Avatar

    Thanks for clearing that up Lurch:wink:. I have another question though that I can't figure out, why does the flame go out when you blow hard on it? My guess is it's b/c your blowing CO2 and not molecular oxygen, is that correct?
  12. Jul 18, 2003 #11
    The flame goes out because you blow it away, and without the flame there isn't enough heat to keep the reaction going.

    Consider a relighting trick candle: you blow it out, but there is still some vapor coming off of the wax. Then a little spark from the wick re-ignites the vapor.
  13. Jul 18, 2003 #12
    Re: Energy is from bond breaking and bond forming

    Sounds like chemical version of fusion chain-reaction..
  14. Jul 18, 2003 #13
    Don't forget, everytime you have oxidation, you have reduction, so in a fire you have reduction occuring too.
  15. Jul 18, 2003 #14


    User Avatar

    Staff: Mentor

    Re: Re: Energy is from bond breaking and bond forming

    Essentially yes. Given enough reactants and waste removal, an exothermic chemical reaction is self-sustaining.
  16. Jul 21, 2003 #15
    Could someone please straighten out this little misunderstanding for me? Thanks.

    When a fire ignites, is this due to a combustion reaction or a oxidation reaction? Or is Oxidation just another word for Combustion? From what I do understand, oxidation keeps the flame/fire going, as it burns oxygen, but is combustion the same thing?

    I appreciate any and all input.
  17. Jul 21, 2003 #16
    Combustion=Oxidation. Same thing. Just when you see metal rusting it's so slow and only a few atoms at a time, that there is no heat that one can notice without a closed system and good instruments.

  18. Jul 23, 2003 #17
    Re: Re: Re: Energy is from bond breaking and bond forming

    Sustaining fire is exothermic, ie it produces more heat than it needs to keep the process going. When happens that less heat is produced than needed, it dies out. Usually, its because of environment, including waste. Any fire normally sustains because of balance between front where it produces more heat than needed and where due to conditions that heat is not enough. This is flame border.

    I wonder about hypotetical 'fire', where amount of heat needed to sustain reaction is exactly equal to heat produced by reaction. Ie. all energy released by reaction is immediately used up. Suppose waste is no issue and all needed reactants are within fuel itself. Would such flame be 'cold'? Would size of such flame have extent of single molecule size?
  19. Jul 23, 2003 #18


    User Avatar

    Staff: Mentor

    Re: Re: Re: Re: Energy is from bond breaking and bond forming

    No, thats just a reaction that is balanced. A candle for example. It doesn't constantly get bigger: once lit, it stays the same size.

    Again with the nuclear chain reaction analogy: There are 3 possibilities:

    1. Below critical mass and the reaction doesn't start. (a lump of uranium that isn't doing anything).
    2. Right at critical mass (a controlled reaction such as in a power plant).
    3. An uncontrolled, geometrically increasing chain reaction (until fuel runs out) such as in a bomb.

    A candle (and what you said) is number 2.
  20. Jul 24, 2003 #19
    Is it? Candle has flame that produces more energy than is needed to sustain its burn. It has balance, but it is not exact, it occurs because flame cools and reactant burns out at flame edges. Only fraction of energy is used to sustain the reaction. Excess energy of candle goes into environment. Its different from what I meant.

    I meant more like exact equality of energy needed to cause molecule reaction and amount of energy such reaction releases. Uranium or candle can't do that. heat energy moves away from reaction point. But if some matter could have such balance, that no expansion from heat, and all energy be consumed by nearby molecules (maybe electrochemically?) then I imagine flame front would be only few molecules thick. It would be very unstable balance, but linear response to environment.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: What fire is?
  1. What is fire really? (Replies: 4)

  2. What is Fire? (Replies: 12)

  3. Fire - what is? (Replies: 12)

  4. What actually is Fire? (Replies: 1)

  5. What exactly is fire? (Replies: 17)