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Can Water be flammable?

  1. Mar 22, 2010 #1
    Hi all,

    im aware the moon, orbiting saturn, titan has such an atomsphere and is cold enough for methane to exist as a gas, liquid and a solid mass, this sparked a question in my head...

    would there be any moon/planet where conditions would make water flammable? i know this is most prob a dumb question, but here on earth methane as you all know is highly flammable. its just something i wondered could exist at all..

    some thoughts please guys

    EDIT: I just found my answer, Water is already burnt so there fore would not be flammable :)
     
    Last edited: Mar 22, 2010
  2. jcsd
  3. Mar 22, 2010 #2

    Chronos

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    Water is flammable at temperatures high enough to decompose it into hydrogen and oxygen [e.g. a metal fire].
     
  4. Mar 23, 2010 #3

    russ_watters

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    No, water is not flammable. Water is a waste product of combustion. In a metal fire, as the name implies, it is the metal burning, not the water. The water provides the oxidizer:
    http://en.wikipedia.org/wiki/Magnesium

    Magnesium's combustion reaction is so energetic that it can afford to give up some of its energy to "unburn" the water and steal the oxygen while keeping the overall reaction exothermic.
     
    Last edited: Mar 23, 2010
  5. Mar 23, 2010 #4
  6. Mar 23, 2010 #5

    mgb_phys

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    Not sure if that counts as burning.

    But if you want a really fun oxidiser try Chlorine Tri-Flouride http://en.wikipedia.org/wiki/Chlorine_trifluoride, it burns asbestos!
     
  7. Mar 23, 2010 #6

    russ_watters

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    That's exactly the same phenomena as what I mentioned for magnesium: the activation heat splits the water into hydrogen and oxygen and then the flourine burns with the oxygen. I wouldn't call that water burning, since burning is one element combining with oxygen in an exothermic reaction. At best I would call that water providing the oxidizer for flourine to burn with.
     
    Last edited: Mar 23, 2010
  8. Mar 23, 2010 #7
    I haven't done any chemistry since high school, but IIRC fluorine is a halogen, so the reaction should produce HF and O2 rather than H2 and F2O, right?

    As to when to call something flammable, the answer depends on whether flammable is used as a technical or as a colloquial term, I'd say. I seem to remember my chem teacher using "combustible" and something really awkward like "combustion-promotive" when he wanted to be extra-precise about distinguishing between oxidizable and oxidizing substances.

    *shrug*
     
  9. Mar 23, 2010 #8

    russ_watters

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    You're right - my chemistry isn't that strong and I misread where they listed the reaction in that link: H2O + F2 = 2HF + 1/2O2

    I'm not sure what you call that reaction, then. For technical vs colloquial, I'm not sure what the OP was really thinking, but then s/he probably wasn't either!
     
  10. Mar 24, 2010 #9
    If you think thermodynamics, combustion is the complete decomposition of a substance into Carbon Dioxide,Water and metal oxides/Hydroxides,etc.

    Thus, as water is an end product, you can't really call it combustible.Like the guys said, reacting water with substances like sodium,magnesium, or even yellow phosphorous can't really be called burning it.

    Also, even though methane can be found in solid,liquid and gaseous states on the moon, it is still combustible. Change in state doesn't guarantee a change in properties of the substance.
     
  11. Mar 24, 2010 #10

    mgb_phys

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    No combustion is the (rapid) oxidation of a substance.

    Carbon combusts into carbon dioxide, CO2 can then be reduced back to C and O2 by a more powerful oxidizer but that isn't combustion.
     
  12. Mar 24, 2010 #11

    DaveC426913

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    So, if burning is defined as oxidation, does that mean you could turn water into peroxide (H2O2)? :biggrin:
     
  13. Mar 24, 2010 #12

    mgb_phys

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    And would O2 -> O3 or O3 -> O2 be 'burning' ?
     
  14. Mar 24, 2010 #13
    Again, my memory is a bit hazy, but I think oxidation in this context doesn't necessarily involve actual oxygen-atoms, but is used as a generic term for a certain type of reactions/chemical bondings.
     
  15. Mar 24, 2010 #14

    mgb_phys

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    yes oxidation is basicaly adding electrons, something more reactive than oxygen (like fluorine) can out-oxidize oxygen.
     
  16. Mar 29, 2010 #15
    The kind of reaction you're discussing here is a displacement reaction, a highly reactive element kicking out a less reactive element from a compound and taking its place (like the famous thermit reaction). Oxygen tends to react strongly with many other substances because it needs a couple of electrons to fill its outer shell and is very good at getting them, to the point that the only element that is better at it is fluorine.

    When fluorine reacts with water, rapid oxidation (combustion) of the water will occur and the water will burn with a very hot flame, producing oxygen and hydrogen fluoride. If someone was watching and didn't know what substances were being used then they would probably assume that some very flammable fuel was being burned in plenty of oxygen. The fact that hot oxygen gas and even some ozone are being given off as the water goes up in flames can make the whole thing even more spectacular.
     
  17. Mar 30, 2010 #16

    russ_watters

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    You forgot the other component of what "burning" is: it is exothermic. (also, typically "rapid", which would tend to exclude rust)
     
  18. Mar 30, 2010 #17
    To keep using this thread as a chemistry refresher: Aren't all exothermic reactions rapid? My thinking is that once favourable conditions exist for the reaction to begin (e.g. once activation energy is supplied), any exothermic reaction should be self-sustaining until one of the reagents is depleted. Or is there more to it?
     
  19. Mar 30, 2010 #18

    Chronos

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    The problem with burning water is it does not produce enough energy to be self sustaining.
     
  20. Mar 30, 2010 #19

    DaveC426913

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    Are you asking if all exothermic reactions are rapid, or if all exothermic reactions are self-sustaining?
     
  21. Apr 12, 2010 #20
    Not all exothermic reactions are rapid (think rusting or the setting of concrete), but brutally exothermic reactions tend to go very quickly, especially if gases are involved. This is due to the energy supplied by the reaction raising the kinetic energy of particles in the system, and so increasing the frequency and violence of collisions. There are exceptions to this, like the dehydration of sugar by concentrated sulphuric acid, that involve a high activation energy and so keep the rate of reaction down even at high temperatures.

    Exothermic reactions are generally self-sustaining, but if their rate drops below a critical level then the system may be able to disperse the heat of reaction more quickly than the reaction can replace it (think of a match sputtering out before the flame has consumed all of the wood).

    Some exothermic reactions will go to completion whereas others form a dynamic equilibrium in which the rate of the reverse reaction gradually catches up with the forward reaction as the concentration of the products increases, although if the products are able to escape then this isn't a problem and the reaction will go to completion anyway. The reactions that don't form an equilibrium are both exothermic and also involve a rise in the entropy of the system. Those reactions that are exothermic but decrease the system entropy will form an equilibrium. Also, they will not be spontaneous above a certain temperature at which the effects of entropy come to dominate the effects of the enthalpy change.
     
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