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Why is water not flammable?

  1. Oct 2, 2008 #1
    why is water not flammable, because H20 is hydrogen and oxygen. I have learned that hydrogen is very flammable and oxygen speeds up a fire. so wouldn't that mean that it is very flammable
     
  2. jcsd
  3. Oct 2, 2008 #2

    mgb_phys

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    It is already burnt, the hydrogen has burnt in the oxygen to make water.
    The molecule H2O means that the hydrogen and oxygen have combined - it's very different from a mixture of oxygen and hydrogen.
     
  4. Oct 3, 2008 #3
    mgb_phys is right

    You can think of it in terms of energy: hydrogen and oxygen gases are at a higher potential energy level than water. THAT change in energy level is what is released (in the form of explosion/fire etc.) when going from (2)H2 + O2 -> (2)H20 (burning hydrogen in presence of oxygen to form water.

    Since water is at a lower energy level there isn't any more energy that can be released by "burning" it.
     
  5. Oct 3, 2008 #4

    russ_watters

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    Water is like the ash that is left in the fireplace (and soot lining the chimney) after wood is burned.
     
  6. Oct 4, 2008 #5

    Borek

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    Agreed.

    Disagreed. Soot burns pretty well.
     
  7. Oct 4, 2008 #6

    russ_watters

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    Is soot unburned particles carried away? I guess I figured soot was like airborne ash.
     
  8. Oct 4, 2008 #7

    mgb_phys

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    I said that but edited it - I thought somebody would say that soot is unburnt fuel.
     
  9. Oct 4, 2008 #8

    Borek

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    Soot is mostly product of wood distillation - ie when you heat up wood it gets degassed. These gases are in general highly flammable. Some of them have low molar mass (like CO and methanol), some are much heavier. Depending on the situation they will be either burn in the flame - or not. If they were not burnt, they form tar, which covers inside of the chimney. It dries there (and it doesn't mean just loosing water, but loosing many low molecular mass, volatile substances). What is left is soot.

    If you burn coal situation is very similar, although there are not so many low molecular mass products, so there is much less soot produced, and it is much drier (less tar-like).

    As I wrote before, soot is higly flammable. This is one of the reasons why you need to clean the chimneys. If soot starts to burn temperature in the chimney can go very high, much higher then during normal exploatation. That's dangerous. It is very difficult to put down burning soot, as you usually have no access to chimney, and just closing the air access at the chimney bottom is not enough; strange things can happen. I have seen several soot fires. The most spectacular one ended with chimney erupting with clouds of sparks and pieces of burning soot every 30 seconds for about 10 minutes. Evidently it was burning without air access, periodically sucking air through the chimney top.
     
  10. Nov 13, 2008 #9
    To put it simply: hydrogen is very flammable when it is in the form of hydrogen molecules, and oxygen speeds up a fire when it is in the form of oxygen molecules. But when the two are combined in the chemical compound water H20 then the molecules are neither of hydrogen, nor of oxygen, but of a different thing altogether (i.e. water), so they take on the properties of water rather than those of molecular hydrogen or molecular oxygen.

    Elements combined as compounds always perform differently from the consituent elements. For example, calcium compounds are found in certain foods (e.g. milk) and are good for teeth. The element calcium is a metal - probably not ideal for eating !
     
  11. May 12, 2011 #10
    so is this reaction endothermic or exothermic?
     
  12. May 12, 2011 #11
    The reaction creates fire which is hot, heat is given off... therefore exothermic.
     
  13. May 12, 2011 #12

    Borek

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    Which reaction?
     
  14. May 12, 2011 #13
    so when the reaction takes place it creates a completely new product which is completely different to H or O there for not flammable
     
  15. May 12, 2011 #14
    the reaction when water is created with H and O
     
  16. May 12, 2011 #15
    i would really like to know if H2O2 is flammable...
     
  17. May 12, 2011 #16

    SpectraCat

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    H2O2 is not combustible .. i.e. it does not burn. However, it is rather reactive and is a strong oxidizer. If you take pure H2O2 from which all water has been removed and heat it above about 80°C, it decomposes spontaneously (and potentially explosively) into water and molecular oxygen. If you happen to have other combustible materials around, the combination of oxygen and high temperature could lead to combustion, but the H2O2 itself never burns.

    Does that answer your question?
     
  18. May 12, 2011 #17
    so it is not something you want to play with? otherwise yes thank you
     
  19. May 12, 2011 #18

    SpectraCat

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    Well, dilute aqueous solutions of hydrogen peroxide ... the stuff they sell in pharmacies for basic first aid (I think these are around 3%) .. is pretty safe. I remember having to gargle with it to help soothe the inflammation in my strep throat as a child, and pouring it on open wounds is a safe way to sterilize them. As you increase the concentration however, it gets more and more reactive. I have not worked with concentrated hydrogen peroxide, so I am not familiar with its properties. I would definitely not mess with it until I had read carefully about proper safety precautions, and its reactivity with the other components of my experiment.
     
    Last edited: May 12, 2011
  20. May 12, 2011 #19

    Borek

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    Water synthesis is exothermic.
     
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