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Fire vs. Water

  1. Mar 14, 2008 #1
    Why does water put fire (not oil fire) out? I understand that the bound Oxygen atoms in the H2O molecules can't be used by the fire but doesn't water contain free oxygen impurities; If not, how do fish breathe? Is it just because the amount of oxygen contained in the impurities is insignificant compared to the amount in the atmosphere that the fire was previously in?
     
  2. jcsd
  3. Mar 14, 2008 #2

    mgb_phys

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    Yes - there is a small amount of dissolved gaseous oxygen in water, but you have to filter a lot of water to extract it that's why fish need gills. Fire can't extract the oxygen from the water in sufficent quantitites.

    ps, The reason not to put water on a oil fire isn't any different chemistry it's that the water and oil mix slightly, then the water boils carrying droplets of oil into the air.
     
  4. Mar 15, 2008 #3

    Dale

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    A fire requires three things: fuel, oxygen, heat. Water dramatically reduces the amount of oxygen available as described above, and it also evaporates and removes a large amount of heat.

    On an oil fire water still removes some heat, but by throwing droplets of oil into the air it dramatically increases the oxygen available.
     
  5. Mar 15, 2008 #4

    rcgldr

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    You wouldn't want to throw water onto a sodium or potassium based fire though.
     
  6. Mar 15, 2008 #5
    Why?
     
  7. Mar 15, 2008 #6

    mgb_phys

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    Sodium and potassiaum are reactive enough that they can break apart water into hydrogen and oxygen.
    If you drop sodium into water it will give off hydrogen and oxygen which will burn, potassium pretty much explodes.
     
    Last edited: Mar 15, 2008
  8. Mar 15, 2008 #7
    Well just hydrogen. (and a great deal of heat)

    2Na + 2H2O => 2 NaOH + H2
     
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