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Why doesn't wet paper burn

  1. Aug 9, 2010 #1
    Hi

    I was wondering why wet paper doesnt burn and I believe its something to do with the ignition point [The minimum temperature at which a substance will continue to burn without additional application of external heat. Also called kindling point.]

    Is it simply because when paper is dipped in water it doesn't reach a sufficient temperature for the paper to have enough energy to overcome activation energy for the reaction with oxygen?

    Why does water put out a fire?

    thanks
     
  2. jcsd
  3. Aug 9, 2010 #2

    Borek

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    Staff: Mentor

    What is maximum temperature of wet paper? Is it enough to start fire?

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    methods
     
  4. Aug 9, 2010 #3
    Where does someone find out the maximum temperature of wet paper? Its not the type of thing you get lots of answers for in a google search
     
  5. Aug 9, 2010 #4
    What he meant was if you start heating a piece of wet paper at what temperature will it cease being wet? Once it reaches water's boiling point the water will boil away and it won't be wet anymore. After that it will burn normally.

    That explains why wet paper is more resistant to burn than dry paper, it is because you must first boil away all the water. Boiling water consumes quite a bit of energy. That energy would otherwise have gone to heating the paper to its ignition point, but is instead wasted.
     
  6. Aug 9, 2010 #5
    Thanks for your answer Dale. However I feel it just raises a different version of the original question: why do you have to evaporate the water from the paper for the paper to burn?
     
  7. Aug 9, 2010 #6

    alxm

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    You don't have to. But as long as there's liquid water present, it will be able to vaporize and lower the temperature.
     
  8. Aug 9, 2010 #7

    turbo

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    At what temperature does paper ignite? You can Google that. If you don't evaporate the water from the paper, can you reach that temperature?
     
  9. Aug 9, 2010 #8
    @alxm - thank-you for giving me a direct answer.

    So are you saying that while the paper is wet, any heat added to the paper will evaporate the water rather than raising the temperature of the paper
     
  10. Aug 9, 2010 #9

    turbo

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    You're catching on. The temperature of the paper can't rise (globally) until the water has been evaporated. How much heat will it take to vaporize water? After the paper is dry, how much heat will it take to ignite the paper?
     
  11. Aug 14, 2010 #10
    This is probably a dumb question but what if we put the wet paper in a pressure cooker and raised the pressure by pumping in oxygen and raised the boiling point of the water significantly .
     
  12. Aug 14, 2010 #11

    Borek

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    Staff: Mentor

    This is not a stupid question. I would expect paper to burn.

    Many years ago I was told about wood that was put on fire with superheated steam from the huge boiler in one of Polish powers stations. This was done in a controlled way, just to show the audience that hot steam is in no way similar to water.
     
  13. Aug 14, 2010 #12
    interesting
     
  14. Nov 22, 2010 #13
    The answer is because of the heat capacity of water, which is enormous. Energy applied to the paper gets used up in the warming and subsequent evaporation of the water.
    In a sense it's not really about "temperature," but rather about energy.
    So the reason water is used to put out a fire is its ability to absorb energy (heat capacity).
     
  15. Dec 5, 2010 #14
    Another reason I myself came up with is that vaporised water molecules surround the paper, preventing it from coming into contact with oxygen in the air. As such, combustion of paper cannot take place.
     
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