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Burning paper in ethanol

  1. Aug 21, 2010 #1
    Hi

    When you burn paper dipped in ethanol why doesn't the paper catch fire? I would have thought that the energy released from the ethanol combustion would raise the temperature of the paper to its ignition point?

    thanks
     
  2. jcsd
  3. Aug 21, 2010 #2

    chemisttree

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    It will burn eventually. The evaporating alcohol keeps the paper cool enough to not catch fire until you run out of ethanol.
     
  4. Aug 21, 2010 #3
    So the ethanol would evaporate and not burn? Why does it do this?
     
  5. Aug 21, 2010 #4
    The ethanol would burn, but it has to evaporate first--otherwise it can't mix with oxygen.

    Of course, evaporation will cool the remaining liquid because of the latent heat of vaporization.
     
  6. Aug 21, 2010 #5
    Oh right. Why wouldn't liquid ethanol mix with gaseous oxygen?
     
  7. Aug 21, 2010 #6

    alxm

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    It doesn't have to evaporate first (didn't I just have this discussion?). It's just that as long as there's liquid ethanol present, the temperature gets reduced by evaporation, and thus the temperature it takes to set it all on fire gets increased.

    You don't have a neat sequence where the water or ethanol first evaporates and then combusts.

    Paper dipped in ethanol burns fine, it just has a higher effective autoignition temperature.
     
  8. Aug 21, 2010 #7
    Yes, but the burning ethanol is the vapor, not the liquid.

    It's not that the process ocurs in several distinct steps--the heat from the flame causes more alcohol to evaporate, while the oxygen in the air mixes with the existing vapor, while the existing fuel-air misture burns.

    But when there is no more liquid fuel to absorb the heat, the paper warms up to its ignition temperature.
     
  9. Aug 21, 2010 #8
    alxm - we had a similar discussion about why wet paper doesn't burn but i wasn't sure what would happen with ethanol as this can evaporate as well as burn. I was wondering if it would burn first or evaporate. However I didn't say it had to evaporate first, someone else did so i was responding to them.

    phaseshifter - why does the ethanol have to evaporate to burn? You said it has to vaporise to mix with oxygen (but i note alxm says this is not the case but he might noe be addressing this specific point)
     
    Last edited: Aug 21, 2010
  10. Aug 21, 2010 #9

    alxm

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    Both vapor and liquids (and solids) can burn. Unless there is zero oxygen at the liquid ethanol/air boundary, it will burn as well.
     
  11. Aug 21, 2010 #10
    ok, i see now why i was confused. I saw something where paper dipped in ethanol wasn't burning. But this was an ethanol and water mixture so that the paper wasn't burning because the water in the alcohol water mixture was evaporating keeping the paper below its ignition temperature
    So the paper wasn't burning but the ethanol was
    However that makes me thing why was the ethanol burning because its autoignition temperature is higher than that of paper? Unless its gaseous ethanol burning and gaseous ethanol has a lower autoignition temperature than liquid ethanol?
     
  12. Aug 21, 2010 #11
    also why is the ignition temperature of paper dipped in water so much higher than paper dipped in ethanol. The boiling points of water and ethanol arent that much different (78 to 100) so if you put paper dipped in water and paper dipped in ehtanol in a bunsen flame both the water and ethanol will evaporate and lower the temperature of the paper yet the paper dipped in (pure) ethanol will burn but the one dipped in water won't
     
  13. Aug 21, 2010 #12

    alxm

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    This is possible, but I doubt it'd stay that way for long before the paper caught fire.

    Ethanol ignites at a lower temperature than paper. (Ray Bradbury was wrong; it's actually closer to Celsius 451 than Fahrenheit 451)

    I wish people would forget about evaporation. If you heated the thing very slowly, so that it was in equilibrium the whole time, then yes, the ethanol/water would evaporate completely before it caught fire. But that's not how it usually is. An open flame is not at all in thermodynamic equilibrium! You have evaporation and burning at the same time.

    Anyway, the difference in ignition temperature is simply that ethanol has a lower ignition temperature. And water doesn't burn.
     
  14. Aug 22, 2010 #13
    I think you are referring to a film but i've never seen it.

    I read that the ignition point of water is 230 degrees C and ethanol is 450 degrees C? This was on a reputable website (royal society chemistry)
     
  15. Aug 22, 2010 #14
    I understand what you are saying about evaporation but when you are hoping to explain these concepts to children (which is my point in asking these questions), children need neat models and sequence of steps. Sorry if I am trying to make the chemistry too 'neat'
     
  16. Sep 3, 2010 #15
    Hi

    Can I resurrect this thread as I don't think i got a final answer that I understand.

    Can anyone confirm the ignition point of water and ethanol because my source says the ignition point of paper is 230 degrees C and ethanol is 450 degrees C. If this is so I don't understand why when you dip paper in a water/ethanol mixture the paper doesn't burn but ethanol vapour appears to burn on the paper. My data suggests the paper would burn before the ethanol.

    thanks
     
  17. Sep 3, 2010 #16
    No. Even if it was pure ethanol, it wouldn't burn the same, until the ethanol has gone away through vaporization. Furtermore, what burns is ethanol in the gaseous state, not liquid ethanol.
     
  18. Sep 3, 2010 #17

    alxm

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    Not this again. Solids and liquids burn just fine. Ethanol does not need to be vaporized before burning. The ignition temperature of ethanol vapors is lower than that of liquid (or solid) ethanol, which means that at low temperatures, it will vaporize and burn before the liquid ethanol vaporizes and burns.

    Edit: Let's get to the bottom of this: Why don't you think that a liquid can burn? What is it that you think stops the oxygen molecules from colliding and reacting with the ethanol molecules at the air/liquid boundary?
     
  19. Sep 4, 2010 #18
    :smile:
    Did you read what you have written?
    <<it will vaporize and burn before ... vaporizes and burns>>
    A liquid can certainly react with other chemicals even at room temperature, but a liquid cannot stay in the liquid state, above its boiling point (I'm assuming atmospheric pressure), so liquid ethanol cannot exist above 78.4°C. Do you really think ethanol could burn at such temperature? Ethanol autoignition temperature is 363°C:
    http://www.cdc.gov/niosh/ipcsneng/neng0044.html
     
    Last edited: Sep 4, 2010
  20. Sep 4, 2010 #19

    alxm

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    Obviously I meant to write "it will vaporize and burn before the liquid itself burns".

    Yes it can. A liquid won't stay a liquid above its boiling point if it's in thermodynamic equilibrium. A fire/flame is not a system in thermodynamic equilibrium. It's pretty damn far from that.
     
  21. Sep 5, 2010 #20
    Do you mean that you have there *liquid* ethanol at temperatures > 363°C?
     
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