Burning paper in ethanol

  • Thread starter Moogie
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  • #26
alxm
Science Advisor
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This is true for every real system, but chemistry still exist.
What's that supposed to mean?
And I believe you are wrong: first ethanol vaporizes, then paper burns.
Yes you keep saying that, but you haven't actually explained why they can't both happen concurrently.
Temperature of the liquid ethanol is not well defined? Well, it's not well defined globally, but it is for small portions of the liquid, and those portions wich will get hotter than 78.4°C will boil.
Only if they're at the boundary layer, where they have the ability to form gas. Otherwise they'll explode. Those are called 'microexplosions'. But it doesn't matter what the temperature is inside the paper/droplet, whatever. Combustion occurs exclusively at the boundary layer of the liquid, and is dependent on the temperature, the volatility and the gas flow.
Instead, I wanted to ask you if you took your master by telephone
Says the guy who thinks boiling points and ignition temperatures are related...
Have a read to these, regarding liquid wax instead of ethanol and candle wick instead of paper
A candle is the classical example of laminar flow combustion; the liquid does not burn, not because it can't, but because the oxygen can't reach it.
 
  • #27
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What's that supposed to mean?
That people can make precise enough computations on those systems without the need of a non-aequilibrium treatize.
Yes you keep saying that, but you haven't actually explained why they can't both happen concurrently.
And where did I say they can't? Of course you will have some oxygen diffusing through the liquid, of course you will have some liquid ethanol reacting with oxygen and thousands of other processes, but I simply say that *in this case* that it's not the reason of the ethanol combustion, the reason is the ethanol in the gas state reacting with air oxygen. If you prevent ethanol vaporization in some way, you won't have combustion at all. In this link:
http://www.cdc.gov/niosh/ipcsneng/neng0044.html
it's written that ethanol's flash point is 13°C. It means that if you have ethanol at temperatures lower than that, you cannot ignite it *even with a flame*. Why? Please, answer this question first.
Only if they're at the boundary layer, where they have the ability to form gas. Otherwise they'll explode. Those are called 'microexplosions'. But it doesn't matter what the temperature is inside the paper/droplet, whatever. Combustion occurs exclusively at the boundary layer of the liquid, and is dependent on the temperature, the volatility and the gas flow.
And have you asked yourself why is dependent on the volatility?:smile:
 
  • #28
I think people are making this out to be way more complicated than it is.

The ethanol burns first partially because it can vaporize and better mix with air, and partially because it coats the paper. Ethanol rapidly evaporates, and it's much easier to ignite as a gas than as a liquid as it's better mixed with oxygen in the air. It's also all about surface area - the combustion reaction only takes place at the surface of anything. Therefore, if your paper is covered in ethanol, the ethanol must first burn away before the paper can start reacting with the air and also burn.

This also has nothing to do with autoignition, I'm not sure where that's coming from. Autoignition temperature is the temp. at which the material will ignite spontaneously, without any ignition source. Since we are talking about dipping paper in ethanol and intentionally lighting it on fire (not gradually heating it to high temperature), autoignition doesn't come into play here.
 
  • #29
1,914
46
I think people are making this out to be way more complicated than it is.

The ethanol burns first partially because it can vaporize and better mix with air, and partially because it coats the paper. Ethanol rapidly evaporates, and it's much easier to ignite as a gas than as a liquid as it's better mixed with oxygen in the air. It's also all about surface area - the combustion reaction only takes place at the surface of anything. Therefore, if your paper is covered in ethanol, the ethanol must first burn away before the paper can start reacting with the air and also burn.

This also has nothing to do with autoignition, I'm not sure where that's coming from. Autoignition temperature is the temp. at which the material will ignite spontaneously, without any ignition source. Since we are talking about dipping paper in ethanol and intentionally lighting it on fire (not gradually heating it to high temperature), autoignition doesn't come into play here.
Which is what I tried to explain too...:smile:
 
  • #30
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I think it is simple, because ethanol evaporates, it insulates paper from oxygen, or it burns and deplete oxygen around paper. Paper burns only with oxygen react with it, remember heat is not the only reason that paper burns.
 

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