Why Doesn't Paper Catch Fire When Dipped in Ethanol?

[Moogie]

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

 

[chemisttree]

It will burn eventually. The evaporating alcohol keeps the paper cool enough to not catch fire until you run out of ethanol.

 

[Moogie]

So the ethanol would evaporate and not burn? Why does it do this?

 

[PhaseShifter]

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.

 

[Moogie]

Oh right. Why wouldn't liquid ethanol mix with gaseous oxygen?

 

[alxm]

The ethanol would burn, but it has to evaporate first--otherwise it can't mix with oxygen.

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.

 

[PhaseShifter]

Paper dipped in ethanol burns fine, it just has a higher effective autoignition temperature.

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.

 

[Moogie]

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)

 

[alxm]

Yes, but the burning ethanol is the vapor, not the liquid.

Both vapor and liquids (and solids) can burn. Unless there is zero oxygen at the liquid ethanol/air boundary, it will burn as well.

 

[Moogie]

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?

 

[Moogie]

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

 

[alxm]

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

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

However that makes me thing why was the ethanol burning because its autoignition temperature is higher than that of paper?

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

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)

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.

 

[Moogie]

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)

 

[Moogie]

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'

 

[Moogie]

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

 

[lightarrow]

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

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.

 

[alxm]

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.

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?

 

[lightarrow]

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.

Did you read what you have written?
<<it will vaporize and burn before ... 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?

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

 

[alxm]

Did you read what you have written?
<<it will vaporize and burn before ... vaporizes and burns>>

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

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.

 

[lightarrow]

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.

Do you mean that you have there *liquid* ethanol at temperatures > 363°C?

 

[Moogie]

I know you two are debating a different point about whether liquids can burn but could someone address my point as it is my thread :)

My point was about the autoignition temperature of ethanol and paper. Specifically my source says the ignition point of paper is below that of ethanol. If this is correct then the paper would burn before the ethanol but in the picture I have the paper is not burning and the ethanol is burning on the paper.

 

[lightarrow]

I know you two are debating a different point about whether liquids can burn but could someone address my point as it is my thread :)

My point was about the autoignition temperature of ethanol and paper. Specifically my source says the ignition point of paper is below that of ethanol. If this is correct then the paper would burn before the ethanol but in the picture I have the paper is not burning and the ethanol is burning on the paper.

The paper cannot burn until the ethanol has completely gone away and leaved the "naked" paper which it was covering. Paradoxically as it seems, the liquid here protect the paper from burning.

 

[alxm]

Do you mean that you have there *liquid* ethanol at temperatures > 363°C?

I'm saying that 'temperature' does not have a well-defined meaning for a system not in equilibrium. Heat transfer occurs (mostly) via intermolecular collisions, which is the same process chemical reactions occur by. There's no distinguishing the two. You can't say "first it gets hot, then it evaporates, then it reacts". Vaporization is itself a reaction. These reactions will occur at the same time if it's hot enough, which is what I've been saying from the start.

The paper cannot burn until the ethanol has completely gone away and leaved the "naked" paper which it was covering. Paradoxically as it seems, the liquid here protect the paper from burning.

You're making stuff up. Seriously, have you studied chemical thermodynamics and kinetics at all? Beyond the introductory, equilibrium-thermodynamics level?

Again, explain at the molecular level what about the ethanol is stopping energetic oxygen molecules from reacting with the paper? You can't. And the reason you can't is because you're trying to apply simplified macroscopic, equilibrium thermodynamics to explain a non-equilibrium situation.

 

[alxm]

My point was about the autoignition temperature of ethanol and paper. Specifically my source says the ignition point of paper is below that of ethanol. If this is correct then the paper would burn before the ethanol but in the picture I have the paper is not burning and the ethanol is burning on the paper.

Again, if the temperature is high enough, the paper drenched in ethanol will burn before the ethanol evaporates. Stick it in an acetylene flame and see what happens.

An ethanol-air mixture burns at a lower temperature than liquid ethanol, and a lower temperature than paper. If the temperature is low, then the ethanol will evaporate and burn in the air before the paper and liquid ethanol does, because ethanol has a low boiling point.

 

[lightarrow]

I'm saying that 'temperature' does not have a well-defined meaning for a system not in equilibrium.

This is true for every real system, but chemistry still exist.

Heat transfer occurs (mostly) via intermolecular collisions, which is the same process chemical reactions occur by. There's no distinguishing the two. You can't say "first it gets hot, then it evaporates, then it reacts". Vaporization is itself a reaction. These reactions will occur at the same time if it's hot enough, which is what I've been saying from the start.

And I believe you are wrong: first ethanol vaporizes, then paper burns. 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 which will get hotter than 78.4°C will boil. Yes, you can have small portions of superheated substance, but not up to 363°C.

You're making stuff up. Seriously, have you studied chemical thermodynamics and kinetics at all? Beyond the introductory, equilibrium-thermodynamics level?

Instead, I wanted to ask you if you took your master by telephone

Again, explain at the molecular level what about the ethanol is stopping energetic oxygen molecules from reacting with the paper? You can't. And the reason you can't is because you're trying to apply simplified macroscopic, equilibrium thermodynamics to explain a non-equilibrium situation.

Have a read to these, regarding liquid wax instead of ethanol and candle wick instead of paper:
http://www.helium.com/items/1165845-how-a-candle-burns
http://www.helium.com/items/1118747-candleburnflamewicksmokewaxparaffin
http://www.helium.com/items/1163220-how-a-candle-burns
http://www.helium.com/items/1190340-how-a-candle-burns
http://www.helium.com/items/1120994-how-a-candle-burns

 

[alxm]

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 which 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.

 

[lightarrow]

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?

 

[danman5000]

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.

 

[lightarrow]

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...

 

[John_5696]

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.