# Air Pressure, Fire "Consuming" Oxygen

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Hey hey guys,

The other day I posted a science experiment where I show how water rise inside a glass with fire in it. I explained the process wrong, though, and Shane Killian, along with some other fellas, corrected me.

That was actually quite cool - I think it's one of the best things about science and learning, to have people think about things and criticize them. That's how we learn.

First off, the videos are here (both): http://www.smarterthanthat.com/experiments/a-physics-party-trick-that-sucks-liquid/

Anyhoo, I was thinking about this quite a lot, and I am not sure if what causes the effect is the heat..

What I was thinking, is that the fire consumes the O2 in the air and emmits out CO2.. that would mean that there are supposed to be less moles of gas in the confined space of the glass, and therefore the pressure is dropping, ad the water are pushed inwards until the pressure is stabilized between both environments.

That would also explain how the water start rising before the candle is fully out.

What do you guys think, though? Is it really the heat? Is the air cooling off THAT fast after the candle is out? and .. even if it is -- why would the water stat getting in before the candle is out? won't the heat from the candle first create HIGHER pressure that push everything out, and THEN lower pressure as the air cooled?

Anyhoo, this is really interesting, and beyond the interest, I would like to post something RIGHT in that experiment post

So.. help?

~moo

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It's not heat; heat would make the gas expand. I don't think it's CO2, either. A molecule of oxygen has two oxygen atoms, and so does one molecule of CO2. Hint: Candle wax is not carbon; it's a hydrocarbon.

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"As the candle burns, it uses up the oxygen in the air. The candle wax melts to form the elements carbon and hydrogen, which combine with the remaining oxygen to form water and carbon dioxide. As this oxygen is used up, the water level in the jar rises."

That's the explanation I found in a book. I think it assumes that the water will condense somewhere.

Heat wouldn't make the gas expand if we assume that we're putting the glass over some already warmed (and therefore slightly less dense) air.

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That's the explanation I found in a book. I think it assumes that the water will condense somewhere.

... or be absorbed by the milk. Mooey, you might want to try the experiment with cooking oil instead of water. Use caution if you do attempt this; cooking oil is flammable.

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... or be absorbed by the milk. Mooey, you might want to try the experiment with cooking oil instead of water. Use caution if you do attempt this; cooking oil is flammable.

To see if water are condensed inside?

Why cooking oil?

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Cooking oil doesn't absorb water vapor. Milk does.

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Water and cooking oil are different densities, so they won't mix together. You'll be able to see the water that condenses.

edit: or, as D H said fractionally quicker...

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Hm.. okay, I'm confused though.

If I try this with cooking oil, and see the water condense. What next? Measuring it is a problem in the tools I have.... and... what "good" does it do? know what i mean? err.

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I doubt the oil will rise the way the milk did because the milk is what absorbed the water vapor (that's my hypothesis). If you do decide to try this, please be careful. I will feel terrible if I find out the fire department had to be called to put out mooeypoo's house as a result of a poorly conducted science experiment.

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yeah is there any other fluid that is less flammable and not absorb water you could think of? seriously... maybe another option, and i'll try, definitely

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I'd just try it on a plate outside on concrete where a cooking oil fire will be able to burn itself out. Make sure the candle floats well above the oil-line, and if anyone asks what you're doing, you're making a sacrifice to the great Jebus under the concrete.

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And on rewatching the video you can see that the majority of the liquid enteres the glass after the candle has gone out (or is about to go out) which is when the air would cool down significantly, so maybe that suggest evidence that it is a temp related process.

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Okay, something quite cool is happening --

I'm in touch with a professor in the Chemistry department. He responded to my request for help and seems very happy to help.

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For simplicity, I'll assume the candle is paraffin wax, C25H52. Burning one mole of paraffin wax consumes 38 moles of oxygen molecules but produces 25 moles of CO2 and 26 moles of H2O, or a net gain of 13 moles of gas for every mole of paraffin wax that is burned. Not only is the quantity of gas increasing, it is getting hotter. Were it not for condensation, burning the candle would push the liquid level down rather than drawing it up. This doesn't happen because the water vapor condenses somewhere. My bet is on the milk for two reasons. First, I expect the milk is cooler than the glass, and second, water (milk) has a much greater affinity for water vapor than does the glass.

So why does the majority of the liquid enter the gas after the candle has gone out? While the candle is burning, the condensation process fights against gas production. If the two processes are nearly balanced there will be little change in the liquid level while the candle is burning. Once the candle goes out there is no more gas production. The condensation process proceeds as there remains some remnant water vapor in the gas part of the glass.

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I think the thing to remember here are a few basic chem ideas.

a Long chain alkane such as Wax, will still follow the the rule is that for every carbon atom, there are 2 hydrogen atoms attatched to it that will then give you a number, add 2 to that number and then you have the total of Hydrogens used.

then you have to think of the Gas itself, a mole of CO2 will occupy the same volume as a mole of O2 (just over 22 litres/mol).

so why is there a vacuum created? well we can see from the 1st idea that Hydrogen is always a little more abundant than the carbon (over 50% always).

2 Hydrogens will combine with half an O2, or better 4H+O2=2H2O

this will condense.

and we know that at room temp Water exists as a liquid, so the Gas law doesnt apply, the Vacuum is created

in an Ideal you Should find that if you burned Pure Hydrogen in there you would lose 21% of the total air volume, leaving 78% N2 and ~1% Argon.

Hmmm... seems DH and I were typing at same time LOL

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I don't think that expansion has been sufficiently ruled out. The fact that the liquid isn't forced in until the flame goes out is quite important, and supports the thermal expansion hypothesis.

I would suggest trying this with a larger container (like an old milk bottle), so that the flame continues longer. Use a deeper dish (i.e. more fluid) and observe if there is any gas forced out from expansion due to heating. If there is, you should see bubbles.

edit: then try it again while holding the bottle above the fluid. As soon as the flame goes out, lower it. If the pressure imbalance is due to a change in the number of moles of gas, this should be thwarted by having the bottle above the fluid. If, however, it is due to P and T changes you should still see fluid forced into the bottle.

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I'd go for a combination of both effects. The consuming of O2 into water could only cause an ~10% contraction, and would depend on the composition of the flame. If the CO2 can dissolve quickly, then you can get an ~20% contraction. I did this test in lab and got slightly over 20%, so I concluded that it couldn't all be due to oxygen consumption (more contraction than oxygen concentration, and the flame will go out before all the oxygen is consumed). But everything happened too quick to tell properly.

Another control is to change the quantity of oxygen, to see what effect that would have.

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I'd go for a combination of both effects. The consuming of O2 into water could only cause an ~10% contraction, and would depend on the composition of the flame. If the CO2 can dissolve quickly, then you can get an ~20% contraction. I did this test in lab and got slightly over 20%, so I concluded that it couldn't all be due to oxygen consumption (more contraction than oxygen concentration, and the flame will go out before all the oxygen is consumed). But everything happened too quick to tell properly.

Another control is to change the quantity of oxygen, to see what effect that would have.

Or find a source of flame that burns at a higher or lower temperature.

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I don't think that expansion has been sufficiently ruled out. The fact that the liquid isn't forced in until the flame goes out is quite important, and supports the thermal expansion hypothesis.

If you watch Mooeypoo's original video, however, a significant portion of the milk is sucked up while the flame is still burning.

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If you watch Mooeypoo's original video, however, a significant portion of the milk is sucked up while the flame is still burning.

but whilst the flame is dieing and becoming cooler...

If it was just condensation surely the water level around the edge would not change, and if you place the jar over the top and removed all the outside liquid the liquid inside would still increase by the same amount...

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You could measure the volume of liquid in the dish before and after the experiment.

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You could measure the volume of liquid in the dish before and after the experiment.

Indeed, more data is needed.

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If you watch Mooeypoo's original video, however, a significant portion of the milk is sucked up while the flame is still burning.

Ah, yes, I see that your are correct, but most of the effect is after the flame falters and is extinguished. Certainly it can be a combination of effects.

You could measure the volume of liquid in the dish before and after the experiment.

Measure? The hell you say. We want our dogma, and we want it NOW!

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I don't think that expansion has been sufficiently ruled out. The fact that the liquid isn't forced in until the flame goes out is quite important, and supports the thermal expansion hypothesis.

Watch the video. There is no outgassing. In fact, as Cap'n noted, the fluid level starts rising well before the flame goes out.

Outgassing is required if this was primarily a temperature-related phenomenon. The burning of the candle wax increases the temperature of the gas and increases the molarity of the gas (thirteen net moles of gas are created for each mole of burnt candle wax if the wax is pure paraffin). Both of these factors argue for outgassing. The observed fact that there is no outgassing can mean only one thing: The molarity of the gas must be decreasing rather than increasing. The only way that can happen is by condensation of the H2O on the sides of the glass, condensation of the H2O into the liquid, or solution of the CO2 by the liquid.

Which one? Some assumptions:

1. The glass is a typical drinking glass with a volume of 0.5 liters.
2. Mooey poured 0.05 liters of liquid into the plate.
3. The gas in the glass starts out at 300K.
4. The flame raises the gas temperature to 310K.
5. 75% of the oxygen is consumed before the flame goes out.

Some observations and facts:

1. The sides of the glass remain clear throughout.
2. Absorbtion doesn't start immediately, but does start before the flame goes out.
3. The liquid rises about one centimeter up the glass.
4. Air is 20.85% oxygen.
5. The partial pressure of CO2 at 310K, 1 atm pressure is about 0.023 moles/liter.
6. The water vapor pressure at 310K is 6300 pascals.

Some calculations:

• The glass contains 0.0043 moles of oxygen molecules (ideal gas law assumption)
• Burning consumes 0.0032 moles of oxygen but creates 0.0021 moles of CO2 and 0.0022 moles of H2O.
• Ignoring condensation, the post-combustion partial pressure of the H2O is about 13,000 pascal, well over twice the partial pressure listed above. If the assumptions are correct, more than half of the generated H2O must condense.
• The water can dissolve 0.0011 moles of CO2, or about half of the quantity of CO2 produced.

Bottom line: Temperature cannot account for the phenomenon for the simple reason that the temperature rise and molarity increase would make the gas expand, not contract. That the glass remains clear throughout argues against H2O condensation onto the sides of the glass. Both CO2 solution and H2O condensation into the water can account for the phenomenon. H2O condensation matches the observation that the water does not start rising immediately. The partial pressure must rise close to the vapor pressure before condensation commences.

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The jar is placed over already hot air, and teh outgassing might be very small and therefore difficult to see...

surely if you just place the jar over the top, without water, or with the water outside removed (suck it up using kitchen roll when the flame is still lit), then you should still get ALOT of water inside if it's condensation...

And the temp being the reason argument is about the temperature drop when the flame goes out not the temperature rise...

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