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It's a very popular experiment (eg), from elementary school : put a burning candle on a dish filled with water, cover the candle with an inverted glass: after a little while, the candle flame goes out and the water level inside the glass rises.

The standard explanation (as I recall it) was that combustion "burns" oxygen, and the consummed volume accounts for the extra water that goes inside the glass. Is this correct? I remember feeling (years later) uncomfortable with the explanation, because "to burn" is certainly not "to dissapear": I thought that oxygen combustion produces (mainly) $CO_2$ and hence one oxygen molecule would produce another $CO_2$ molecule, and the volume would remain basically the same. Perhaps $CO_2$ dissolves into the water? I would doubt that.

To add to my confusion, others state that the main cause is not the oxygen combustion but the changes of air temperature, that decreases when the flame goes out and makes the air inside the glass contract... which would rather invalidate the experiment as it was (and is) traditionally taught to students.

What is the right explanation?

enter image description here

(image from here)

Update: As from webpage linked in accepted answer, there are several effects here, but I think it's safe to say that the "traditional" explanation (consumption of oxygen) is just wrong: oxygen turns into $CO_2$ and water and -even assuming that this is the complete chemical picture (it's not) and that all water condenses- that would produce a slight decrease of volume ($CO_2$ dissolves in water poorly and very slowly). The main effect here is thermal expansion-contraction of air.

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Is there a stackexchange for chemistry? Maybe they could provide better help. –  sidht Jan 4 '12 at 1:58
    
@jak Not yet. –  Manishearth Mar 15 '12 at 7:21
    
@Manishearth Yes there is - chemistry.stackexchange.com It is in beta, though. –  Dave Coffman Jul 28 at 22:06
    
@DaveCoffman look at the date on that comment. I moderate Chem.SE, I know about it :P –  Manishearth 2 days ago

3 Answers 3

up vote 11 down vote accepted

I found two web pages that explain the phenomenon quite well, and even looks into the misconceptions people have.

The candle flame heats the air in the vase, and this hot air expands. Some of the expanding air escapes out from under the vase — you might see some bubbles. When the flame goes out, the air in the vase cools down and the cooler air contracts. The cooling air inside of the vase creates a vacuum. This imperfect vacuum is created due to the low pressure inside the vase and the high pressure outside of the vase. We know what you're thinking, the vacuum is sucking the water into the vase right? You have the right idea, but scientists try to avoid using the term "suck" when describing a vacuum. Instead, they explain it as gases exerting pressure from an area of high pressure to an area of low pressure.

A common misconception regarding this experiment is that the consumption of the oxygen inside of the bottle is also a factor in the water rising. Truth is, there is a possibility that there would be a small rise in the water from the flame burning up oxygen, but it is extremely minor compared to the expansion and contraction of the gases within the bottle. Simply put, the water would rise at a steady rate if the oxygen being consumed were the main contributing factor (rather than experiencing the rapid rise when the flame is extinguished).(1)

The page from Harvard goes into more detail on the argument versus the error for the incorrect statement.

Argument: Oxygen is replaced by Carbon dioxide. So, there is the same amount of gas added than taken away. Therefore, heat alone most be responsible for the water level change.

Source of the Error: A simplified and wrong chemical equation is used, which does not take into account the quantitative changes. The chemical equation has to be balanced correctly. It is not true that each oxygen molecule is replaced by one carbon dioxide molecule during the burning process; two oxygen molecules result in one carbon dioxide molecule and two water molecules (which condense). Remember oxygen is present in the air as a diatomic molecule. [A reader clarifies the water condensation in an email to me as follows: If the experiment were done with the sealing fluid able to support a temperature greater than 212 F and the whole system held above this temperature then the water product of combustion would remain gaseous and the pressure within the vessel would increase as a result of three gaseous molecules for every two prior to combustion and the sealing fluid would be pushed out.]

Argument: Carbon dioxide is absorbed by the water. Thats why the oxygen depletion has an effect.

Source of the Error: This idea is triggered from the fact that water can be carbonized or that the oceans absorb much of the carbon dioxide in the air. But carbon dioxide is not absorbed so fast by water. The air would have to go through the water and pressure would need to be applied so that the carbon dioxide is absorbed during the short time span of the experiment.

Argument: The experiment can be explained by physics alone. During the heating stage, air escapes. Afterwards, the air volume decreases and pulls the water up.

Source of the Error: the argument could work, if indeed the heating of the air would produce enough pressure that some air could leave. In that case, some air would be lost through the water. But one can observe that the water level stays up even if everything has gone back to normal temperature (say 10 minutes). No bubbles can be seen.

Argument: It can not be that the oxygen depletion is responsible for the water raising, because the water does not rise immediately. The water rises only after the candle dims. If gas would be going away, this would lead to a steady rise of the water level, not the rapid rise at the end, when the candle goes out.

Source of the Error: It is not "only" the oxygen depletion which matters. There are two effects which matter: the chemical process of the burning as well as a physical process from the temperature change. These effects cancel each other initially. Since these effect hide each other partially, they are more difficult to detect. (2)

It clearly has more to do with the temperature differences than any conversion of gases. Especially considering that a volume of oxygen and carbon dioxide will be nearly identical to human eye observation.

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I'd trust Harvard (second footnote I am guessing). –  Skava Jan 4 '12 at 3:11
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Yes "Skava", now go to bed! –  Larian LeQuella Jan 4 '12 at 3:12
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This answer is useful in pointing the best explanation I've seen (the second link), but the text is plainly copied other pages (should be formatted as quotes) and does not make clear the general summary/conclusion. –  leonbloy Jan 4 '12 at 13:49
    
I'd question one thing from that answer, though: Nowhere is a vacuum created. There's always air in the glass, and it always fills the whole space not occupied by water. When the air cools down, it doesn't contract by itself, only its pressure goes down (intuitively: Since the molecules get slower, they hammer less onto the water surface). As result the water is pressed more in by the air outside than out by the air inside, and thus flows inside. This rising water compresses the air inside, which causes air density and thus pressure inside to rise again until equilibrium is reached. –  celtschk Jan 18 '12 at 5:47

I have not actually tried this experiment, but I will make at least a few observations:

Hypothesis 1: The burning of oxygen is responsible for the reduced air pressure.

Prediction - if the burning of oxygen is the sole cause of the change in pressure, we should expect to see the water in the glass rise at a more or less constant rate from the moment the environment is sealed until the burning stops. After the candle extinguishes, there should be no more change in water level.

Hypothesis 2: The reduction in temperature after the candle extinguishes is responsible for the reduced air pressure.

Prediction - if the temperature change is the sole cause of the change in pressure, we should expect to see no change in water level while the candle is burning (in the limit that the glass was lowered very slowly). After the burning stops, the water should rise at a rate related to the temperature drop and eventually stop as the experimental setup comes to room temperature.


In order to test which explaination is correct, you should be able to merely perform the experiment and match the observation with the prediction. Of course, in real life it may be a combination of these two factors or perhaps include other reasons not listed here.

Additional measures such as putting an oxygen indicator in the glass (say a fresh slice of apple) or a thermometer would provide further insight.

Good luck!

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As oxygen is burned - how many moles of CO2 do you get for each mole of O2 used? –  Martin Beckett Jan 3 '12 at 23:15
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@MartinBeckett: Not to mention it's mostly carbon monoxide because it's imperfect burning. –  Mike Dunlavey Jan 4 '12 at 3:15
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@MartinBeckett: The pertinent equation seems to be something like $C_{25} H_{52} + 38 O_2 => 25 C O_2 + 26 H_2 O$. So for 1 mole of oxygen we have 0.65 moles of $C O_2$ - a moderate reduction, and this assuming water condenses. –  leonbloy Jan 4 '12 at 14:40
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@leonbloy - although with a smoky candle you do get a lot of CO. Plus since O2 is only 20% of air it would at most be a (1-0.65)*0.21 = 7% change in volume even with full combustion –  Martin Beckett Jan 4 '12 at 16:26
    
@MartinBeckett: you are right, of course. See the Harvard link in the other answer for the complete picture. –  leonbloy Jan 4 '12 at 16:36

I will make this into an answer because the idea behind this question is used in an ancient medical method which was still used by practical nurses and even prescribed by old fashioned doctors when I was a child more than half a century ago in Greece. It is now used in alternative medicine practices

The air inside the cup is heated and the rim is then applied to the skin, forming an airtight seal. As the air inside the cup cools, it contracts, forming a partial vacuum and enabling the cup to suck the skin, pulling in soft tissue, and drawing blood to that area.

I think it was the invention of antibiotics which diminished rapidly its use, which was mainly for bronchitis pneumonia and similar afflictions, at least in Greece.

As far as the question goes, no liquids to confuse the issue of its being a strongly temperature dependent effect.

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Indeed, the practice is known as "cupping" and is often offered at spas and other health resorts. –  AdamRedwine Jan 4 '12 at 13:15
    
+1 In spanish: "ventosa". I've seen it applied by my grandmother many years ago. –  leonbloy Jan 4 '12 at 13:37

protected by Qmechanic Mar 21 '13 at 19:08

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