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Combustion and expansion of gases?

  1. Jul 31, 2011 #1
    I think most of us have probably seen the little trick in which somone will place a burning candle in a bottle and get something like an egg to be pushed inside the bottle.

    How does this happen?

    I understand that a vacuum is created inside the bottle, and the normal outside air pressure pushes the egg in.

    Problem: I always thought the vacuum was because the oxygen in the bottle was combining with the wick material (i.e. less oxygen molecules flying around the inside of the bottle).

    However, the oxygen atoms are still there. They are simply attached to carbon, forming carbon dioxide. Right?

    Furthermore, oxygen and carbon combine with a great deal of energy, so that the CO2 molecules should be vibrating internally (source of the heat), bumping stuff around and thus creating MORE pressure.

    So why the vacuum? I know the vacuum exists, so that means my thinking/physics is screwed up somewhere.

    Can anyone help?
     
  2. jcsd
  3. Jul 31, 2011 #2

    SpectraCat

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    I am not familiar with this experiment, but I from your description, I don't think your analysis is correct. My guess is that flame inside the bottle makes the air expand, so that it is forced out around the edges of the egg (which I assume is hard-boiled, and perhaps had the shell removed as well). Eventually, the flame will go out as it is starved of oxygen. At this point, the air in the bottle will cool, creating negative pressure to suck the egg in. One point is that you probably need a bottle with a mouth that is only very slightly larger than the egg in order for this trick to work.

    Do you know of a link to a video of this experiment? If I could see it done, I could understand if my analysis might be correct.

    [EDIT] One final point ... for most fuels, combustion reactions actually increase the number of molecules in the gas phase. For example:

    C3H8 + 5 O2 --> 3 CO2 + 4 H2O

    There are 6 molecules on the left, and 7 on the right, so the total number of gas phase molecules actually increased. This is important because at constant temperature and volume, the pressure is directly proportional to the number of molecules in the gas phase. Note that the fuels volatilized from heating candle wax are likely much larger than the propane molecule in the above example, so the pressure change associated with increasing the number of molecules is even greater. There is a further increase in pressure due to the heating of the gas. Note also that it is this pressure change that can result in an explosion if a combustion reaction is carried out in a closed (or partially closed) container (think firecracker).
     
    Last edited: Jul 31, 2011
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