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A minor swindle

  1. Jun 16, 2010 #1
    To amaze and entertain my 8 year old I did the candle under the drinking glass experiment.

    I took a bowl of water, put a lit candle in the middle and placed a glass over the top. Sure enough, the water rose up the candle went out. For him it was an unexpected and wonderful result What made the water go up?! I was very happy to see the amazement in his face. :bugeye: Wonderful!

    However, I told him knowingly--and knowing I had no idea what I was talking about, "The candle burned the oxygen in the air, so there was less air, and so the water had to rise up because now there's less air under the glass." Its what I was told when I was about his age :grumpy: --but now I question this patent answer.

    What's the real story? I'm burning wax, which is fairly close to (CH2)n, and combining it with oxygen in the air.

    2CH2 + 3O2 → 2CO2 + 2H2O

    For every three molecules of oxygen, we get 4 molecules of gas. But the water level rose. Now I know, that air has about 21% oxygen and the water rose to reduce the initial volume by about 10 to 15%. To be sure, I'd have to try it again for better numbers. It's a bit warmer inside the glass then when the experiment started so the vapor is a bit less dense So what really happened?
    Last edited: Jun 16, 2010
  2. jcsd
  3. Jun 16, 2010 #2
    The volume of the candle decreased slightly and the steam condensed to water.As the air cooled the pressure dropped.
  4. Jun 16, 2010 #3


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    Staff: Mentor

    Change in the candle volume is neglectable if you ask me. Other than that I mostly agree with Dadface. There are two products of the combustion - water and carbon dioxide. Water condenses, CO2 is well soluble in water. But my bet is that change in water level is mostly because when you cover the candle you trap hot (expanded) air, which gets colder after candle stops to burn, sucking water up.
  5. Jun 16, 2010 #4
    Well, when the glass is first placed over the candle, there's not a great deal of hot air inside. It should be a contributing factor though. It could be tested though. Hovering the glass over the candle for a while could cause the water to rise higher in the glass.
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