I've been reading this book, page 149. In chapter 15 the following experiment is suggested,

Fix a candle in a container. Fill the container with water. Light the candle and then cover it with an inverted glass.

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It is said that the oxygen in the space of inverted glass will be used up and to fill the space that was previously occupied with oxygen the water level will rise.

My question is, when the wax burns since $\ce{CO2}$ is emitted, won't this $\ce{CO2}$ fill up the space of used up $\ce{O2}$? What amount of $\ce{O2}$ is used up and what amount of $\ce{CO2}$ is released in the burning of wax of candle?


Surely $\ce{CO2}$ will fill some of the space of used up $\ce{O2}$. Candles these days are commonly made out of paraffine, which is basically $\mathrm{C_{many}H_{many}}$. The equation may look as follows: $$\ce{C31H64 + 47O2 -> 31CO2 + 32H2O}$$ You see that we get approximately 2 moles of $\ce{CO2}$ for 3 moles of spent $\ce{O2}$, hence the decrease in volume.

Another factor is that $\ce{CO2}$ is much better soluble in water than other components of air, so over time (though not immediately) it will probably vanish from the glass and thus free some more space.

Yet another factor is that air in the glass is quite hot while the candle is burning. As soon as it cools down, it shrinks.

  • $\begingroup$ Are the 32 $\ce{H_2O] in liquid form or vapor form? $\endgroup$ – user31782 Nov 5 '15 at 13:53
  • $\begingroup$ Initially in vapor form, but mostly turns to liquid pretty soon. $\endgroup$ – Ivan Neretin Nov 5 '15 at 14:04
  • $\begingroup$ Another factor is that the combustion is incomplete, so there is even less $\ce{CO2}$ produced per mole of carbon atoms in the candle. $\endgroup$ – Jan Nov 5 '15 at 15:19
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    $\begingroup$ This factor does not necessarily work in favor of the effect in question. Incomplete combustion may end in C (soot) and produce less gases per 1 mole of used $\ce{O2}$, or it may go to $\ce{CO}$ and thus produce more gases. $\endgroup$ – Ivan Neretin Nov 5 '15 at 15:30

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