| MadSci Network: Chemistry |
This experiment is something that is regularly used by chemistry teachers to show that air is 20% oxygen. It is a hoax! The heat causes the air around the candles to expand. When the air cools down, it contracts and water rises up the jar until the air pressures inside and outside are at equilibrium. With two candles, the air will be hotter and will expand further. The higher temperature of the air around the pair of candles allows for greater cooling when the candles go out and therefore more contraction. As a result, two candles will cause the water to rise further up the jar.
The amount of oxygen in the jar has little to do with how high the water rises because the candles go out well before the oxygen has been used up. the oxygen is also replaced with carbon dioxide molecules. I did see a picture of this set-up in the journal of chemical education which showed a mouse still alive in the bell jar after the candle had gone out!
Richard Kingsley
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