A burning candle consumes oxygen
If I put a small burning candle on the middle of a dish,
fill the dish with water, and cover the candle with a jar — why does the candle go
out and the water flow into the jar? Shaquille,
Blandon, Pennsylvania
First,
a word about how a candle burns. A candle is a cylinder of solid fuel — paraffin
wax — that surrounds a wick. Bringing a lit match to a wick melts and then
vaporizes the wax coating the wick. The wax vapor combines with oxygen, and
burns. As it burns, it consumes oxygen.
Now, to answer your question:
The candle goes out, because it eventually consumes all the oxygen in the
jar. The candle must have oxygen to burn.
The water level rises in the jar, because, as the burning candle consumes the
oxygen, the pressure drops inside the jar. The greater air pressure
outside the jar, pushes on the water, and forces more water into the jar.
The increased water volume reduces the jar's air volume until the pressure is the same
inside and outside the jar. Water continues to rise in the jar, as the
candle consumes oxygen.
The experimental setup: A quart-size peanut butter jar and a burning
birthday candle, surrounded by a moat of water, which flows into the inverted
open jar, and rises .
Then the candle goes out. But the water level continues to rise in the jar.
This surprised me when my assistant and I tried your experiment. The
volume of air is no longer losing oxygen, because the candle is out. But
the pressure still drops because the hot gases in the jar are cooling now that
the candle has gone out.
As the temperature of a gas decreases, it's pressure drops (for a fixed gas
volume and number of gas molecules). So, again,
water flows in to equalize inside and outside pressures.
A caveat: Candles burn nicely, and can catch other things on fire.
Keep a bucket of water or, better still, a fire extinguisher handy, if you
experiment.
Further Reading:
Ideal gas law, HyperPhysics
(Answered May 19, 2008)
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