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Macaque monkey,  Crab-eating macaque (Macaca fascicularis) in Lopburi, Thailand.  Photo courtesy of 'Chris huh' and Wikipedia.

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How to Offer Wild Birds Shelter in the Winter

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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 seeps into the inverted open jar.

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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