A burning candle needs oxygen
If I put a small burning candle on the middle of a dish,
fill the dish with water, and cover the candle with an inverted glass — why does the candle go
out and the water flow into the glass? Shaquille,
Blandon, Pennsylvania
First,
a word about how a candle burns. A candle is a cylinder of solid fuel — paraffin
— that surrounds a wick. Lighting the wick melts and then vaporizes the wax
coating the wick. The wax vapor combines with oxygen by burning. Within
the bluer, hotter region near the base of the wick, hydrogen separates from the
wax vapor, burns and forms water vapor. Within the brighter, yellower part
of the flame, carbon soot oxidizes, and forms carbon dioxide. As the
candle burns, it converts oxygen into carbon dioxide and water.
The experimental setup: An inverted water glass and a burning
votive candle, surrounded by a moat of green-colored water, which flows into the inverted
open glass, and rises slowly at first. The arrow shows the depth of water forced
in, so far. Photo courtesy of Lanney Atchley.
Now, to answer your question:
After I invert the glass over the candle, the candle eventually goes out,
because the burning process converts all the oxygen in the glass into other
products. The candle must have oxygen to burn.
At first, the water level slowly rises in the glass, because, as the burning candle
converts gaseous oxygen and hydrogen into liquid water, condensation forms on
the glass, and the gas density inside the glass decreases. 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
burning candle converts oxygen and hydrogen into water.
The candle goes out and the water-level inside the glass rises sharply.
Photo courtesy of Lanney Atchley.
Then the candle goes out. But the water level rises even faster in the jar.
This surprised me when my assistant and I tried your experiment. Gaseous oxygen
is no longer reacting with hydrogen to form liquid water because the candle is
no longer burning. 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 well, 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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