This Week's Experiment - #304 Observation of a Candle
With the holiday season, I am once again rereading Michael Faraday's Chemical
History of a Candle. You can find the entire book online at http://www.bartleby.com/30/7.html. This week we will take a few minutes to
look at some of the marvels involved in the burning of a candle. You will
need:
a candle
a candlestick or holder
a lighter or matches
metal tweezers or needle nosed pliers
an adult (as this experiment uses fire)
Place the candle in the holder. Light the wick. Watch carefully for a
minute or so. What do you see? You probably noticed that the wick does
not burn up. If you removed it from the candle and lit it, the wick would
be consumed by the fire very quickly. Why does the wax make it last so much
longer? To find out, use the tweezers to pinch the wick, just below the
flame. What happens? The flame goes out. Why? The candle flame melts
the wax, which soaks up into the wick, just as water soaks up into a paper
towel. As this melted wax gets closer to the flame, it breaks down into a
variety of chemicals, producing a very flammable gas. It is this gas that
you see burning. When you pinch the wick, the melted wax cannot rise up to
the flame. The fire runs out of fuel and goes out.
You may also have noticed that the melted wax forms a nice pool around the
wick. How does the candle form the cup shape to hold the liquid wax? The
flame heats the air. This air expands and rises. Air from below the flame
moves up to take its place, where it will also be heated and rise. This
forms a steady flow of air up the sides of the candle. The flow of air
cools the outside of the candle, keeping the edges solid as the center melts.
That forms the bowl to hold the melted wax. This only works well if there
are no breezes blowing across the candle. Wind blowing from one side cools
that side while disturbing the rising air which would cool the other side.
The side towards the breeze forms a high wall of unmelted wax. On the side
away from the breeze, the rising air is disrupted. This lets the wax melt
all the way to the side of the candle. The melted wax can then run out,
dripping down the side of the candle.
The upwards flow of air has an impact on the flame itself. Most of the
flame is yellow, indicating that it contains many particles of carbon burning
in a low oxygen environment. If there were plenty of oxygen, the carbon
would all burn and the flame would be blue. The very base of the flame is
getting plenty of oxygen, due to the upwards flow of air. If you look
closely, you will see a small blue area of flame at the very lowest part.
This is only be beginning of the science involved in burning a candle. To
learn more, read some of Faraday's lecture. It is fascinating science and
amazing how well Faraday could explain it. In the mean time, have a
wonderful holiday season.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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