Experiment of the Week -#199 Blowing Out a Candle
Since it is mid-December, it is time once again for me to read Michael
Faraday's Chemical History of a Candle. This wonderful book is one of
best of his Christmas lectures for children and it is still in print
years later. If you have never read this book, I highly recommend it,
both for the information and to experience Faraday's marvelous style of
science. In keeping with my annual tradition, I have an experiment for
you that uses a candle. What is involved in blowing out a candle? You
that if you blow on the flame, it goes out, but why? To find out, you
* a candle
* a lighter or matches
* a candle holder or some aluminum foil
* a dinner plate
WARNING! THIS EXPERIMENT USES A LIT CANDLE. Be safe and use good
judgement. Never experiment with fire when you are alone. You should
always have one or more adults with you, in case there is an accident.
Place your candle in a holder. If you don't have a holder, you can make
a temporary one by crumpling a ball of aluminum foil around the base of
the candle and then pressing the bottom of it against a table to
flatten it enough for the candle to sit upright. Light the candle and
watch it burn for a minute or so. Then blow it out. Great! It worked. I
hope you liked
Wait a minute! WHY did it work? If you ask different people, you will
get different answers. Some people say that blowing on it cools
the flame enough to put it out. Others say that the carbon dioxide in
your breath smothers the flame. Lets experiment with each of these to
see if we can find out.
First, lets see if cooling the flame will put it out. If cool air will
put it out, then cold air should definitely do it. Relight your candle.
If it is a cold day, take your candle outside. If the weather outside
is not cold, open your freezer and hold the candle in the cold air.
Does it go out? No. Even in very cold air, the candle continues to
burn. That tells us that it is not cooling that lets us blow out the
OK, then maybe it is the carbon dioxide in your breath that puts out
the flame. When you breath air in, you absorb some of the oxygen from
the air and put back some carbon dioxide. Carbon dioxide itself does
extinguish a flame. Instead, it is the lack of oxygen which can cause a
flame to go out. To see if that is the reason you can blow out a
candle, we need to try blowing out the flame with air that has the
normal amount of oxygen in it. To do that, you need something that you
can use to fan through the air. I tried a plastic dinner plate and
found that it worked very well. Light the candle again. Place it on a
flat surface. Hold the dinner plate in your hand and use it like a fan
to blow air at the candle flame. With one quick snap of your wrist, you
should be able to get enough of a blast of air
to put out the candle. Now we know that blowing out a candle is not due
to changes in the amount of oxygen or carbon dioxide.
Then what does make the flame go out? To find out, relight your candle.
Now start blowing gently on the flame. You don't want to blow it
out. Instead, you want it to begin flickering. Watch the flame
you blow. Observe carefully as you blow a bit harder and a bit
harder. You should notice that the flame always seems to stay attached
to the candle wick. If it breaks away from the wick, then the flame
That is why you can blow out a candle. The wick itself is not burning.
Instead, as the wax gets hot, it melts and then comes apart to form
several new chemicals. One of these chemicals is a gas that will burn.
This burning gas is what makes the flame. As long as the flame is
around the wick, it continues to heat more wax and form more gas to
burn. If you blow the flame away from the wick, then it runs out of gas
to burn and goes out.
to Krampf Index
Including permission from Robert
Krampf to post his
experiments on my web site