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Krampf #483 Why Wet Things Don't Burn?
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KRA-@aol.com
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Apr 17, 2007 20:58 PDT
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Robert Krampf's Experiment of the Week: #483 Why Wet Things Don't Burn?
Greetings from Fort Lauderdale, Florida. I had a wonderful weekend of shows
at the Museum of Discovery and Science. Thanks to all of you that came by
to say hello. This week I have more school shows for Florida Power and Light,
and Omar and I are taping some new science videos. This time, we will tape
some that include ice cream.
I finally put up my blog, The Happy Scientist, for anyone that wants to know
more about me, follow my travels or see my photography. Be sure to check out
the photos of Nancy and me wearing rabbit costumes to the grocery store.
In this week's science video, see how to make an object that only has one
side.
This week's experiment was inspired by my recent science video about how to
hold a balloon in a candle flame without popping it. It started me thinking
about why water is so effective at stopping fires.
*WARNING* This experiment involves the use of fire. Be safe, and be sure
that there is an adult with you to help.
To try this, you will need:
paper towels
kitchen tongs or pliers
a lit candle or a lighter
a bowl of water
Begin by tearing a strip of paper towel about an inch wide and a couple of
inches long. Hold it in the tongs and place it over the flame of the lighter.
What happens? It quickly catches fire, just as you would expect. Put the
burning paper into the water to put out the flames.
Then tear another strip of paper towel the same size. Hold it with the
tongs, but this time dip in into the water first. Once it is wet, hold it in the
flame. What happens this time? The paper turns black, but it does not
burn. Even the black color does not come from the paper. Instead, it is
carbon soot that comes from the flame. As long as the paper is wet, it will not
burn. Why?
Understanding the Science
Some things are so common that we just accept them without wondering why.
Water puts out fires, so of course wet things don't burn. But why? There
are several things that add together to give water its fire quenching ability.
First, water does not burn. Why not? After all, it is made up of hydrogen
and oxygen, both very flammable gases. When you burn fuel, it combines with
oxygen. Last week, we saw that when we burned iron, it bonded with oxygen
to become iron oxide. The carbon found in many fuels will bond with oxygen to
become carbon dioxide. The hydrogen in water is already bonded to oxygen,
so it is the same as if it had already been burned.
Second, water absorbs a lot of heat. Every material has a property known as
specific heat. That is the amount of energy that it takes to raise the
temperature of one gram by one degree. Water has a high specific heat, which
means that it can absorb a lot of heat energy before it gets hot enough to boil.
At the point where it boils, it absorbs even more heat, to give the
molecules enough energy to change from a liquid to a gas. With the water absorbing
all that heat, the wet fuel does not get enough heat for it to burn. That is
how the balloon in the video was protected from the flame. It is only when
the water has evaporated that the fuel can finally get hot enough to catch
fire.
Water also separates the fuel from the oxygen it needs to burn. A coating
of water provides a barrier to keep oxygen away from the fuel. Without that
oxygen, the fuel won't burn.
All these things combine to explain why wet things don't burn.
Safety
With that said, it is important to mention that there are some fires that you
should never use water on.
Never put water on an electrical fire. Water conducts electricity, and it
could provide a connection between you and the electric wires.
Never use water on grease or oil fires. Water is heavier than the oil, so
instead of floating on top, where it would block the oxygen, it sinks to the
bottom. There it absorbs enough heat to change into steam. The rapidly
expanding steam throws the burning oil in all directions, spreading the fire
instead of putting it out.
That is why you should always have an approved type ABC fire extinguisher in
your kitchen. They are inexpensive and can save your kitchen and your life.
Have a wonder-filled week!
Robert Krampf
Robert Krampf's Science Education Company
www.krampf.com
The Happy Scientist Blog
Who is Robert Krampf?
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