This Week's Experiment - #288 No Dented Cans
One of the things that I have enjoyed on this trip is doing the cooking. I
really like to cook, both for the fun of making things that taste good and
for the science involved. We went into Whangarei to get groceries, and while
we were shopping, I noticed some dented cans. That gave me the idea for this
week's experiment. Why is it a bad idea to buy dented cans? Because they
don't look as nice? Because some of the food inside might be dented too? To
find out, you will need:
a penny or other copper coin
some aluminum foil
some catsup
a plate or saucer
Tear a strip of foil about an inch wide and 3 inches long. Place it on a
plate. Put the penny on one end of the strip. Squirt a blob of catsup on
top of the penny and then fold the other end of the foil over, so that it
squishes into the catsup. Crease the fold enough so that it will stay in
place. This should give you sort of a foil and penny sandwich. Looking from
the side, you should have foil on the bottom, with a penny on top of that,
then catsup and then the other end of the foil. Set the plate in a warm
place where it will not be disturbed.
The next day, rinse the catsup off the foil and examine it carefully. In the
area where it was touching the catsup, it is full of holes! Oh no! If
catsup does that to aluminum foil, think what it is doing to my stomach! I
can never eat French fries again!
Yes, the catsup does contain acid. ACID!!!! Don't get too excited. The
acid is vinegar. Read the label and you will see. How did vinegar do that?
Well, the acid is not responsible, or at least, not directly. The culprit is
electricity.
Electricity? I did not tell you to plug it into the wall. DON'T DO THAT!!!
It would be very dangerous, and besides, you don't need to. This experiment
makes its own electricity. The copper penny and the aluminum foil, connected
by a conducting solution (catsup) form an electric cell, which will produce
about 1/2 of a volt of electricity. You can't light a light with it, as the
amperage is very small, but it does produce an electric current. If you cut
through the foil strip in the center and connect a probe from a volt meter to
each piece of foil, you should be able to measure about 1/2 of a volt.
In the process of producing the electric current, the penny is stealing
electrons (tiny, negatively charged pieces of atoms) from the aluminum. This
changes the aluminum and lets it dissolve in the acid in the catsup.
OK, so now we know how to dissolve holes in aluminum, but what does that have
to do with dented cans. Cans are made of steel. The problem with steel is
that it rusts when it comes in contact with water and oxygen. Since most of
the foods that we put into cans are wet, they would quickly rust the can. To
solve this, the steel cans were plated. That means that the inside was
covered with a thin coating of the metal tin. Tin is not strong enough to
make a good can, but it does not rust. The tin coating on the inside keeps
the can from rusting and the steel give the can its strength.
If a can is dented, the tin layer can be cracked, letting the liquid come in
contact with the steel. Now you have two metals (steel and tin) in contact
with a conducting solution (the liquid in the can). The can of food begins
to produce electricity. It also starts to oxidize (rust) the metal in the
can, producing a hole and letting the food inside spoil. That is why we
don't buy dented cans.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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