Experiment of the Week - #155
This week's experiment came from
a shopping trip. Lisa wanted a new cake pan and as I had just gone to
the bank, I paid with a $50 bill. Most stores test large bills with a
special pen, to see if they are counterfeit, but this store had run out
of the special pens. Instead, they put a drop of iodine on the bill to
be sure that it was good. To see why, you will need:
* tincture of
iodine (from the pharmacy)
* a dollar bill
* bread or a
Warning! Iodine is poisonous and
will stain skin and clothing. Also, some people are allergic to iodine.
Be safe and careful.
Usually the bottle of iodine has
a plastic stick attached to the cap, to make it easy to apply. Use this
or a toothpick to put a tiny bit of iodine on a piece of white paper.
Notice that the paper turns black. Put a drop of iodine on a piece of
bread or cracker and it will also turn black. This is an indicator
test. Iodine turns black when it comesin contact with starch. Most
paper contains starch and will turn black with iodine.
Now put a drop of iodine on a
dollar bill. It stays a brownish color. The special paper used from
printing U.S. money does not contain starch, which is one of way that
you can spot a counterfeit bill.
Try testing small pieces of
different fruits and vegetables, to see which ones contain starch. Be
sure to throw away the things you test, to be sure that no one
accidentally eats any of the iodine. You might also try testing
different kinds of paper, to see if you can find any others that are
starch-free. Any of you that are outside the U.S., let me know if the
bills from other countries are starch-free too.