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This Week's Experiment - #255 Why We Use Soap
Today I spent some time rebuilding a transformer for my Tesla coil. I got
the idea for this week's experiment as I was cleaning up, trying to get the
tar and grease off of my hands. Have you ever wondered why we use soap when
we wash things? What is so special about soap? To find out, you will need:
soap
water
some oil, butter, or grease
Be sure to do this experiment in the sink, as it could be messy. Also, be
sure to clean up afterwards, so you don't get in trouble and I don't get
e-mails from unhappy parents.
The first thing we need to do is to get your hands dirty. Instead of using
dirt, we are going to use something oily. Cooking oil, butter or petroleum
jelly will work very well. I used cooking oil. Put some into your hand and
rub it into your hands, just as if you were washing them. You want your
hands to be covered with the oil.
Next you will try to wash the oil off of your hands. DO NOT USE SOAP!
Instead, just use water. Keep washing until all of the oil is gone. Do not
wipe the oil off onto the towel. You hands should be clean and oil-free
before you dry them.
Did it work? No, you probably could not get all of the oil off of your
hands. Now use a small amount of soap and try washing your hands again. Did
it work this time? Yes, it worked much better this time. Why?
Generally, oil and water do not mix. Water is a polar molecule, which means
that it has a slight charge at each end. Because of these charges, water
molecules tend to stick to each other. It will also stick to other polar
chemicals, such as alcohol. You can easily mix water and alcohol because
they are both polar.
Oil is not polar, so water molecules do not stick to them and they do not
stick to the water. When you pour oil and water together, the water
molecules stick to each other and the oil molecules stick to each other.
Since the two liquids stick to themselves but not to the other, they form two
layers, with the denser one on the bottom.
This is where soap comes in. Soap is a wonderful chemical which has
molecules that are polar at one end and nonpolar at the other. That means
that one end will stick to water and the other end will stick to oil. The
soap forms a link to connect the water to the oil, letting them mix.
That is what happens with the oil on your hands. When you just used water,
the oil stayed on your hands. When you added the soap, it let the oil stick
to the water, pulling the oil away from your hands. That is why your mother
is always reminding you to use soap when you wash your hands. Dirt sticks to
the natural oils of your skin and you need the soap to help remove it.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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