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Why We Use Soap

Today I spent some time rebuilding a transformer for my Tesla coil. I got the idea for this 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.