Home School Life Journal

Home School Life Journal ........... painting by Katie Bergenholtz
"Let us strive to make each moment beautiful."
Saint Francis DeSales

What does Soap do to Water? or Using a Series of Experiments

For our first experiment, we cut out four "boats" out of cardboard. We cut notches in the back of them to hold some things we were experimenting with for "motors": a piece of chewing gum, a small wad of tissue and a small lump of soap, all about the same size. We placed them, one at a time, in the bathtub of water. We had expected the one with the soap to move the most, but as it turned out for us, they all moved about the same. I am guessing that the experimental variables such as the size of the boats, the places we placed them and the size of the "motors" must have made a large enough of an effect to change the results of the experiment.
So, we did another experiment to see if we could see the effects of soap as compared to the other motors on surface tension. For this experiment, we filled a bowl with water and then allowed the water to become still. Then we shook pepper on the surface of the water until it had a coat of it on it's surface. We took turns holding the various things we had used in the previous experiment as the motors on the surface of the water to see how the pepper on the surface reacted. Each item made the pepper on the surface move aside, but when we put the soap in, there was a dramatic effect of much more pepper moving aside.

It is hard to tell from these pictures because it is hard to tell the pepper on the surface from the pepper on the bottom of the bowl and I forgot to take pictures until after the boys enthusiastically held the bowl to get a closer look. The moving aside of the pepper for the soap was quite dramatic.
For the third experiment, we had to carefully place a needle of the surface of another bowl of water with pieces of thread. Now we used the same items we used as motors. We dipped them into the water. When we used the wad of tissue and the piece of chewing gum the needle moved around a bit but stayed on the surface, but when we put the piece of soap in, after the needle moved around a bit, it sank. I am sorry I do not have photos for this, but I could not get good ones that showed what was happening. I will try to do it again and make a small movie to show the differences in what happens.
This series of experiments all deals with surface tension. An explanation of surface tension is that it "is caused by the attraction of water molecules to each other, just as a magnet is attracted to things of metal. A molecule is a single unit or a single part of something that goes in making up the completed whole, like a house being built brick by brick. Each brick laid goes to make a whole house. This attraction that the molecules have for each other is particularly strong on the surface or the top of the water, because the molecules have nothing above them to be attracted to, they pull harder to the sides. This pulling creates a"skin" on the surface of the water." This "skin" allows us to place things on the surface of water that it otherwise would not hold up. Soap decreases the surface tension of water, but only where the soap touches. This decrease in surface tension around the soap, but not around the other areas of water, making the skin pull. This makes a boat move, pepper move or the needle to sink.
It helps to do a series of experiments with the same variables to get a true explanation of what is happening in the experiments. Sometimes the experiments, like our first experiment, do not turn out the way you expect them to. Sometimes this is due to experimental variables that you do not mean to have. Further experiments showed us what the different items do to surface tension. "Comparing the results of the experiments helped us explain what soap does to the surface of water."- Exploring Creation Through General Science by Dr. Jay Wile

source: Exploring Creation Through General Science by Dr. Jay Wile

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