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Today I was so busy having fun that I put off working on this week's experiment. I had several ideas bubbling around in my brain, but nothing quite came together. Then, as I often do when I am looking for an idea, I picked up the first thing I saw and began playing with it. It happened to be a bottle of liquid soap. I turned it over and watched as the bubble rose slowly through the thick liquid. I knew that I had this week's experiment. You will need:

  • several clear bottles of liquid. You can use cooking oil, vinegar, shampoo, honey, window cleaner, cough syrup, or any other liquid that comes in a clear bottle.

We are going to test the different liquids for their viscosity. What is viscosity? It is a liquid's resistance to flowing. If a liquid has a low viscosity then it flows very easily, like water. If it has a high viscosity then it flows very slowly, like honey.

One way to test viscosity is to drop objects into the liquid and time how long it takes for them to fall to the bottom. You could do this test by dropping marbles or small stones into each bottle of liquid, but whoever does the cooking at your house would not be pleased to find rocks in the bottles of oil, honey, etc. A much neater way to test viscosity is to use rising air bubbles instead of falling rocks.

First, check the top of each container, to be sure that it is securely in place. This is the voice of experience talking, as the top for my liquid soap was not very secure during my first test. Turn the bottle upside down and watch what happens. You should see one or more air bubbles rising through the liquid. Depending on which liquid you used, it may have moved quickly or slowly. Try other liquids and compare the speed of the rising bubbles.

As you test different liquids, arrange them in order of their viscosity with the highest viscosity at one end and the lowest at the other. You can even make your own viscosity chart, allowing you to quickly rate the viscosity of each new liquid.

Viscosity is very important. Motor oil commercials talk about viscosity frequently. If an oil is not viscous enough, then it will not lubricate the engine enough. This lets friction heat the engine and possibly damage it. If the oil is too viscous, then it is difficult for the engine parts to move, which is also not a good thing. Viscosity changes with temperature, so motor oils must keep the right viscosity in very hot and very cold weather.

Viscosity is also important in foods. Each gravy or sauce has its own proper viscosity. Imagine spaghetti sauce as thin as water or soup as thick as honey. Different gravies have different viscosities. The "Red Eyed" gravy made from good, southern, country ham is almost as fluid as water, while some white gravies are much thicker than molasses. Interestingly, they both taste wonderful over fresh made biscuits. A stick of butter can easily show you how viscosity changes with temperature. As with motor oil, the warmer the butter gets, the lower its viscosity is. Just be sure to remember that butter is MUCH better on biscuits than motor oil. You can get the opposite result by pouring hot fudge sauce onto very cold ice cream. You can actually see the fudge sauce get thicker and thicker as it is chilled by the ice cream. Don't take my word for it. That is one experiment that you definitely will want to try for yourself.