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WHAT IS SURFACE TENSION?

To answer the question, surface tension is the tendency of liquid molecules to be attracted to each other (almost like magnetism) and to stick together - to adhere (pronounced "add-HEER"). Have you ever heard your mom complain about not being able to get things clean, and she sometimes says it is because the water is hard? The difficulty in getting things clean is very often caused by surface tension. Surface tension sometimes keeps water from soaking into the fabric of clothes. Detergents break up the surface tension, and allow the water to soak into the fabric.

Have you ever put water from the hose onto really dry soil outside in the yard? Have you noticed that at first, the water just runs along the top of the soil and doesn't sink
in? Why do you think that is? Right! Surface tension! If you put a little water onto the soil, then turn off the water and wait a few minutes, you would notice that the soil does begin to absorb some water. At that time, more water can be put onto the soil, and it will go into the soil--the water molecules will follow and try to adhere to the molecules that finally soaked in. Why did they begin to penetrate the soil? Because the surface tension is strongest at the surface, and the water under the surface was trapped, until the soil molecules expanded a little and took in some of the water. The rest of the water followed it.

We have discussed surface tension several times. This will be your lucky day, because now, you'll get to feel it. We're going to make tensiometers.

Each of you has some wrapping paper and a small bowl of water. Cut and fold the wrapping paper as shown.

You see, it has a base and a stem. It is important to keep the stem part dry.

The base of our instrument has to be wet, so put some water on it. Don't drown it--just make sure it is wet.

Now, lower the instrument so that it is close to, but not touching, the surface of your water. Do you see that the base is being drawn (pulled) to the water? Can you feel it? That's pretty neat, isn't it?

Let's make more tensiometers and see if other liquids draw them to their surface like the water does. We can use cooking oil, dishwashing soap, and syrup. We'll moisten the bases of our tensiometers with whatever liquid we're testing.

I'm wondering if we could actually measure the pull of surface tension. Let's attach a tensiometer to one end of an equal-arm balance scale, and bring a bowl of liquid up close to the tensiometer. We have to remember that the base must be horizontal. It has to "lie down" onto the surface of the liquid. When we lower the liquid, the surface tension of the liquid will pull on the tensiometer. Watch the scale. Start putting washers, one at a time, onto the other side of the scale, and count them, until the balance is equal.

If we know how much each washer weighs, we can say that the surface exerted so many units of weight of tension.

Some things need to be considered that may make a difference in our measurements. If the tensiometer touches the edge of the bowl, it would change the weight of the surface tension's pull. That would mean that our measurement was not accurate. Also, air bubbles may be trapped under the flat base of our tensiometer, which would be another way to cause our measurement to be incorrect.

When scientists test things, they have to be VERY, VERY careful, and sometimes make the same test several times, to be certain that their results are accurate.

It is probably more important that we learn to be careful, and to be serious about these kinds of experiments than it is to get answers to our questions. Answers are no good if they are wrong. You certainly would want the doctor to be careful when he tests your blood. If he made a mistake in his testing, he might tell you that you are really a buffalo!

We can make tensiometers from other materials, such as tagboard, card-board, stiff plastic, or glass (if we use glass, we'll have to glue some thread to it to attach it to
the balance arm, or tape it).

I wonder if the shape of the tensiometer makes any difference. We should make some with different shapes to see if they get the same results. If they do not, how do we know which one is the most accurate? Of course, when we compare different liquids, we should use the same shape and material for each tensiometer.

 

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