Surface Tension of Water
PURPOSE Observe the surface tension
of water by placing as many drops of water on a penny as you can without
them falling off.
PERFORMING THE EXPERIMENT 1. Place a clean dry penny on the tray provided. 2. Partially fill a dropper with water. Note that you take water into the dropper by squeezing the plastic or rubber bulb, placing the tip of the dropper well under the surface of water in a cup, and releasing your squeeze to let water into the dropper. You let drops out of the dropper by gradually squeezing the plastic bulb. Before you begin the experiment practice using the dropper over the cup of water until you can let the drops out only one at a time. 3. Hold the plastic dropper vertically about 1 centimeter above the penny in order to place drops of water on the penny one by one. If you arenít sure how high 1 centimeter is, check with the ruler. 4. You and your adult partner should both count the drops carefully as you drop them on the penny. Donít lose count! Watch from the side as the water builds up. The surface tension of the water will finally break, causing the water to spill off the penny. 5. Carefully empty the water from your tray back into the cup. 6. Dry your penny and tray with a paper towel, see if you can pile more drops on during a second try, and record your result. Then let your adult partner try the experiment. 7. On the data
sheet, write your first and last name neatly and the maximum number of
drops of water that you were able to get onto the penny at one time.
QUESTIONS TO THINK ABOUT 1. Could you coat the penny with something that would help you add more drops of water before it spills off? Could you coat the penny with something that would make the water spill off when fewer drops are added? 2. How would you modify the
dropper so that you could release smaller drops and thus get more drops
on the penny?
EXPLANATION The structure of water molecules
makes water act in some interesting ways. This activity shows you
one of the wonders of water, namely its ability to cling to itself and
to pile up on a surface. Because of the way hydrogen and oxygen atoms
are joined within a water molecule, water molecules are attracted to or
pulled toward one another. This is called cohesion. There is
a difference in this attraction among water molecules at the surface of
water and in all the rest of the water. A water molecule at the surface
is attracted mostly by water molecules beneath it. A water molecule
below the surface is attracted in all directions by the water molecules
around it. Thus, water molecules can form a kind of "skin" on the
surface of the water. This results in what scientists call surface
tension. Surface tension helps a drop of water hold its shape.
It also lets some insects walk around on the surface of the water.
EQUIPMENT NEEDED Pennies; plastic or glass medicine
droppers; paper cups of water; trays to catch water; paper towels; 15-cm
rulers.
REFERENCE Modified from WonderScience, Fun Physical Science Activities for Children and Adults to Do Together, Volume 7, Number 1, January, 1993, American Chemical Society/American Institute of Physics, page 7.
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