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# Surface Tension: A Liquid's Skin-like Property (page 2)

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Author: Janice VanCleave

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Detergent is called a surface-active agent, or a surfactant, because it accumulates at a liquid's surface and reduces the surface tension of the liquid. What effect would a surfactant have on the results of the experiment? Repeat the experiment using water in which teaspoon (2.5 ml) of liquid dishwashing detergent has been dissolved.

1. A drop of surfactant in water immediately weakens the surface tension where the surfactant enters the water. Design a way to show how differences in surface tension can cause a liquid to flow. One way is to add just enough water to cover the bottom of a plate. Let the plate stand for about 1 minute to allow motion of the water to stop. Place one drop of food coloring on the edge of the water. Wet one end of a toothpick with liquid detergent and touch this wet end to the colored spot in the water. Observe any motion of the colored water.
2. Surfactants weaken surface tension. How does the concentration of a surfactant affect surface tension? Design a way to determine how the concentration of a surfactant affects surface tension. One way is to cover the bottom of a small plate with water. Place two round toothpicks on the surface of the water. With a third toothpick, move the floating sticks so they are close together. Wet the end of the toothpick you're holding with liquid detergent and then touch the wet end to the water between the two floating toothpicks (see Figure 4.2). Observe the speed at which the two toothpicks move. The greater the speed, the greater the substance's effectiveness at lowering the water's surface tension. Repeat the investigation four times, using clean water, new toothpicks, and different concentrations of detergent. The detergent concentrations can be made using the combinations in Table 4.1. The materials can be measured with an eyedropper, combined on a sheet of waxed paper, and each stirred with a separate toothpick.
3. Rate the effect of the different concentrations on their ability to lower surface tension using a distance scale from 0 to 5 to compare the distances the toothpicks move, with 0 being the least distance. Record the distance rating for each detergent concentration in a Surface Tension Data table like Table 4.2.

Use this information to prepare a graph comparing detergent concentration with its effect on lowering surface tension. For more information about the effect of concentration of surfactants on surface tension, see Carl H. Snyder, The Extraordinary Chemistry of Ordinary Things (New York: Wiley, 1998), pp. 322-330.

4. Surface tension is also the reason why unconfined liquids form drops. The net inward force on the surface molecules of water causes the surface to become as small as possible. A sphere is the shape that has the least surface for a given volume. Design a way to show the shapes of drops of liquids that have weak surface tension. One way is to use different concentrations of a detergent solution (as listed in Table 4.1). Use an eyedropper to place 1 drop of each solution onto waxed paper, and compare the roundness of each drop. Rate the roundness of each drop on a scale of 0 to 5, with 0 being the least round.
5. Detergents reduce the surface tension of water, which makes the surface of water stretchier. This is why bubbles can be made with soapy water. Determine the best soap-and-water combination for blowing the largest bubbles. Does glycerin (found in most pharmacies) have an effect on the surface tension of water? Try using glycerin in your bubble solution to see what effect it might have.

### Get the Facts

1. Water molecules have hydrogen bonds. How do these bonds affect surface tension? For information, see a chemistry text.
2. Water is not always wet. What can this mean? What does the wetness of water depend on? How can the wetness of water be increased? For information about making water wet, see Robert L. Wolke, What Einstein Didn't Know (New York: Dell, 1997), pp.194-195.
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