How to Demonstrate the Concept of Surface Tension

Surface tension is a basic principle of science and can be seen in operation all around us. It determines the shape of raindrops, allows us to blow beautiful soap bubbles and lets insects skim across the surface of a pond. You can easily demonstrate the principle of surface tension using just a few household objects.

Things You'll Need

• Bowl
• Water
• 2 paper clips
• Pepper
• 1 drop of dish detergent or bar soap

Instructions

1. Pour water into the bowl until it is about half full.

2. Sprinkle a small amount of pepper on the surface of the water and observe what happens. The pepper will float. This may not be too surprising, because we think of pepper as being very light.

3. 

   Take one of the paper clips and unbend it, making it into an L-shape that can be used to lower the other clip into the water.

4. Use the L-shaped clip to lower the other paper clip into the water. You need to do this because if you try to just put the paper clip into the water with your fingers, you are likely to break the surface tension and the paper clip will sink.

5. Very slowly pull the L-shaped clip out from under the floating clip. Now you should have a paper clip and some pepper floating on the surface of the water.

6. Explain to the students that the pepper and the paper clip are actually heavier that the water and should sink. The reason they are floating is because of the way the water molecules are acting on the surface of the water. The molecules at the surface are very strongly attracted to each other and bunch together tightly, forming a very thin "skin." This "skin" is strong enough to support light objects such as paper clips or insects.

7. Add one drop of dish detergent and observe what happens. The surface tension will be destroyed, and the paper clip will sink. The pepper will at first be pushed to the edges of the bowl, and then it should eventually sink.

8. Explain to the students that the dish detergent destroyed the strong attraction that the water molecules had for each other, thus destroying the surface tension. Because the thin skin of surface tension was gone, the paper clip could no longer float. The pepper was driven away from the detergent because of the "negative" chemistry between them. The pepper will eventually prove itself to be more dense than water by gradually sinking to the bottom.

Resources

• Read this Article in Spanish