Surface Tension

Lesson on Surface Tension

There are so many interesting surface tension-based science activities that the kids can enjoy and some might even surprise you.

First, you might want to start with gently introducing the concept of surface tension. The homework sheet had some discussion of surface tension and drew an analogy with a balloon. The analogy to a stretched surface can be useful. Show your child a stretched rubber band and then cut it. Show them how the two ends of the cut band pull away. This exhibition might come in handy if you want to explain what happens in the boat and the paper clip activities mentioned below.

The following three activities are probably the most widely mentioned activities associated with surface tension. I wouldn't even know how to pick one source to cite since I have found these activities described in so many websites and books.

Boat powered by surface tension You can remind your child of what happened when you cut a stretched rubber band. The tension in the rest of the band pulled the end away from the place where it was cut. In the same way, the surface tension of the water is lowered by the detergent/soap. Lowering it enough is like cutting the water surface. The rest of the water surface pulls away from this point, and pull the paper boat along.
Floating a paper clip While the paper clip metal is denser than water, it is light enough for the water surface to hold it up. That's surface tension at work. When soap/detergent is added, the paper clip falls into the hole on the surface "cut" by the detergent.
Mixing colors in milk Actually there is a more than surface tension at work here. There is also emulsification that brings the incompatible oil/fat and water together. But still it is the weakening of the surface tension that starts it going and besides this is fun to watch. It will look better with higher fat content in the milk. Skim milk, for instance, is almost all water and hence the food coloring will disperse quite a bit even before you start with the soap/detergent.

Here is another quick thing you could try. It might perhaps work better with older children or even adults. Fill a glass with water (say a little more than half full). Take a cork (or anything else that floats about that size) and ask your child to float it in water. Ask your child to try to make it float in the center. The cork should slowly always move away from the center and towards the sides. Now, put on your magician's hat. All you need to do is start pouring more water. As the water reaches the rim, slow down and add more water carefully so that a convex water surface forms but it doesn't spill over. The cork will move to the center on its own.

(I found this "experiment" in Martin Gardner's book "Entertaining Science Experiments with Everyday Objects" published by Dover publications.)

To explain why this happens, ask your child to look at the water surface closely when the glass is not full. You could point out that although the water molecules attract each other, the glass attracts the water molecules more. So the surface should be slightly concave. In contrast, when you added more water, the surface becomes convex. The bouyancy makes the the cork seek the highest point. (You could mention how if you throw a tennis ball into the ocean, the ball always rises and rides the waves.)