Surface Tension Chapter

Surface Tension

We can start off by trying to give to your child a rough idea of what surface tension is. Water molecules like each other. They attract and pull at each other. Think about the molecules at the surface. Nearby molecules are pulling at them. This causes the surface to become taut. To make it a little more concrete, you can take a paper towel or piece of cloth. Ask your child to imagine there are molecules at the 4 corners of the cloth/paper. Each corner will then try to pull the paper so as to pull the molecules at the other corners closer to it. So with 4 hands at the corners, let each hand pull and show how the cloth/paper becomes taut.

While the following activities are based on surface tension, most of them also illustrate how soap/detergent reduces the surface tension of water. Since this is a common theme, I will attempt a rough explanation of this phenomenon here rather than repeat it in the "explanation" part of these activities.

The soap molecule has one end that likes water molecules (hydrophilic) and an other end that can't stand them (hydrophobic). Suppose you like to get your feet wet. But let's also suppose that you don't like getting your head wet. So what will you do when you get into a swimming pool. You will probably spend a lot of time standing with your feet under water and your head above it. Well, a soap molecule is like you then. The soap molecules near the water surface have one (the hydrophilic) end under water and the other (hydrophobic) end sticking out. when you have a lot of such soap molecules on the surface like this, they are going to try to take some space between the water molecules, causing them to become further apart. Then the water molecules, which are now a little bit further apart, pull each other with less force and hence we have weakened surface tension.

Surface Tension Related Activities

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