Activity: Surface tension

I picked up a new salt and pepper set at the post-Christmas sales and it reminded me of a simple activity.

You will need:

• A bowl
• Water
• Finely ground pepper (it doesn't need to be freshly ground, store-brought pepper is normally fine enough)
• A pencil
• Detergent

What to do

1. Pour some water into the bowl
2. Grind or sprinkle pepper onto the surface of the bowl. Most of it should stay on the surface, although a little will sink.
3. Dip the tip of the pencil into the detergent.
4. Now dip the pencil into the middle of the bowl
5. Watch what happens to the pepper

What's happening

The pepper should move away from where the pencil touches the water. The pepper moves because the detergent on the tip of the pencil reduces the surface tension of the water at the point where it touches.

In many liquids, the molecules of the liquid are attracted to each other. This attraction makes the surface of the liquid act like a stretched-out balloon skin. Any point on the surface of a liquid is under tension. In water, the tension is only very slight and it is fairly easy to break through the surface, but if you have ever done a "belly-flop" into a swimming pool, you have felt the effect of surface tension.

Different liquids have different amounts of surface tension. The surface tension of Mercury is more than six times as great as water. A mixture of water and detergent (or soap) has much less surface tension than water.

Normally, surface tension doesn't make things move around on the surface, because there is the same amount of force pulling in each direction and it cancels out. When you put a little bit of detergent into the water, the surface tension at that spot was reduced. When this happened, the force on the surface around it didn't cancel out any more, so the water on the surface pulled away from that spot. As it moved, the pepper was carried with it.

Another famous demonstration of surface tension is the "detergent-powered boat". If you put a plastic tag from a loaf of bread onto the surface of the water, then put a drop of detergent into the hole in the tag, the surface tension near the hole will be reduced and the tag will be pulled in the other direction. It's not really powered by detergent, but by the surface tension of the water.

When you sprinkled the pepper onto the surface, the pepper wasn't actually floating, it was sitting on the surface because it was too light to break through the surface tension of the water. You can tell because if you stir the water up, the pepper on the surface will sink. There are some insects, called water skimmers, which use this same effect. They land on the surface of the water, but are so light they don't break the surface tension.