This week, a classic demonstration of one of the basic properties of water.
You will need
Cardboard or thin plastic (like an old margarine container lid)
A shallow container of water
Detergent
Toothpick, pencil or dropper
Scissors
What to do
Cut a flat, boat shape out of the cardboard.
At the back of the boat, cut out a triangular notch, with the triangle pointing towards the back of the boat.
Gently place your boat on the surface of the water.
Using the pencil, toothpick or dropper, place a drop of detergent into the notch at the back of the boat.
The boat will zip along the surface of the water. When it stops, add another drop of detergent and it may start again.
If you have a plastic tag from a loaf of bread, you can use it as your boat.
What's happening?
It looks like the boat is being pushed along by the detergent, but in fact it is being pulled by the water in front of it.
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, an object on the surface feels an equal pull in every direction, so it doesn't move. When you put the detergent in the water, the surface tension at that point was reduced. This stopped it cancelling out the force from surface tension on the front of the boat, so the boat was pulled forwards. Having the triangular notch helps the boat drag a little bit of the detergent with it, so it travels further.
Normally, surface tension pulls the boat in every direction.
When you add the detergent, the surface tension at the back is reduced. The tension at the front pulls the boat forward.
You may find the boat only works once or twice. This is just because the water mixes with the detergent you have dropped in, so the surface tension of the water in the container is reduced and it can't pull the boat as well.
Applications
There are insects called water skimmers, which use surface tension to float on water. They land on the surface of the water, but are so light they don't break the surface tension.
A simple boat shape with the notch at the back.
Place the boat in the water and add a drop of detergent in the notch.
