"Soap boats" Return to list of pracs

Aim: This is to illustrate the use of surface tension to create a soap boat.

Equipment: The equipment includes...  

• detergent;
• large basin of water;
• cardboard;
• scissors;
• eye dropper?

Method: Cut out a small square piece of cardboard about the size and shape of the object below...

Place gently on the surface of water. A large tub or basin of water would be good. Use the eyedropper to drops 2-3 drops of detergent into the round reservoir at the back of the boat and watch it move along.

Background knowledge: This strange phenonema is due to surface tension. On the top surface of water in any container the water molecules are crowded together packed up against each other. The detergent molecule is a much much larger and denser molecule than water. So when the drop of detergent drops into the reservior all the detergent molecules that are packed together much much more densely than the water realise that outside the boat there is a lot more room to move. They literally "push" the water molecules out of their way as they burst out the back end of the boat. This effect propels the boat forward. It is like a whole class going on an excursion by train to the city. As soon as the doors of an almost empty train open at a station the students burst in and spread themselves along the full length of the carriage in a hurry.