Soap powered boat

Make a boat that uses detergent to move through the water and learn about surface tension.

You will need

  • thick cardboard or thin plastic (like the lid of a margarine container) or bread bag clip
  • scissors
  • large shallow bowl or a baking dish with shallow water
  • dishwashing liquid
  • toothpick, pencil or eye dropper.

What to do

  1. Carefully cut out a boat shape from the cardboard (around 4 centimetres long by 2 centimetres wide).
  2. Cut out a triangular notch at the back of the boat with the triangle pointing towards the back of the boat.
  3. Gently place the boat on the water in the dish (using a bath tub instead of the dish will give the boat more room to move). 
  4. Using the pencil, toothpick or dropper, place a drop of detergent into the notch at the back of the boat.
  5. Watch your boat zip across the water!

Note: you could use a bread clip as your boat instead of cutting one out of cardboard.

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. A mixture of water and detergent (or soap) has much less surface tension than water.

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. The triangular notch helps the boat drag a little bit of the detergent with it, so it travels further.

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.

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