Power a boat with soap

Power a boat with soap

Sail the soapy seas!

What you need:

  • Scissors
  • Cardboard
  • Water
  • Washing up bowl. Or you can do it in the bath!
  • Liquid detergent or washing up liquid
  • Toothpick
  • Cup

How to:

  1. Cut out a small boat shape from the card.
  2. Using the scissors, make a little notch in the cardboard at the back of the boat.
  3. Fill the washing up bowl with water.
  4. Place the boat gently on the water, so that it floats.
  5. Pour some of the liquid detergent into the cup.
  6. Use the toothpick to get a drop of liquid detergent on the end.
  7. Place the drop of detergent in the notch in the end of the boat.

What happens?

You should see your boat sail across the water!

N.B. The experiment won't work if the water already has soap or detergent in it.

Watch this video to see a soap boat in action. Instead of cardboard, the Questacon Science Squad use a plastic bread tie to make their boat.

How does it work?

Water molecules are strongly attracted to each other and stick close together. This creates a strong but flexible "skin" on the water's surface called surface tension. Surface tension allows the cardboard boat to float on top of the water.

Adding soap disrupts the arrangement of the water molecules. The water molecules near the detergent are attracted to the detergent as well as to other water molecules, so the surface tension of the water behind the boat decreases. Water molecules move from areas of low surface tension to areas of high surface tension. The boat is pulled towards areas of high surface tension by the water in front of the boat.

Curriculum information

  • Summary

    Investigate forces and movement as well as surface tension. A simple demonstration that can stimulate students to try and explain why a detergent-powered 'boat' moves over the water's surface. A simple explanation involves the detergent generating a 'push' whilst a more complex explanation looks at the effects of detergent on the water's surface tension.

    Curriculum key wordsOther web links
    • Forces and movement
    • Surface tension
    Moving on Water in Nuffield Primary Science
    Science Curriculum Links
    National CurriculumCurriculum for excellenceNorthern Ireland Curriculum
    Forces and their effects (KS1 SC4 2b,c). Forces can affect motion (KS3 3.1b). Explanations can be developed using scientific ideas (KS4 1.1c).Forces on toys and other objects (SCN 1-07a). Relate forces to motion (SCN 4-07b).Pushes can cause movement (KS2 Strand 2). Forces and Energy (KS3). Explanations can be developed using scientific ideas (KS4).