What you need:
- Washing up bowl. Or you can do it in the bath!
- Liquid detergent or washing up liquid
- Cut out a small boat shape from the card.
- Using the scissors, make a little notch in the cardboard at the
back of the boat.
- Fill the washing up bowl with water.
- Place the boat gently on the water, so that it floats.
- Pour some of the liquid detergent into the cup.
- Use the toothpick to get a drop of liquid detergent on the
- Place the drop of detergent in the notch in the end of the
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.