Soap-Powered Boat

SPARK Intern Gabi leads an activity to learn all about the surface tension of water by making a soap-powered boat!

Once you’ve followed Gabi’s instructions in the video and made your observations, do you have a hypothesis about why the soap moves the boat?

• Water is made up of molecules that stick to each other; the molecules’ “attraction” to each other (especially at the top) is known as surface tension. Surface tension is what allows things to float on water and certain animals to walk on water.
• In this activity, you can observe something called the Marangoni effect, which is when an object in an area of low surface tension moves to a place with higher surface tension. The soap that you dipped into the notch of your boat is a “surfactant”, which means that it is able to break the surface tension of water. The soap attaches to the water, and gives the water molecules something else to stick to, other than themselves. The soap reduces the surface tension of the water in that area, but the rest of the water molecules in the container are still attracted to each other. The force of the surface tension pulling on the front of the boat is greater than the force pulling on the back of the boat, so the boat moves forward.
• You may have noticed that if you tried dipping the toothpick with soap back into the notch, the boat may not move forward again. This is because the surface tension of the water has already been broken in that area. In order to do this experiment multiple times, you need to replace your water with each test!

To learn more about the Marangoni effect, head over to Khan Academy.