Soap Boat Experiment

One day, my 4-year-old daughter asked, “What is science?”

“Well, remember you always ask me Why — ? Why is the sky blue? Why is sunset orange? Why are bubbles round?” I answered.

Science helps us find out answers to these whys!”

My daughter’s question came at the perfect time when I received the new book Ada Twist, Scientist* whose creator wrote the New York Times bestselling picture books Rosie Revere, Engineer and Iggy Peck, Architect.

My four year-old couldn’t read yet, but she was intrigued when I told her about this book.

Like her, scientist Ada is always curious and wants to know why to everything.

She likes to conduct scientific experiments and is not afraid of failure.

She also asks lots of questions and searches for the answers.

What better way to illustrate the spirit of Ada Twist than doing a science experiment ourselves to explore, discover and learn?

Soap-Powered Boat

A few weeks ago, we did an experiment using pepper, water and liquid soap.

After sprinkling some pepper in the water, we were able to use liquid soap to move and scatter the pepper.

We wondered if liquid soap could do the same thing to a boat, making it move across the water.

Can soap power a boat?

soap-powered boat science experiment

Soap Powered Boat

Prep Time: 40 minutes
Active Time: 30 minutes
Total Time: 1 hour 10 minutes

Today, we are building a soap-powered boat to learn about an important water property.


  • scissors
  • a sheet of clear PVC plastic or styrofoam (I used a clear PVC gift box)
  • (Optional) color paper and a stick
  • liquid dish soap


  • a wide container or a tray
  • a dropper or a drinking straw
  • adult supervision


  1. Cut the sheet of plastic or styrofoam into the shape of a boat.
  2. At the end of the boat, cut a small notch.
  3. (Optional) Decorate the boat with a stick and a triangular piece of paper. I also added some foam shapes for fun. boat floats on water
  4. Fill the container or the tray with a thin layer of water.
  5. Place the boat in the water.
  6. Using a dropper or a drinking straw, put a drop of liquid soap into the notch at the end of the boat and watch your boat move forward!

Did you try this project?

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Do you have a hypothesis on why the boat moves?

After the boat has stopped, repeat the experiment a few more times in the same pool of water and see what happens.

Are you able to prove or disprove your hypothesis?

Soap powered boat explanation

If your boat is lightweight (not too many decorations), it should float and move forward quickly when you put the first drop of soap into the notch.

The boat moves because the soap alters the water’s surface tension.

Water molecules (the tiny little pieces of water) tend to stick to one another (cohesion).

On the surface of the water, water molecules are more attracted to other water molecules than to the air resulting in surface tension.

In this experiment, the “boat” can float on water due to this surface tension.

Water has high surface tension, which means the molecules are pulling each other on the surface very strongly.

Soap is a surfactant that breaks down water’s surface tension by giving water molecules something else to be attracted to (soap molecules).

When a drop of soap is placed in the notch, the surface tension of the water in that small area is reduced.

The high surface tension in the rest of the surface of the water pulls the water away from the area with low surface tension dragging the boat with it.

After repeating the experiment several times, the boat stops moving despite adding more soap.

By then, the surface tension in the entire pool of water has been broken.

When the surface tension difference between the front and the back of the boat disappears, the water will not be pulled and the boat will not move.

The Book Ada Twist, Scientist

Ada Twist, Scientist is an amazing picture book for children. Ada is a budding scientist who asks Whys, proposes hypotheses, overcomes failure and looks for answers. Ada Twist, Scientist will be released on September 6th, 2016.

Ada Twist Scientist book cover and soap boat experiment explanation

More Experiments

For more Ada Twist, Scientist experiments, visit


Questacon, The National Science And Technology Center

* My daughter received a free copy of Ada Twist, Scientist and a signed print.

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