Sail Away with Soap: Build a soap-powered boat

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Surface tension can be challenging to explain, but it’s really easy to demonstrate!

The Soap-Powered Boat STEM experiment demonstrates how the surface tension of water can be disrupted using the power of dishwashing liquid. The dishwashing liquid reduces the surface tension at the back of the boat, and the stronger surface tension at the front of the boat pulls and propels the boat forward.

Let’s get sailing!

How to make the Soap-Powered Boat experiment

Supplies you will need

For this experiment, you’ll need:

A small leaf
Scissors
Pipette
Dishwashing liquid
A large tray of water

Before you start

We found that we had to use a new leaf each time we wanted to do the experiment, otherwise it wouldn’t work properly.

Instructions

Here is how to do this experiment with your child:

Step 1: Cut a small notch in the leaf

In the back of the leaf, cut a small notch (see picture). This is where we will add the dishwashing liquid.

We chose to cut the notch at the heaviest point of the leaf, where the stem is.

Step 2: Add dishwashing liquid to the boat

Use your pipette or dropper to gather some dishwashing liquid.

In the notch of the leaf, add a small drop of the soap and watch your boat take off!

Here are some extension activities, if you want to keep the fun going:

Time the boat to see how fast it makes it across the water (turns it into a math experiment!)
Measure the distance the boat travels (also turns it into a math experiment!)

Leaf in the water, showing where to add the dishwashing liquid

The STEM behind the Soap-Powered Boat experiment

This experiment teaches:

Surface tension
Observation
Cause-and-effect

How it works

Water molecules are polar (oppositely charged) and therefore very attracted to other water molecules. That attraction causes a high surface tension in our container of water.

Dishwashing detergent has two ends, one of which is polar and the other is hydrophobic (water-hating). That polar end is attracted to the polar water molecules, and that reduces the surface tension.

So when we add soap to the back of the boat, it reduces the surface tension there. The stronger surface tension at the front of the boat pulls it forward, propelling the boat.

Surface tension

We can demonstrate surface tension by observing how the soap affects the boat’s movement.

The water’s surface acts like a stretchy skin due to surface tension. When soap is added, it breaks this “skin” at the back of the boat, where we cut the small notch and added the soap.

This imbalance in surface tension creates a force that propels the boat forward. By experimenting with different amounts of soap and boat designs, kids can visually see how surface tension influences the boat’s behavior.

Observation

Have your child make some observations about the boat:

How did the dishwashing liquid affect the boat?
Did the shape affect the boat’s speed? (if you made multiple boats to compare to one another)
Why did we place the dishwashing liquid at the back of the boat? What would have happened if we placed it in front of the boat instead?

Making observations about the experiment helps to solidify the lesson!

Cause-and-effect

Kids can observe cause and effect through this experiment by changing something in the experiment (adding dish detergent) and watching how that affects the boat.

Cause-and-effect thinking empowers children to make sense of their world, learn from their experiences, and develop problem-solving abilities.