Pepper and Soap Experiment

Pepper and soap experiment

With this easy-to-do pepper and soap science experiment, kids can easily understand complicated science concepts like buoyancy and surface tension. As a bonus, you can add a lesson on hygiene and use the experiment to explain how soaps work.

STEM concept: Buoyancy, surface tension, how soap works
Challenge: Easy
Messiness: Low


• black pepper
• a plate or bowl
• water
• liquid soap

Steps to follow

1. Fill the plate with a little bit of water, enough to cover the entire surface with a thin layer.
2. Sprinkle the pepper all over the water. Cover as much of the water surface as possible.
3. Dip your finger into the liquid soap.
4. Dip your finger covered with liquid soap into the middle of the pepper-covered water surface.
5. Watch what happens.

The science behind the experiment

What is buoyancy?
Bouyancy is the upward force exerted by a liquid. This force causes objects to float in a liquid. The pepper flakes in this experiment floated because of buoyancy.

What is surface tension?
Water and other liquids have surface tension because their molecules pull towards each other and stick together very strongly. Surface tension causes the pepper to stay on top of the water’s surface.
When you dipped your finger with the liquid soap, the soap broke the surface tension of the water! Some of the pepper flakes sank to the bottom of the plate or bowl. But the water wants to retain the surface tension, so it moves away from the soap, bringing the pepper flakes with it. This caused some of the pepper flakes to move towards the edges of the plate.

How does soap work?
This experiment can also be used to teach kids why soap is an effective cleaning agent. Soap breaks down water’s surface tension. Washing our hands with water alone will not effectively clean it. Using soap will do the trick.

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