Pepper and Soap Experiment!

Key Concepts

Activity Directions:

Pour 4 cups of water into a large bowl.
Sprinkle pepper into the water until there’s a thin layer of pepper on the surface.
Squirt a little bit of soap into a small bowl.
Take your index finger and dip it into the soap.
Take your index finger and dip it down into the center of the large bowl.
Notice that the pepper “runs away” from your finger!

Discussion Questions & Science Explanations:

Why do the flakes float instead of sinking into the water?
1. The water molecules at the surface of the water like to stick together, making the surface of the water tight as if it were covered with plastic wrap. This property of water is called surface tension—you might have seen this cool effect in raindrops on a leaf.
2. The pepper flakes are hydrophobic, meaning that they repel water and do not like to mix with it. Because of a combination of the fact that pepper flakes are light and hydrophobic and that water exhibits surface tension, the pepper flakes float on top of the water.
Why do the pepper flakes seem to “run away” from my finger?
1. Soap disturbs the bonds between water molecules, and therefore disrupts the surface tension of water when they’re mixed. Because water naturally wants to preserve its surface tension, water molecules near the soap will move away from the soap to another place where they can maintain surface tension.
2. When the water molecules move away from the soap, they carry the pepper flakes with them. This creates the effect of the pepper flakes “running away”.

Additional Experimentation:

Instead of soap, try using other liquids like oil or juice. Do these liquids also cause the same effect?
Instead of pepper flakes, you can try to use other substances like sprinkles. Do these still float on top of the water? Do they run away just like pepper?

Published 6.19.2020

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