Pepper and Soap Experiment

Can Soap Push Things Across Water?

Water may look calm and simple, but its molecules are actually holding tightly onto each other. In this quick experiment, you will see how adding soap can suddenly change the behavior of water.

Materials

• A shallow bowl or plate

• Dish soap

• A cotton swab or your finger

Experiment Steps

1. Fill the bowl

Pour water into the bowl until the bottom is covered.

2. Add the pepper

Sprinkle black pepper evenly across the surface of the water. The pepper will float.

3. Prepare the soap

Dip a cotton swab or your finger into a small drop of dish soap.

4. Touch the water

Gently touch the soapy swab or finger to the center of the water.

5. Watch what happens

The pepper will quickly move away from the soap and rush toward the edges of the bowl!

The Science Behind It

Water molecules are attracted to eachother. This is called cohesion. As a result of this water molecules form a thin layer called surface tension.This is why light objects like pepper can float on the surface.

When dish soap touches the water, it disrupts these forces between the water molecules. Soap molecules have one end that is attracted to water and another end that is attracted to oils and fats. When the soap spreads across the surface, it weakens the surface tension in that area.

The water molecules then move away from the soap toward areas where the surface tension is stronger. As the water moves, it carries the floating pepper with it, which is why the pepper suddenly rushes away from the soap.

This experiment also helps show why soap is so good at cleaning. By breaking the surface tension of water and interacting with oils and fats, soap allows water to wash away dirt, grease, and certain germs more easily.

Microwonder Question

What do you think would happen if you added more soap to different parts of the bowl? Would the pepper move again? Why?