Pepper and Soap

This demonstration visualizes the effect dish soap has on the surface tension of water, and it helps to explain why soap is good for cleaning dirty dishes and germy hands.

Pepper sits on the surface of the water. This is because water has a high surface tension, meaning the water molecules have a powerful attraction to each other and they like to stick together. As the pepper is so light, it sits on the surface of the water rather than sinking to the bottom. The pepper is also hydrophobic, which means water is not attracted to it, and the pepper does not dissolve into the water, it just remains resting on the surface. When soap is added, the surface tension is reduced, and the water wants to spread out flat (water normally bulges up slightly, like when you overfill a glass of water.) As it spreads out, it flattens on the dish and carries any pepper that’s floating on the surface with it, away from the source of the soap and to the edge of the water.