What You Need:
- Tray, plate, or dish
- Liquid detergent
To Do and Observe:
Put some water into a clean tray, plate or dish. Sprinkle some pepper on top of the water. With a cake of soap, touch the water. What happens to the pepper? What happens when you take the soap away? Rinse the tray thoroughly to remove traces of soap. Put some water into the tray and sprinkle some pepper on top. Place a drop of detergent on the water. What happens to the pepper?
What's Going On:
Surface tension on water makes a strong 'skin' which is tight and not very elastic. The pepper stays on the water until soap or detergent touches the skin. Soap and detergent reduce the attractive forces between water molecules--that is, they reduce surface tension. The skin stretches, scattering the pepper.
Use a toothpick dipped in detergent to touch a water drop resting on waxed paper. What happens?