Powder Dunk

Prepared by David Brandt

Purpose: To observe the wetting effect of soap and detergent.

Materials:
Shampoo
Liquid dish soap
Toothpicks
Talcum powder
2 bowls

Procedure:
1. Fill both bowls with water
2. Sprinkle a thin layer of talcum powder on the surface of the water in each bowl
3. Dip the end of one toothpick in the shampoo, and touch the end in the center of the powder in one bowl.
4. Observe the movement of the powder.
5. Dip the end of a second tooth pick in the liquid dish soap and touch the end in the center of the powder in the second bowl.
6. Observe the movement of the powder.

Why? Talcum powder if water resistant. The grains of powder float on top of the water. The water molecules on the surface are pulling equally in all directions before the shampoo or dish soap is added. The addition of the shampoo or dish soap breaks the attraction between the water molecules wherever it touches, causing the water to move outward and take the floating powder with it. The shampoo is a moderate wetting agent while the liquid dish soap is a great wetting agent. A wetting agent allows water to spread rapidly over the surface of a solid and penetrate the surface of some solids. The liquid dish soap dissolves in the water and the water quickly covers the grains of talcum powder, causing them to sink to the bottom of the bowl.
Other Ideas: Have students bring in other soaps and detergents and try to figure which are better/stronger than others. Could be used in a chemistry setting with water polarity or in a physics lesson dealing with forces and movement.
Credit: VanCleave, Janice, Chemistry for Every Kid: 101 Easy Experiments That Really Work, Wiley and Sons, New York, 1989.