Saturday, August 25, 2007

COHESION

WATER ON A PENNY

MATERIALS

One penny
Eyedropper
Water
Liquid detergent

PROCEDURE

Ask, "How many drops of water do you think
you can put on this penny without the water
dropping off the edge?"
Place the penny flat on a smooth surface. With
eyedropper, drip the water onto the penny one
drop at a time, counting the drops, until the
water spills off the edge.
Take same penny and rub detergent on it.
Predict the number of drops of water you can
put on the penny without the water spilling
over the edge.
Using the eyedropper, drip the water on the
penny and count the drops.

EXPLANATION

Water molecules have a strong cohesion, or
attraction, to other water molecules. This
cohesive force of the like-molecules forms the
skin-like surface of the water, called surface
tension. Soap reduces surface tension, causing
less drops to stay on the penny with the soap.

ADHESION

PASS THE PEPPER

MATERIALS

Cereal bowl
Pepper
Liquid dish detergent

PROCEDURE

Fill dish with water
Sprinkle some pepper on top of the water
Ask, "What do you think will happen to the
pepper if we add detergent?"
Drop several drops of detergent into the
center of the water.

EXPLANATION

Adhesion is the attractive molecular force
that holds together UNLIKE bodies in
contact. The water is pulling on the pepper
evenly from all directions. The detergent
reduces the adhesive force between the
pepper and water. The water around the
bowl's edges, untouched by the detergent,
still has its full pulling strength.