Water wetting
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Materials needed:
Wax paper
Spoon
Water
Dishwashing liquid
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In this experiment you will explore why water wets (spreads out on) or does not wet a surface.

Place a sheet of wax paper about the size of a piece of notebook paper on a flat surface. Use a spoon to add about 1 teaspoon (5 ml) of water to the wax paper to make a blob of water about the size of a quarter. Slightly lift the edges of the paper and move the blob of water around on the surface of the wax paper. Observe how the water moves across the surface. Touch the wax paper where the water has been. Does the wax paper feel wet?

Now, move the blob of water to the center of the wax paper. Add one drop of dishwashing liquid to the center of the water blob. What happens?

Lift the edges of the wax paper to make the water move across the surface. Touch a spot where the water has been. How does it feel?

You should find that the water blob moves freely across the surface of the wax paper without wetting it. The water molecules in the blob pull together to stay in the shape of a drop. The wax paper does not feel wet even where the large drop of water has passed across the surface.

After the drop of dishwashing soap is added to the water, the behavior of the water should be much different. The soapy water mixture should spread out across the surface rather than staying in a small blob. When you touch the surface over which the water has passed, it probably feels wet. The water did not wet the wax surface. However, the soapy water does.

Water molecules are polar, which means that they each have a positive and a negative side. Molecules of the wax on wax paper are nonpolar, which means that they do not have a positive and negative side. Polar molecules will mix with other polar molecules, just as nonpolar molecules will mix with other nonpolar molecules. However, polar molecules do not mix with nonpolar molecules. The water and wax paper do not mix. The water molecules stay together in the form of a bead or large drop to minimize their contact with the wax paper. They do not spread out on the surface.

Soap molecules are unique because they each have a polar and a nonpolar part. A soap molecule has a long part that is like a wax molecule and is called a tail. It also has a short part that is like a water molecule and is called a head. The head of a soap molecule is polar, while the tail of it is nonpolar.

When soap is added to water, it causes the water to wet the surface of the wax paper. The heads of the soap molecules attract, or mix with, the water. The tails of the soap molecules are attracted to, or mix with, the wax. Since the soap is attracted to both the water and the wax, it causes the water to spread out across the surface of the wax paper.

Scientists have made a drop of water move uphill against gravity by making one side of a surface underneath the drop polar and the other side nonpolar. The drop of water moves toward the polar side even if it is uphill from the water.

Sometimes we want to make surfaces that cannot be wet with water. For example, car wax is used to make a nonpolar surface on which water will bead up rather than wet the surface. Glass is sometimes coated with a nonpolar substance so water will not stick to the surface and the glass will not "fog up" (become covered with tiny droplets of water).

Sometimes we want to change liquids so that they can be more easily wet by water. Spills of oil into the ocean can damage the environment and harm living things. One of the ways used to clean up oil spills is to add surfactants (soaplike molecules) to the oil. A surfactant molecule has a polar and nonpolar part, and thus a surfactant can help the oil mix with the water so it does not wash up on beaches and kill animals.