Water covers about three-fourths of our planet, yet is still a fascinating substance.
"Cohesion" is a phenomenon in which a substance is attracted to itself. With water, the oxygen part of the molecule is negatively charged, while the hydrogen is positively charged. Thus, two water molecules will be magnetically attracted to each other. "Adhesion" describes when water is attracted to another substance, such as clothing. Where water does not adhere to another substance, it will cohere to other like molecules.
Surface Tension
Water coheres to itself in what is called surface tension. To test this, have your students place a drop of water on a piece of wax paper. The water does not adhere to the wax, instead forming a bead or droplet. Because water molecules bond together very tightly, they form the shape with the least surface area, a circle. Have your students experiment with what happens when another water bead meets the first.
Overfill a Glass of Water
Break your students into groups, each with a glass of water that they fill as close to the rim as possible. Have each student in the group guess how many more droplets of water can fit in the glass without spilling over. The water will form a tension and will eventually overfill the glass, with a small dome of water held by surface tension. You may also substitute paperclips in place of adding water, and students can guess how many clips will fit in the glass before the water spills out. The paperclip experiment also demonstrates water displacement.
Pinching Water
With your students at their own lab sinks, give each group a plastic cup and a nail or needle. Have each group punch four holes in their cup close together, along a line. You may want them to mark their holes prior to perforating the cup. When they fill the cup, the water will pour from the holes. Then have them "pinch" the water between their fingers and watch the water form a single stream. The water does not adhere to their fingers, but the water molecules will cohere when pushed close enough together.
Oil and Water
Have your students break into separate groups, each with a graduated cylinder, a sink and some cooking oil. Have them measure 8 ml of water and then try to predict what will happen if they add some oil to the water. Will the oil sink or float? The water will not adhere, and the oil floats. Then try the opposite. With a new cylinder, pour in 8 ml of oil. Will the water now float on top of the oil? In fact, the water will sink due to the greater density of water. The water will not mix with the oil because the two substances do not adhere.
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