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Objective: Students will experiment with water, its cohesive qualities, and its surface tension.
Key Concept: Water has intermolecular attraction which holds it together (cohesion) and this creates surface tension.
Procedure and Results:
1. Explore water’s desire to stick to itself, called cohesion. Fill a cup of water to the brim. Add drops of water and you’ll see the water bulge over the top without spilling. This is because surface tension forms a “skin” holding the additional water drops in. Adding water by the drop allows the “skin” to stretch, until it reaches capacity, breaks and spills the water.
2. Test how a waxy surface affects surface tension. On plain paper, use a crayon to color a small area, applying a thick layer of crayon. Use the dropper to drop a number of drops onto each other, forming the largest rounded droplet you can. Then pile water droplets on top of each other on an uncolored part of the plain paper. Which area could hold more drops before breaking the surface tension?
See surface tension in action. Gently sprinkle crayon shavings into a bowl of water. They should float on the water. Pour a few drops of liquid soap into the bowl. Where do the crayon shavings run to and why? The soap weakens the surface tension and the shavings pull to the sides of the bowl. The ingredients (surfactants) in the soap weaken the intermolecular attraction of the water molecules which causes the water to spread out, moving the shavings.
Try this: Water drops can act like tiny convex lenses. Put a drop of water on a thin piece of transparent plastic or glass and place it on a picture. The image under the drop will seem much larger.