Experiment 3

Water Tension

 

In this experiment we will discover if Ms. Crowler was telling the truth when she said she didn’t have time to wash the soap out of her hair.

You may remember from the story that Ms. Crowler’s hair, when it touched the water in the bowl of floating suckelberry seeds, caused the seeds to scatter towards the edge of the bowl.

I. Hypothesis Soap will break the surface tension of water causing floating objects to either scatter to places where there is surface tension or to sink.

II. Materials neededWater Tension Bowl Experiment

  • a bowl
  • water
  • pepper
  • soap (dish soap or shampoo)
  • paintbrush

III. Procedure/Observation

  1. Place a bowl of water on the table. Let it rest until the water is calm. Sprinkle some pepper on the surface of the water.
  2. Gently dip the tip of the paintbrush into the water. What happens?
  3. Dip the paintbrush into some dish soap or shampoo.
  4. Gently dip the tip your soap-covered paintbrush into the bowl. What happens?

 

V. What is the surface tension of water.
You may know most things are slippery when wet. But water molecules, when it comes to other water molecules, are sticky. This is because water molecules have a strong attraction to each other.*

In the center of a glass of water, the molecules are sticking to other water molecules in all directions because they are all attracted to each other. Think of all the water molecules pulling towards each other, imagine a glass of magnets all attracting each other.

At the surface, there are no water molecules above for them to stick to, so there is nothing pulling the surface molecules up. These surface molecules therefore stick more to the molecules next to them than other water molecules below them. This extra stickiness forms a “skin” at the surface of the water that is called surface tension. Surface tension is what causes water to form circular drops on the surface of a clean car. Surface tension is even strong enough to support some insects like the water strider. 

Check out this water strider filmed by KQED!