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.
If you want, you can see a video on how to do this experiment on the bottom of this page. However for the best science discovery experience it best to do the experiment first before watching the tutorial.
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 needed
- a bowl
- water
- pepper
- soap (dish soap or shampoo)
- paintbrush
III. Procedure/Observation
- 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.
- Gently dip the tip of the paintbrush into the water. What happens?
- Dip the paintbrush into some dish soap or shampoo.
- Gently dip the tip your soap-covered paintbrush into the bowl. What happens?
Experiment 3:
Materials you’ll need, procedure and detective results are listed above!
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!
Why water molecules stick together.
Water molecules stick to each other because of their molecular makeup. Each water molecule is made of two hydrogen atoms (H) and one oxygen atom (O). The two hydrogen atoms are smaller and positive. They are attached to a larger, negative oxygen atom in a water molecule. A water molecule’s shape is easy to remember if you think it looks like Mickey Mouse™.
Water Molecule
The hydrogen side of a water molecule tends to be slightly positive while the oxygen side tends to be slightly negative. Positive and negative charges attract each other, so water molecules’ positive sides are attracted to other water molecules’ negative sides. This attraction, known as hydrogen bonding, is why water is sticky at its molecular level.
Soap breaks water tension!
Soap molecules have a polar head on one edge that attracts water and a polar head on the other edge that repels water. These opposing heads interfere with hydrogen bonding in the water molecules and weaken the strength of the skin on the surface, breaking the tension.