How to Demonstrate Soap Tension to Kids

"Soap tension" occurs when the surface of a body of water is "broken" by its coming into contact with soap. A demonstration of soap tension shows that soap is a surfactant: a substance that releases surface tension.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Bowl of water
• 1 paper clip
• 1 disposable plastic fork
• 1 toothpick
• Liquid soap

1. Drop the paper clip into the bowl of water from about five feet high. This demonstrates that it is heavy enough to sink.

2. Place the paper clip on the spoon and use it to gently lower the clip into the water. The clip will float, demonstrating the "surface" tension of the water (in contrast to the soap tension). Describe this surface tension as the "skin" of the water.

3. Show the kids closely that the paper clip is pressing down against the skin of the water, and that the water is pressing back up against it.

4. Apply some soap to the toothpick. With a steady hand, lightly touch the water with the soapy toothpick. It is important to avoid making any ripples as these would only add extra variables to the test for soap tension.

5. Have the kids watch closely as the skin of the water is broken by the soap tension.

6. Point out that once the skin of the water is broken, the paper clip sinks to the bottom of the bowl, demonstrating tension.

Tips & Warnings

• The cleaner the water and paper clip, the better the paper clip will float.