Soapy Surface Tension

3.7 based on 57 ratings

Updated on Nov 18, 2013

Grade Level: 8th - 10th; Type: Physics


Surface tension causes the molecules at the surface of a liquid (such as water) to cling together. This project examines the effects of soap on the surface tension of water.

Research Question:

How does soap affect the surface tension of water?

Did you know that the molecules on the surface of a cup of water stick together? This is because of a force called surface tension. In fact, if you carefully place a paper clip on the surface of a cup of water, it will seem to float because of surface tension. But what happens when a drop of soap hits the water’s skin? Find out with this science project.


  • Pennies
  • Plastic Cup
  • Water
  • Eye dropper
  • Liquid soap

Experimental Procedure:

  1. Fill a plastic cup with water.
  2. Dip the dropper into the cup, and suction in some water.
  3. Place drops of water from the dropper onto a penny, counting how many it takes for the water to spill off the penny. This is the number of drops it takes until the surface tension breaks.
  4. Dry off the penny.
  5. Repeat this process several times to make sure that you have an accurate count of how many drops it takes to break the surface tension.
  6. Dilute the water in the cup with a drop or two of liquid soap.
  7. Now see how many drops of water it takes until the water spills off the penny. How can you explain the difference?

Terms/Concepts: Surface tension; Water skidder (insect); How does soap interact with water?


  • Experiments With Bubbles, by Robert Gardner. Pp 18-20.
Keren Perles has worked as an educational writer, editor, teacher, and tutor of all ages. Her experience spans the subject areas, from science and math, to English and the Hebrew language.

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