Comparing Surface Tension of Liquids with Pennies

The Seasons

(Physics for ages 5+)

Liquids can be interesting to play with because they can take nearly any shape you put them in, and they have special properties not seen in different states of matter, like gaseous or solid. One of these special properties is surface tension, which is a force found in the surface layer of a liquid that holds the molecules together. Insects like water striders are able to walk on water because of its surface tension, but not all liquids are the same. The video above walks through one activity that compares the surface tension of different liquids. Here’s how it’s done:


Pennies (you’ll want at least one penny for each liquid you test)
Paper towels
Eye dropper
Test liquids that you have around the house (rubbing alcohol, cooking oil, water, liquid soap, honey, etc.)
Adult supervision (Adult supervision at all times please)


  1. Since this activity involves comparing different liquids, it will be easier to compare if you keep track of your results. Start by making a simple table with a row for each liquid and a column to keep track of the number of drops.
  2. Lay your pennies out on some paper towels to minimize any messes.
  3. To test your first liquid (in the video they test water with food coloring first), fill your eye dropper with it.
  4. Next, add one drop of the liquid at a time. Do you see how the liquid bubbles up? How many drops can you add to the penny before this bubble bursts? Write this number down in your table.
  5. Repeat steps 3 and 4 for all of your test liquids. Be sure to clean the eye dropper between each liquid to be as accurate as possible. How do the number of drops added to each penny compare by liquid?


When solutions (like water, rubbing alcohol, oil, etc.) are in liquid form, their molecules are constantly moving around within the liquid. As they move, they create bonds and tension between each other to hold themselves together.

This allows the bubble to form on top of the penny. Liquids like water and cream have strong surface tension, so you can add more drops before their bubbles burst.

On the other hand, rubbing alcohol and syrup have weaker surface tension so fewer drops are able to be added to the surface of the penny.