What happens if you fill a glass to the very top with water, then add more drops?

You can actually overfill the glass and still not spill any water. That's because water molecules have a property called SURFACE TENSION—they stick together and almost make a "skin" on top of the water.

Add something like soap or salt, and you change the surface tension. Soap reduces the surface tension. Think of it as soap making water "stretchy." What do you think happens when you add salt to water?

Try this surface tension activity and record your data below.

  • Water
  • Measuring cup
  • Tablespoon
  • Salt
  • Dish washing liquid or liquid soap
  • 3 Droppers
  • 1 – 3 Dry Pennies (washed and dried)
  • Paper towels
  • 3 Cups

Get ½ cup of water.

Put a clean, dry penny on a paper towel, on top of a flat surface like a counter or table.

Fill one dropper with water from the cup, then drip one water drop at a time onto the penny. Count how many drops you can fit on the penny before water spills over onto the towel. Enter the drop count from this first try below. Dry the penny really well and repeat this step two more times. Enter the 2nd and 3rd drop counts below, and see what the average count is. (Instead of 3 tries with the same penny, you can use three clean, dry pennies.) As you’re dripping and counting, watch how each new drop fuses with earlier drops. Observe the shape of the water on the penny—does it change?

Now fill a new cup with ½ cup of water. Gently stir in 1 tablespoon of dish washing liquid or liquid soap. (You don’t want too much foam). Get a clean, dry penny and a new dropper and see how many drops of soapy water you can fit on the penny. (You can do 3 tries with the same penny or use 3 pennies.)

Next fill a new cup with ½ cup of water. Add 1 tablespoon of salt and stir until the salt is dissolved. Get a clean, dry penny and a new dropper and see how many drops of salty water you can fit on the penny. (You can do 3 tries with the same penny or use 3 different pennies.)

What did you notice about the water drops as you dripped each different solution on the penny? What does that tell you about water’s surface tension?