Testing Surface Tension

Surface Tension is the ability of a liquid to allow objects to float on top of it. Here’s how it works:

  • The molecules in a liquid pull at each other from all directions. This means they have 0 force.
  • The molecules on the surface of a liquid can’t be pulled in all directions, though, because the top or surface molecules don’t have anything but air to pull or push against.
  • Because the surface molecules are only being pulled down by the liquid’s other molecules, the surface has more tension—the molecules are tighter and can hold up any object that is lighter or less dense.

Surface tension is why feathers, canoes, and even snakes can float on top of the water. But what about the surface tension of other liquids? Or what happens when you add soap to water? Let’s find out!

HYPOTHESIS: When the surface tension of liquids is changed, it changes a liquid’s ability to allow objects to float.

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Supplies needed:

  • 3 or 4 glass bowls that are the same or similar in size
  • Water
  • Dish soap
  • Vinegar
  • Cooking oil
  • Soda-any kind will do
  • Small plastic bowl
  • Feathers or cotton swabs
  • Empty soda can
  • Top of a tin can
  • A handful of pebbles (small rocks)
  • Objects of your choice
  • Towel
  • Spoon
  • Pen and paper

Here is what you do:

  • Fill one container ¾ full of water
  • Fill one container ¾ full of vinegar
  • Fill one container ¾ full of Cooking oil (you can use less oil to save money if you are using large bowls).
  • Fill one container ¾ full of Soda.
  • Place a feather or cotton swab on the top of each surface. What happens? Do they react the same on top of each surface? Write down the results.
  • Place an empty soda can on top of one surface. Write down what happens.
  • Remove the can, wipe it dry, and repeat the process with every liquid. Write down what happens.
  • Repeat this process with the top of the tin can and the small plastic bowl; writing down what happens each time.
  • Place the small plastic bowl in the water again. Add pebbles ONE AT A TIME.

How many pebbles can the bowl hold until it sinks?

  • Repeat this process with the vinegar, oil, and soda.
  • Using the objects of your choice, place them one at a time on the surface of different liquids. Can some liquids hold heavier objects than others? What is the heaviest floating object you used? Did any objects sink? Write down what happens.
  • Add dish soap to the water (a few drops to a tablespoon—depending on how much water you have). Stir well.
  • One at a time, place the objects on the surface of the soapy water. Does the water react differently? How? Can you add more or fewer pebbles to the bowl before it sinks? Write down what happens.


  • Do all liquids have the same surface tension?
  • If not, which liquid has the most surface tension?
  • If not, which liquid has the least amount of surface tension?
  • Does the size of an object always determine whether or not it will float? Explain.


Write a paragraph or two to tell what you have learned about the surface tension of different liquids.


How is it possible for people to float on top of water AND sink or drown?