Water. You can drink it, splash it, dive into it, float things in it. But it has its strange side: it flows, but then on some surfaces, it won’t flow, it’ll just roll up into a little bead. And on still ponds outside, in shady areas, you can sometimes see small insects with long legs walking around—on top of the water. What’s up with that? Today you’re going to learn about how water has something called “surface tension” that you can see and play around with by doing some very easy experiments.
Water molecules like to cling to one another through the bonds in their hydrogen atoms. The strength of this cohesive force allows the molecules to act similar to an elastic membrane on the water’s surface. This creates surface tension, a property of a liquid that allows it to resist external forces including the weight of a small bug or the force of gravity trying to pull water into a flat puddle! If you drop a small amount of water on a piece of wax paper, you can see a great example of surface tension in action. Instead of splashing or flattening, the water will form small, hemispherical droplets on the paper. These water droplets can hold their shapes because water molecules are more attracted to one another than they are to the wax paper. The strength of that attraction helps hold a water droplet together.
In this activity we’ll be exploring surface tension with water and some household products. Get ready to make a splash!
- Ground black pepper—at least five teaspoons
- A shallow bowl or aluminum pie tin (Something light in color works best.)
- Liquid dishwashing soap
- Cooking oil
- Glass cleaner
- Five toothpicks
- An adult helper
- Pencil (or pen)
- Access to a sink
- Use your paper and pencil to create a table with two columns and six rows. Label the first column “household product” and the second column “behavior of pepper.” Add the following words to the rows in the left column: “oil,” “dishwashing liquid,” “glass cleaner,” “milk” and “toothpaste.” This will help you record your observations during this activity.
- Gather your materials on a surface that can withstand spills.
- This activity uses household chemicals that, if handled incorrectly, can be dangerous. Please have an adult help you!
- Fill your bowl or pan two thirds full of water.
- Sprinkle one teaspoon of black pepper over the water. Observe the behavior of the pepper. Does the pepper sink or float? Does it spread out or clump together? What else do you notice about the pepper?
- Carefully dip the end of your toothpick into the cooking oil. You only need a tiny bit of oil at the end of the toothpick!
- Dip the oil-coated end of the toothpick into the water with the pepper. Observe what happens when the oil comes in contact with the water. Does the movement of the pepper flakes on the surface of the water change when you add oil? What else do you notice about the oil and the pepper?
- Write your observations in your table. Discard the toothpick.
- Empty and rinse your bowl.
- Again, fill the bowl two thirds full of water.
- Sprinkle one teaspoon of black pepper over the water. Again take a minute to observe how the pepper moves in the water.
- Carefully dip the end of a clean toothpick into dishwashing liquid.
- Dip the dishwashing liquid end of the toothpick into the water with the pepper. Observe what happens when the dishwashing liquid comes in contact with the water. Does the movement of the pepper flakes on the water’s surface change when you add the dishwashing liquid? In what way?
- Write your observations in your table. Discard the dishwashing liquid toothpick.
- Empty and rinse your bowl with water.
- Again, fill the bowl two thirds full of water.
- Sprinkle a teaspoon of black pepper over the water. Again take a minute to observe how the pepper moves in the water.
- Repeat these steps with each of the remaining testing ingredients. Rinse and refill the bowl with clean water between each ingredient. Record your observations in your table.
- Extra: Repeat this activity, testing with other household supplies. Do you notice a pattern in which products affect the pepper and which don’t?
- Extra: Repeat this activity testing what happens when you use juice or soda instead of water.
Observations and results
During this activity you tested five different household products to see how they affected the movement of pepper flakes in water. The first thing you may have noticed is that at least some of the pepper flakes floated on the water’s surface. Pepper is hydrophobic, which means water is not attracted to it. Therefore, unlike salt or sugar pepper will not dissolve in water. The pepper is able to float on the surface because water molecules like to cling to one another. They arrange themselves in a way that creates surface tension on the top of the water. This tension keeps the pepper flakes floating on top instead of sinking to the bottom of the bowl.
You should have observed a change in the behavior of the pepper flakes when you added different household products. Adding three of the products—the dishwashing liquid, glass cleaner and toothpaste—to the water should all have caused the pepper flakes to instantly dart away from the toothpick. In contrast the oil and milk should have had very little or no effect on the pepper’s behavior, although you could probably see the oil droplet floating on the surface. Before we break down why this happens can you think of anything that dishwashing liquid, glass cleaner and toothpaste have in common?
If you said that they all clean things—you’re right! And that important trait helps explain why the pepper was chased away by each of those three products. Soaps and cleaners are designed to break down the surface tension of water. This helps make them good cleaning tools. When you add the dishwashing liquid, toothpaste or glass cleaner to the water it breaks up the surface tension. The water molecules, however, want to stick together and maintain that tension, so they move away from the soap, carrying the pepper with them!
Discard any remaining liquid down the drain and throw away any used toothpicks.
More to explore
Measuring the Surface Tension of Water, from Science Buddies
Build a Raft Powered by Surface Tension, from Science Buddies
Surfactant Science: Make a Milk Rainbow, from Scientific American
Make a Paper Fish Swim with Surface Tension, from Scientific American
Science Activities for All Ages! from Science Buddies
This activity brought to you in partnership with Science Buddies
Editor’s Note (5/6/20): This story has been edited after posting to correct descriptions of how surface tension works.