Key Concepts
Cohesion, surface tension, hydrophobic

Introduction

You are probably used to seeing things float, be it a boat on the water, or a rubber duck in your bathtub. But did you ever wonder how the same water that you drink, splash, and dive into – how can that support the weight of giant boats? In this activity we’ll learn about surface tension and how it helps us keep afloat!

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

Water molecules like to cling to each other (through hydrogen bonds), and the strength of this cohesive force allows the water molecules to act similar to an elastic membrane on the surface of the water. This creates surface tension, a property of a liquid that allows it to resist external forces, including the weight of boats and rubber ducks! 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, spherical droplets on the paper. These water droplets can hold their shape because water molecules are more attracted to each other than they are to the wax paper. The strength of that attraction helps hold the water droplet together.

In this activity we’ll be exploring surface tension with water and some household products. Get ready to make a splash!

Materials

  • Black pepper (at least 5 teaspoons worth)
  • A shallow bowl, or aluminum pie tin (something light in color works best)
  • Liquid dishwashing soap
  • Cooking Oil
  • Milk
  • Toothpaste
  • Glass Cleaner
  • Water
  • 5 toothpicks
  • An adult helper
  • Paper
  • Pencil or pen
  • Access to a sink

Preparation

  1. Use your paper and pencil to create a table like the one below, to help you record your observations during this activity.

Household product

Behavior of Pepper

Oil

 

Dishwashing Liquid

 

Glass Cleaner

 

Milk

 

Toothpaste

 

Procedure

WARNING: This activity uses household chemicals that, if handled incorrectly, can be dangerous. Please have an adult help you!
  1. Fill your bowl or pan 2/3 full of water.
  2. Sprinkle 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?
  3. 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!
  4. 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?
  5. Write your observations in your table.
  6. Empty and rinse your bowl with water. Throw the oil coated toothpick away.
  7. Again, fill the bowl 2/3 full of water.
  8. Sprinkle black pepper over the water. Again, take a minute to observe how the pepper moves in the water.
  9. Carefully dip the end of a clean toothpick into dishwashing liquid.
  10. 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 surface of the water change when you add the dishwashing liquid? In what way?
  11. Write your observations in your table.
  12. Empty and rinse your bowl with water. Throw the dishwashing liquid coated toothpick away.
  13. Again, fill the bowl 2/3 full of water.
  14. Sprinkle black pepper over the water. Again, take a minute to observe how the pepper moves in the water.
  15. Repeat steps 3-8 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 experimented with 5 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 surface of the water. Pepper is hydrophobic, which means that 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 of the water 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 surface 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 3 of the products; the dishwashing liquid, glass cleaner and toothpaste, 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 behavior of the pepper, although you could probably see the oil droplet floating on the surface of the water. 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 3 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 of the water. However, the water molecules want to stick together and maintain that surface tension, so they move away from the soap, carrying the pepper with them!

More to Explore

Credits

Megan Arnett, PhD, Science Buddies

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Key Concepts
Cohesion, surface tension, hydrophobic
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