Key concepts
Chemistry
Surface tension
Surfactant
Molecules
Chemical bonds

Introduction
If you’ve ever washed dishes, you know the right dish soap can make a dirty job a lot easier. Have you ever wondered how dish soap is able to clean dishes so much more effectively than water alone? Like many household cleaners, dish soap is a surfactant—it helps break up leftover food on plates by making it easier for food particles to dissolve in water. This soap also breaks up the water molecules themselves, which leads to some pretty interesting kitchen science! In this activity you’ll be observing some surprising properties of dish soap in water!

Background
Giant ships, people and rubber ducks can all float on water’s surface, thanks to one very important trait of water molecules: hydrogen bonds! Water molecules cling strongly to one another by forming these bonds from one molecule to another. They allow water molecules on the surface of water to behave like a membrane, which can even support the weight of small objects, such as water strider insects. This property of water is known as surface tension. You have probably observed this phenomenon many times in your life. Have you ever noticed a single drop of water sitting on your car windshield? Instead of flattening out or splashing, these raindrops are able to hold a spherical shape because the water molecules comprising the raindrop are more attracted to one another than they are to the windshield of your car. As a result, those molecules hold tightly to one another, forming the raindrop’s spherical shape.

Surfactants such as dish soap break up water’s surface tension. As a result, objects floating in water will sink or change shape as the surface tension changes. In this activity you’ll explore how surface tension affects the behavior of objects in water—and why it’s so important!

Materials

  • A bowl
  • Water, enough to fill the bowl at least halfway
  • Liquid dish soap
  • A rubber band
  • A metal sewing pin


Preparation

  • Fill your bowl at least halfway with water.
     

Procedure

  • Place your rubber band on a table or other flat surface. Notice its shape when it sits on the table.
  • Place your rubber band into your bowl of water. Notice the shape of the rubber band when it is in the water. Does it look different than it did when it was on the table? If so, what is different about it?
  • Place your pin in the water in the center of the rubber band. Does the pin sink or float?
  • Drop a few drops of dishwashing liquid into the water surrounded by the rubber band. Notice the rubber band’s shape. Did anything about it change when you added the liquid soap?
  • Observe the pin. If it is still floating, add a few more drops of liquid dishwashing soap. Does the pin continue to float or does it sink? Why do you think this is the case?

Observations and results
In this activity you used dishwashing soap to examine how surface tension affects the behavior of objects floating in water.

In the beginning of the activity you should have noticed the rubber band and the pin floated on the water’s surface. They float because water molecules hold on to one another in a way that creates surface tension. This property allows many things to float on water, including your rubber band and pin. You should also have noticed the rubber band had a loose, or irregular, shape. It did not hold a perfectly round circle but instead floated in a shape similar to what it looked like before you put it in the water.

When you added the dishwashing soap, however, you should have observed a change in the behavior of the rubber band and pin: The band should have suddenly popped out into a circle. In addition you should have noticed that adding the dish soap caused the pin to sink to the bottom of the bowl instead of floating.

Both of these results happen for the same reason—and you probably already guessed it has something to do with dishwashing soap, which is a surfactant and designed to break down water’s surface tension. This helps make it an excellent cleaner. After you added the dishwashing liquid to the center of the rubber band the surface tension on the inside of the band broke down. The water on the outside of the band, however, hadn’t come in contact with the dishwashing soap. As a result, the water on the outside of the band still had strong surface tension, which caused it to pull the rubber band outward in all directions. This made the rubber band pop suddenly into a circle shape.

Similarly, after you added the soap to the water you should have seen the pin sink to the bottom of the bowl. Because the soap broke down the surface tension the water could no longer support the weight of the pin.

More to explore
Measuring the Surface Tension of Water, from Science Buddies
Measure Surface Tension with a Penny, 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

Science Buddies