Does It Sink or Float? Depends on the Soap!
Summary
Introduction
If you’ve ever had to wash dishes, you know that 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. The dish soap also breaks up the water molecules themselves, which leads to some pretty interesting kitchen science experiments! 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 the surface of water, thanks to one very important trait of water molecules – hydrogen bonds! Water molecules cling strongly to each other by forming hydrogen bonds from one molecule to another. These bonds allow water molecules on the surface of water to behave like a membrane, which can even support the weight of small objects like water strider insects. This property of water is known as surface tension. You have probably observed surface tension 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 of the rain drop are more attracted to each other than they are to the windshield of your car. As a result, those water molecules hold tightly to one another, forming the spherical shape of a rain drop.
Surfactants like dish soap break up the surface tension of water. As a result, objects floating in the water will sink or change shape, as the surface tension of the water 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
Instructions
- Fill your bowl at least halfway with water.
- 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 center of the rubber band. Notice the rubber band’s shape. Did anything change about the shape of the rubber band 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 you should have noticed that both the rubber band and the pin floated on the surface of the water. The rubber band and pin float on the surface of the water because water molecules hold on to each other. Water molecules bind together in a way that creates surface tension on the surface of the water. This surface tension allows many things to float on the surface of water, including your rubber band and pin. You should also have noticed that 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.
However, when you added the dishwashing soap you should have observed a change in the behavior of both the rubber band and pin. When you added the dish soap, the rubber band hould 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 on the
surface.
Both of these results happen for the same reason, and you probably already guessed it has something to do with dishwashing soap. Dishwashing soap is a surfactant, and it is designed to break down the surface tension of water. This helps make it an excellent cleaner. When you added the dishwashing liquid to the center of the rubber band, the surface tension on the inside of the band broke down. However, the water on the outside of the band 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, when you added the soap to the water, you should have observed that the pin sank to the bottom of the bowl. Because the soap broke down the surface tension of the water, the surface of the water could no longer support the weight of the pin. Therefore, the pin sank to the bottom of the bowl.
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Additional Resources
- The Chemistry of Clean: Make Your Own Soap to Study Soap Synthesis, by Science Buddies
- Measuring the Surface Tension of Water, by Science Buddies
- Science Activities for All Ages!, from Science Buddies
