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Blow the Best Bubbles

A soapy chemistry challenge from Science Buddies
girl with bubbles



iStockphoto/yuhirao

Key concepts
Chemistry
Water
Fluids
Surface tension

Introduction
Have you ever wondered what makes a bubble form? The secret to making bubbles is surface tension. Adding soap (such as the kind you use to wash dishes in the sink) to water changes the surface tension of that water, and this creates a great solution to make bubbles from.

If you try to make bubbles using normal water, you will quickly see that it doesn't work very well. This is because the surface tension—the forces holding the molecules of a liquid together—of water is too high. When detergent is added to water, it lowers the surface tension so that bubbles can form. Add other things, such as corn syrup or glycerin, to improve the bubbles. Which solution will create the best bubbles?

Background
In a container of water, the tiny water molecules are attracted to each other, which means that they're constantly pulling on each other. At the surface of the water, these water molecules are attracted to the water molecules around and below them. But they have no water molecules above them to be attracted to (since it is just air up there). This is what creates the force known as surface tension. The water molecules at the surface of the water do not want to move up, away from other water molecules to which they are attracted. This gives plain old water a high surface tension. In fact, it's too high to allow big bubbles to form.

When a soapy dish detergent is added to water, it lowers the surface tension so that bubbles can form. The detergent molecules increase the distance between water molecules and reduce those molecules' ability to interact with each other. This decreases the pull—or attraction–that the water molecules exert on each other, lowering the surface tension of the solution. Other substances, such as corn syrup or glycerin, can be added to the solution of water and detergent to make even better bubbles.

Materials
•    Three large cups or jars with a wide opening
•    One-cup measuring cup
•    Tablespoon measurer
•    Three cups of distilled water (which can be purchased at the supermarket)
•    Spoon
•    Liquid dishwashing soap (for example, Dawn or Joy brand)
•    Glycerin, small bottle (available at a drugstore or pharmacy)
•    Light corn syrup
•    Three pipe cleaners
•    Permanent marker
•    Timer

Preparation
•    Label the three cups "Detergent Only," "Glycerin," and "Corn Syrup," respectively.
•    To all three cups, add one cup of distilled water.
•    To the "Detergent Only" cup, add an extra one tablespoon of water.
•    Make a pipe cleaner wand by pinching a pipe cleaner in the middle and bending half of it into a circle, twisting a little bit of the end to secure it. Make two more pipe cleaner wands this way, making sure their diameters are all the same.

Procedure
•    To all three cups, add two tablespoons of detergent. Mix the detergent in each cup with a spoon. You should see small bubbles forming as you mix in the detergent. Why do you think you need detergent in every solution?
•    To the "Glycerin" cup, add one tablespoon of glycerin. And to the "Corn Syrup" cup, add one tablespoon of corn syrup. What is the consistency of the glycerin and corn syrup? Does one seem more viscous (thick and sticky) than the other, or do they have about the same viscosity? Mix the contents of the "Glycerin" and "Corn Syrup" cups.
•    Blow a bubble from one of the solutions (outside is best, but over a kitchen sink or any other place that can stand to get a little sticky is okay). Try to catch the bubble on your wand and time how long the bubble lasts before it pops. This can be difficult to do, so you may need to practice it first. Also, it might be helpful to have another person time you.
•    Catch and time at least four bubbles from each solution and write the times down. Calculate the average bubble life span for each solution. To do this, add the recorded times for each bubble type separately, then divide each total by the number of times you recorded that bubble (for example, if your "Detergent Only" bubble times were 5.1 seconds, 4.5 seconds, 5.2 seconds and 5.7 seconds, the average would be 5.1 seconds). Which solution makes bubbles that last the longest? Which solution makes the shortest-lived bubbles? Why do you think this is? Did some solutions make larger bubbles than others?
•    Extra: How does the concentration of glycerin or corn syrup in the bubble solution change how long the bubbles last? You can try this activity again, using different amounts of glycerin or corn syrup in the solutions. How little is too little, and how much is too much to add? Can you make a bubble solution that results in bubbles that last longer than the ones in the original activity?
•    Extra: Do bubbles always make a spherical shape? Try twisting pipe cleaners into different shapes, such as stars, squares or triangles. What shape are the bubbles made from these differently shaped pipe cleaner wands?


Observations and results
Did the solutions with glycerin or corn syrup produce bubbles that lasted longer? Did the solution with glycerin make the longest-lasting bubbles of all?

Detergent lowers the surface tension of water enough so that bubbles can form. A bubble formed from a solution with water and detergent is a spherical layer of water molecules that is surrounded on either side by a layer of detergent molecules. Parts of the detergent molecules are attracted to water (which means they are known as hydrophilic) and other parts do not want to be near water (they are hydrophobic). Because of this, the detergent molecules in the bubble become oriented so that their hydrophilic parts touch the water and their hydrophobic parts face outward, touching the air.

The solution with only water and detergent probably made smaller, shorter-lived bubbles compared to the solutions with glycerin or corn syrup. Bubbles burst when the layer of water molecules between the detergent molecules evaporates. But Glycerin and corn syrup form weak bonds with the water molecules and slow down the evaporation process, thus improving the life span and durability of the bubble. Glycerin makes stronger, longer-lasting bubbles, but corn syrup is often substituted in bubble solutions because it is cheaper. Can you find other substances around the house that can be added to water to make a bubble solution? If you try products such as shampoo or liquid hand soap, you can check their ingredients to see what might be helping to make the bubbles form.

More to explore
"Bursting Bubbles Beget Itty-Bitty Bubbles" from Scientific American
"Bubbles" from Rob Hipschman at the Exploratorium.
"Soap Bubbles" from Chemical & Engineering News
"No-Soap Bubbles" from NASA’s Saturday Morning Science
"Bubble-ology" from Science Buddies

This activity brought to you in partnership with Science Buddies
ScienceBuddies

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