Key concepts
Chemistry
Surface tension
Water molecules
Introduction
Have you ever tried to blow a bubble, and no matter how hard you try—that bubble just won’t form? Why does this happen? Why do bubbles form at all? And why is it harder to blow bubbles sometimes? In this activity we’re going to explore how bubbles form, and test the limits of how big our bubbles can get!
Background
As you may know, bubbles rely on surface tension to hold together. Water molecules like to cling to one another and, just like magnets line up with and attach to other magnets they’re attracted to, water molecules line up with one another and form hydrogen bonds, creating surface tension. This surface tension allows the surface of water to act like an elastic membrane, stretching and holding a shape. 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 one another than they are to the wax paper. The strength of that attraction helps hold the water droplet together.
The surface tension of water is so strong in fact, that it prevents us from blowing bubbles from just water. Adding soap or detergent lowers the water’s surface tension, allowing bubbles to form. It does this by creating space between the water molecules, decreasing the strength of their attraction to one another. If you think again about magnets, when two magnets are very close to each other, the pull between them is much stronger than when you slowly separate them. Decreasing the strength of the attraction between water molecules lowers the surface tension of the bubble solution, allowing bubbles to form!
Materials
- Two dowel rods (At least half an inch diameter works best.)
- Two screw eyes (a closed loop with a threaded base)
- Yarn or baker's twine (at least 18 feet)
- Two washers
- Bubble solution (Optional make-your-own bubble solution ingredients are listed below.)
- A large bucket to hold bubble solution
- Outdoor area with space to run!
- An adult helper
- A measuring tape
- Scissors
Bubble Solution (Makes approximately one-half gallon.)
- Eight cups of Water
- One-half cup liquid dish soap (Blue Dawn works well.)
- One tablespoon glycerin
- One tablespoon baking powder
Preparation
- This activity could get a little messy. From this point forward everything should be done outside!
- Pour the bubble solution into your bucket. If you’re making your own bubble solution, combine all ingredients in your bucket and gently stir to combine. (Try to avoid creating a sudsy foam on the surface of the solution.)
- Ask your adult helper to help you attach a screw eye to one end of each rod.
Procedure
- Cut a three-foot length of twine and thread it through one washer and each screw eye, making a loop. Tie the end of the twine together (the knot can be anywhere). This is your bubble wand!
- Set your bubble solution bucket in an open area. Place your measuring tape on the ground, starting at your bucket and extending at least 10 feet in the direction the wind is blowing.
- Hold your rods on the ends without the screw eyes. Gently touch the two screw eyes together, and completely submerge the yarn in the bubble solution.
- Gently lift your wand from the bubble solution. Separate the ends of the rods to open the yarn loop.
- Hold the wand away from your body and slowly walk along the measuring tape, in the direction of the wind. How big of a bubble do you think you will be able to create?
- Have your adult helper measure the length of the bubble you create! Try it at least five times, and remember the longest.
- Cut the twine and remove it from the wand. Save the washer!
- Cut a six-foot length of twine, and thread it through one washer and each screw eye, making a loop. Tie the end of the twine together (the knot can be anywhere).
- Repeat the above bubble-making steps, using the bigger wand. Measure the longest bubble you can create with the six-foot twine. Do you think these bubbles will be easier or more difficult to form?
- Cut the twine and remove it from the wand. Save the washer!
- Cut a nine-foot length of twine and thread it through two washers and each screw eye, making a loop. Tie the end of the twine together. (The knot can be anywhere.)
- Have your adult helper assist you while you repeat the bubble-making steps, with both of you holding the wand. Measure the longest bubble you can create with the nine-foot twine. Do you think you will be able to create bubbles with the nine-foot wand? Do you think they could be much larger?
- Extra: Test the bubble solution. Try removing the glycerin or the baking powder. How do these ingredients affect the bubbles you create?
- Extra: Let the bubble solution sit overnight, then retry this activity. Test to see if you get bigger bubbles!
Observations and results
In this activity you created some of the biggest bubbles possible, with just a little string and some patience! You may have found that it was easier to create the long bubbles with the shorter twine. As the twine got longer (especially at nine feet) it may have been more difficult for the bubbles to form.
We know bubbles can hold their shape as a result of surface tension. In fact, the bubble solution mixture that makes up a bubble’s surface is composed of three very thin layers: soap, water and another layer of soap. This is called a soap film. The surface tension of the water layer is what holds the bubble together. Therefore, when the water between the soap layers evaporates, the bubble pops. To make bigger and stronger bubbles, we need to thicken the walls of soap, to prevent the water from evaporating. To do this, we added glycerin, to help thicken those soap walls and hold the bubble together! This makes for stronger, bigger and longer-lasting bubbles.
At a certain point, however, bubbles get too big to hold together. Larger bubbles have larger surface area, which means more places for external forces, such as air pressure and gravity, to have an effect. In addition, the larger bubbles have their own weight to support. Imagine building a tunnel with LEGO. You can make a very long tunnel with a smaller diameter. If you try to make a tunnel that an adult could walk through, however, it’s going to have more trouble supporting its own weight. This is similar to what you experienced with the nine-foot twine wand. The body of the bubble was much larger, and therefore much more fragile.
Cleanup
Pour the soap mixture down the drain.
More to explore
Bubble-ology, by Science Buddies
Blow the Biggest Bubbles, from Scientific American
The Chemistry of Clean: Make Your Own Soap to Study Soap Synthesis, by Science Buddies
Blow the Best Bubbles, from Scientific American
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
