The Floating Soap Bubble

The floating soap bubble¹ is an experiment that is very easy to set up and carry out (Video 1).

Video 1: Floating Bubbles? Tommy Technetium YouTube channel, accessed 8/19/23.

Isn’t that a beautiful effect? My students certainly find this effect to be rather captivating. This experiment is also quite popular with younger students, so it tends to be a real hit at outreach events.

I present the floating soap bubble experiment to my students in class at the beginning of the year when discussing the topic of density. However, because this experiment can be used in a wide variety of ways, I often revisit the experiment throughout the year.

Here’s the way I usually carry out the experiment in class. I first liberally sprinkle baking soda into the bottom of the very large container with transparent sides (like an aquarium). I add at least two cups of baking soda. Next, I pour at least one quart of vinegar into the large container, on top of the baking soda. The following chemical reaction occurs, generating a lot of CO₂ gas:

NaHCO₃ (s) + CH₃COOH(aq) → CH₃COONa(aq) + H₂O + CO₂(g)            Equation 1

I make sure not to disturb the container as the CO₂ is produced. Once the reaction settles down, I carefully blow bubbles straight across the top of the container (Figure 1), perhaps a foot above the container. I make sure not to blow down into the container, which would displace the CO₂ gas.

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Figure 1: Blowing bubbles into an aquarium full of CO₂ gas.

One or more of the bubbles settles into the container, but floats on top of the CO₂ gas that has formed (Figure 2). 

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Figure 2: The floating soap bubble!

It is interesting to continue to observe the bubbles that continue to float. Students will notice that over time, the bubbles slowly sink and concurrently increase in size. After students make these observations, I use my hands to “scoop” out the CO₂ gas in the container. After this, I try to get a bubble to float in the container after the CO₂ gas has been scooped out. This of course fails if enough CO₂ has been removed from the container.

Explanation 

As stated previously, when the baking soda and vinegar are mixed, carbon dioxide gas is formed (Equation 1). Carbon dioxide has a density of 1.8 g / L, while air has a density of 1.2 g / L. Because carbon dioxide is more dense than air, it sinks in air. Therefore, the carbon dioxide formed in this reaction stays “sunk” in the container. If I’m using this experiment when discussing gas laws and the density of gases, I make certain to note that the density of CO₂ is 1.5 times that of the density of air, which is equal to the ratio of the molar masses of CO₂ (44 g mol-1) and air (average 29 g mol-1).  

A bubble blown into the tank floats. Note that the gas inside the bubble is mostly air. The density of the soap bubble, which contains mostly air, is lower than that of CO₂, allowing it to float on the CO₂ in the container. Because many students think the gas we exhale is comprised entirely of CO₂, I think it’s important to tell students that only 4-5% of the gas we exhale is CO₂.²  

Over time, the bubbles blown into the container slowly sink. By observing this process carefully, it can be seen that the bubbles grow in volume as they sink. These processes occur as CO₂ diffuses into the bubbles, causing their density (and size) to increase.

The floating soap bubble indeed relates to several chemical topics: density, gas-releasing chemical reactions, gas diffusion, and the relationship between molar mass and density of gases. If you end up using this experiment in your classroom, be sure to let me know how it worked out for you, and if you learn anything interesting when you do.

Happy Experimenting!

References:

1. Shakhashiri, B.Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol. 2, 1985, The University of Wisconsin Press.

2. Siobal, M. S. Respiratory Care 2016, 61 (10), 1397-1416.