Do you remember that scene from the Wizard of Oz when Dorothy first arrives in Munchkinland and she sees a bubble that seems to float forever (it then turns into Glinda, the good witch of the North)?  If you’ve never seen it, Google “dorothy lands in munchkinland” and hava a look at the videos.  This experiment reminds me of the bubble that just seems to hang around floating for a very long time. 

Materials:

• Bubble solution and wand
• Vinegar
• Baking Soda
• A large container (a fish tank works great but a garbage can or 10 qt stock pot will work)
• Candle

Caution:  Be sure to have an adult around anytime you have an open flame. 

EYE PROTECTION IS REQUIRED FOR THIS EXPERIMENT.  The use of gloves may also be warranted particularly if there are any open abrasions that the vinegar can interact with. Vinegar is typically 5% acetic acid.  Adult supervision is strongly recommended. 

Experiment:

Depending upon the size of your container, empty a substantial amount of baking soda (sodium bicarbonate or NaHCO₃) into the vessel.  Now add a lot of vinegar (acetic acid or C₂H₄O₂) into the container.  What do you see?  Whenever you see the formation of bubbles, you are seeing gas trapped by a liquid.  Now that all of those bubbles have formed and broken, use the wand and bubble solution and blow bubbles over the top of the container and try to get some of your bubbles to land inside the container.  Do not blow into the container.  What you should see is that the bubbles that find their way into the container float and do not land. 

What’s happening?

As was discussed last month in “The Alka Seltzer Challenge”, sodium bicarbonate, NaHCO₃, will react with the acetic acid (C₂H₄O₂) in vinegar to create water (H₂O), NaC₂H₃O₂, and CO₂ as seen here: 

 NaHCO₃  +  C₂H₄O₂    ->    NaC₂H₃O₂  +  H₂O  +  CO₂

Carbon dioxide, CO₂, is a gas and as it turns out is significantly more dense than air. This means that the carbon dioxide will remain in the container for a while and will sit at the bottom of the container.  It may fill the container entirely depending on how much baking soda and vinegar you used.  One way to find out how high up the gas goes inside the container is to light a candle and slowly lower it into the container.  Because CO₂ does not support combustion, the candle will extinguish when it is completely surrounded by CO₂. Because the density of CO₂ is 2.0 grams per liter, whereas the density of air is 1.3 grams per liter, the bubble full of the lighter (less dense) air cannot go underneath it, and so the bubble will just float on the surface of the carbon dioxide.  If you pay close attention, you may notice that the bubble will increase in size as the CO₂ finds its way into the bubble via osmosis, making it bigger.  If the bubble lasts long enough and grows in size due to the CO₂ entering into the bubble, expect it to sink as the air inside the bubble becomes more dense.  When you’ve had enough of the bubbles, see what happens when you pour the contents of the container (CO₂) on top of a lit candle!  Notice how the CO₂ seems to pour just like a liquid. 

References:

https://www.exploratorium.edu/snacks/bubble-suspension
http://www.scifun.org/HomeExpts/SOAPBUBL.html

To view all past “ChemShorts for Kids”, go to:
http://chicagoacs.org/articles/article_category/1

- Paul Brandt