Kids, why does shaking a can of soda make it burst out when you open it? And does tapping on the can stop it from doing this?
Contrary to popular belief, shaking a can of soda does not increase the pressure inside the can. Shaking takes one single pocket of carbon dioxide gas at the top of the can and changes it into thousands of tiny bubbles distributed throughout the entire can. This causes a huge increase in the surface area, so there are more places for the carbon dioxide to dissolve and bubble.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
In an unshaken can, bubbling out happens at the liquid’s surface, but it can also happen anywhere along the inside walls of the can where there is some tiny bump—also called a nucleation site—that can help bubbles to form. For this reason, bottling companies make sure that the inside walls of their cans are as smooth as possible. Otherwise, soda would spray all over the place each time a can is opened.
When a can is shaken and then opened, not only is there a great deal more surface area, but each tiny bubble can act as a nucleation site. This causes a rapid bubbling out of carbon dioxide all throughout the liquid, not just at the top. Voilá - a soda explosion.
Now, does tapping on the top of the can stop it from exploding? The theory is that tapping loosens tiny bubbles stuck on the sides, which are nucleation sites; they float to the top and no longer pose a “soda explosion threat.” Ask an adult partner to confirm it (over a sink!): shake a can of soda at room temperature for five seconds, open it, and watch soda spray fizz out. Then shake an identical can of soda at room temperature for 5 seconds, tap it 10 times, open it, and much less soda should squirt out.
But is this really a well-controlled experiment? Aside from tapping vs. not tapping, the time delay between shaking and opening the can may also make a difference. Now have your adult partner shake two identical cans—one in each hand—for five seconds, set them down and tap just one of them for five seconds. Then open them at the exact same time. Others report trying this experiment with a variety of sodas for a variety of shaking times and delay times, and are unable to observe any consistent difference between the tapped can and the control. No, it wasn’t “Mythbusters”, but this is similar to many of the urban legends that they disprove!
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
April 2008
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Reference:
Bob Becker in “ChemMatters” at http://www.acs.org/chemmatters February 2008 issue (big file, may load very slowly).
Also see other sites such as: http://www.snopes.com/science/sodacan.asp
