Solution to Chemical Mystery #6: The Case of the Collapsing Can

Crush a can with chemistry

In Chemical Mystery #6, I used chemistry to crush a metal can. To do so, concentrated sodium hydroxide solution (about 35% NaOH by weight) was added to a can that was almost completely filled with carbon dioxide gas. The can was then sealed. The carbon dioxide gas in the can reacted with the added sodium hydroxide:

2 NaOH(aq) + CO2(g) --> Na2CO3(aq) + H2O(l)        Equation 1

This chemical reaction removed the gas from the can. Of course this lowered the gas pressure inside the can. Once the pressure inside the can became low enough, the surrounding air pressure was able to push on the can, causing it to implode. The same effect was not observed in a can simply filled with air, because air contains very little carbon dioxide. The video below shows one way you can carry out this experiment.

So that’s how you can crush a can using chemistry, and also present a neat chemical trick! 

Congratulations to Andres Tretiakov and Bob Worley who both figured out how this experiment works. Bob likes to use plastic soda pop bottles as a less expensive alternative for this experiment, but I happen to think it’s a bit more impressive to use metal cans. For those interested, Flinn Scientific has published a procedure on how to carry out this experiment using plastic bottles1.

There are a surprising number of concepts that can be taught while using this demonstration. Certainly one can discuss the gas laws and composition of the atmosphere when using this demonstration. Students could be challenged to identify reactions that might allow one to crush a can filled with air. This demonstration can also provide a reference point for one way chemistry is being used to combat climate change: sodium hydroxide, though application of Equation 1, is used as a reagent to remove carbon dioxide from the atmosphere2-4. Finally, this experiment provides an example of a spontaneous reaction for which both the enthalpy and entropy changes are negative. Although I didn’t mention it in the video, the can becomes quite warm as a result of the reaction (negative enthalpy )and a gas is consumed (negative entropy). 

Are there any other concepts you think might relate to this particular experiment? I'd love to hear your thoughts in the comments below.

References:
1.    https://www.flinnsci.com/media/621027/91423.pdf
2.    http://scholar.harvard.edu/files/davidkeith/files/97.stolaroff.aircaptur...
3.    http://wordpress.ei.columbia.edu/lenfest/files/2012/11/ZEMAN_LACKNER_200...
4.    http://www.popsci.com/molika-ashford/article/2008-10/better-co2-scrubber