Addition/Correction Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX
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Addition to “Gas Law Property Demonstrations Using Plastic Water Bottles”: Safety Comment and Extension Miah M. Montes and Dean J. Campbell* J. Chem. Educ. 2011, 88 (6), 784−785. DOI: 10.1021/ed100745c ABSTRACT: This addition describes how placing a hole in the top of the plastic water bottle used to demonstrate Charles’s law can make the demonstration safer and enhance the presentation of the demonstration.
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gases behave in accordance with Charles’s law. Special thanks to Scott Duda for bringing this safety issue to our attention.
lastic water bottles have been shown to be useful in demonstrations of Boyle’s law, Charles’s law, and Avogadro’s law.1 In a demonstration of Charles’s law, the bottle can be immersed partway into liquid nitrogen. As the cooled gases inside the bottle contract, the bottle collapses with a spectacular crumpling sound. Thin-walled bottles collapse more completely than thick-walled bottles. When the bottle is removed from the liquid nitrogen, it expands again, and the bottle can be reused for a later demonstration. We have used this demonstration at many science outreach events. However, we recently observed a small safety issue with this demonstration and suggest a simple solution. The plastic bottles used for this demonstration can crack as they flex and crumple in the cold liquid nitrogen. Liquid nitrogen can then sometimes leak into the submerged cracked bottle. The liquid nitrogen could then evaporate to produce nitrogen gas within the bottle faster than the gas can leave. If the excess gas pressure is not released quickly, then the bottle could burst open and chips of plastic could break off. To minimize this potential hazard, a 4−5 mm hole pierced into the cap of the plastic bottle could be used as a gas safety vent. A rubber stopper (size #3) containing a hole can also be used in place of the cap. Covering the hole in the cap while immersing the plastic bottle partway into liquid nitrogen prevents air from entering the bottle and allows it to collapse. A bare finger can adequately seal the hole, but concerns about having a hand too close to the liquid nitrogen warrant the use of gloves. If the gloves are made from porous material such as fabric, placing a deflated rubber balloon over the glove finger used to cover the hole will help seal it. If any liquid nitrogen remains in the bottle after it is removed from partial immersion, the finger can be lifted away from the bottle hole before unsafe pressure builds up. It is important to make sure that the hole is not blocked too long. Cracked bottles should be recycled and not reused. The hole in the bottle cap lends itself to an interesting variation on the demonstration: if the bottle is immersed partway into liquid nitrogen and a hole of sufficient size is left uncovered, the bottle does not collapse. If the hole is too small for adequate airflow, e.g., 1 mm, the bottle will still collapse. Doing this immersion demonstration first with the open hole, before reimmersion with the blocked hole, shows the audience that the bottle contraction is NOT a result of how the plastic itself interacts with the liquid nitrogen, but rather, how the © 2011 American Chemical Society and Division of Chemical Education, Inc.
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REFERENCES
(1) Campbell, D. J.; Bannon, S. J.; Gunter, M. M. Gas Law Property Demonstrations Using Plastic Water Bottles. J. Chem. Educ. 2011, 88 (6), 784−785.
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DOI: 10.1021/acs.jchemed.7b00729 J. Chem. Educ. XXXX, XXX, XXX−XXX
