DEMONSTRATION pubs.acs.org/jchemeduc
Gas Property Demonstrations Using Plastic Water Bottles Dean J. Campbell,* Stephen J. Bannon, and Molly M. Gunter Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinios 61625, United States ABSTRACT: Plastic water bottles are convenient containers for demonstrations of gas properties illustrating Boyle’s law, Charles’s law, and Avogadro’s law. The contents of ironbased disposable hand warmer packets can be used to remove oxygen gas from the air within an unfilled plastic water bottle. KEYWORDS: First-Year Undergraduate/General, General Public, High School/Introductory Chemistry, Demonstrations, Physical Chemistry, Public Understanding/Outreach, Hands-On Learning/Manipulatives, Gases
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lastic water bottles lend themselves well to demonstrations of gas properties. In the United States, these convenient containers are composed of transparent, flexible polyethylene terephthalate (PET). They can be readily acquired and are objects of familiar volume (approximately 500 mL) to demonstration spectators. The consistent shapes and sizes of bottles of the same brand enables visual volume comparisons. Finding two water bottles of the same size is perhaps more easily accomplished than trying to inflate two balloons to identical size.
nitrogen; if it does so, it will likely stay in a somewhat crumpled shape even after warming to room temperature. However, if the bottle does not crack, it will return back to its normal volume and shape upon removal from the liquid nitrogen, again making crackling noises as it does so.
’ DEMONSTRATION 3: AVOGADRO’S LAW A third demonstration of gas properties involving water bottles involves the oxidation of finely divided iron with the oxygen in the air of an unfilled bottle. As the oxygen gas is removed from the air and incorporated into solid iron oxide, the bottle contracts. This decrease in volume as a result of the decrease in the moles of gas in the bottle is an illustration of Avogadro’s law.1 This oxygen removal by iron oxidation can happen over long time periods if iron powder is placed within a sealed bottle.2 Others have used steel wool that has been first treated with acid to remove corrosion from the iron surface. The clean steel wool has been used to remove oxygen from the air and draw water up a tube, enabling measurements of the percent oxygen in the atmosphere.3 7 We have also caused unfilled water bottles to contract by adding steel wool, iron filings, or iron powder that has been cleaned with vinegar. Unfortunately, even with a large stoichiometric excess of metal, the bottles can take hours to finish contracting. A relatively fast, easy, and reproducible alternative is to use the contents of disposable hand warmer packets that generate heat by reacting oxygen gas with iron in the presence of charcoal, sodium chloride, and other components.8 Simply cut a hand warmer packet open with a scissors, empty the entire contents into an empty water bottle, and recap the bottle.
’ DEMONSTRATION 1: BOYLE’S LAW One gas property demonstration illustrates the compressibility of gases as distinct from the incompressibility of liquids and illustrates the relationship between gas volume and pressure (Boyle’s law).1 In this demonstration, squeeze or step on a capped water-filled plastic bottle and a capped unfilled plastic bottle. The water-filled bottle will not compress, but the air-filled bottle will be compressed reversibly, decreasing in volume upon application of pressure. ’ DEMONSTRATION 2: CHARLES’S LAW A second demonstration of gas properties involving water bottles is a variation of a classic demonstration of the relationship between gas volume and temperature (Charles’s law)1 in which balloons filled with air contract when placed in contact with liquid nitrogen.1 An unfilled (capped) bottle will also contract when immersed into liquid nitrogen. A narrow water bottle more easily fits into liquid nitrogen Dewar flasks than round balloons. This contraction is dramatic, with the bottle making crackling noises as the cold PET changes shape. The cold plastic bottle sometimes cracks open as it contracts, entraining some liquid Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Published: April 08, 2011 784
dx.doi.org/10.1021/ed100745c | J. Chem. Educ. 2011, 88, 784–785
Journal of Chemical Education
DEMONSTRATION
The visible collapse of the bottle within a half hour shows the consumption of oxygen in the reaction. Measurement of bottle volumes by water displacement in a 2000 mL graduated cylinder before and 1 h after addition of the packet contents indicates that bottle volumes decrease by approximately 20%, roughly the amount of oxygen normally present in the air.1 The volume changes achieved by this method were more reproducible than those achieved by the steel wool method. The exothermic reaction of the packet does not appear to significantly warm (and expand) the remaining gases in the bottle. We also have never observed the contents of any of our packets “smoking” as others have claimed.8 However, the reacting contents produce warmth that can be felt through the bottle walls as the bottle is handled. Note that the packet contents in older hand warmers might have already reacted with oxygen and will produce no heat because they consume no additional oxygen. At the end of the experiment, the heat packet contents (roughly 20 30 g of powder) can be rinsed out of the bottles into the sink or even discarded in the trash as long as the powder is no longer producing heat. As a final point, many PET water bottles are being recycled in many communities. When a water bottle becomes too worn out for use in demonstrations, it can be recycled rather than placed the trash—making these demonstrations, perhaps, a little bit “green”.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We would like to thank the Bradley University Science 101 class for assistance with this study and the Illinois Heartland Section of the American Chemical Society for funding. ’ REFERENCES (1) Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E.; Murphy, C. J. Chemistry: The Central Science, 11th ed.; Folchetti, N., Gilfillan, A., Hart, J., Eds.; Pearson Education, Inc.: Upper Saddle River, NJ, 2009. (2) Campbell, D. J.; Dr. Campbell’s Favorite Demos. http://bradley. bradley.edu/~campbell/demopix7.html (accessed Mar 2011). (3) Birk, J. P.; McGrath, L.; Gunter, S. K. J. Chem. Educ. 1981, 58, 804. (4) Martins, G. F. J. Chem. Educ. 1987, 64, 809. (5) Braathen, P. C. J. Chem. Educ. 2000, 77, 1410. (6) Gettys, N. S.; Jacobsen, N. S. J. Chem. Educ. 2001, 78, 512A. (7) Exton, D. Determination of the Percent Oxygen in the Air. University of Oregon. http://chemlabs.uoregon.edu/Classes/Exton/ CH228/PercentOxygen.pdf (accessed Mar 2011) (8) Wang, L. What’s That Stuff? Hand Warmers. Chem. Eng. News 2010, 88 (4), 36.
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dx.doi.org/10.1021/ed100745c |J. Chem. Educ. 2011, 88, 784–785
