In the Laboratory
Imploding Soda Cans: From Demonstration to Guided-Inquiry Laboratory Activity Jack F. Eichler Division of Natural Science and Mathematics, Department of Chemistry, Oxford College of Emory University, Oxford, GA 30054;
[email protected] The imploding can has always been one of my favorite demonstrations. The experiment is simple, yet elegant. An aluminum beverage can containing a small quantity of water is heated until the water boils, and when it is placed mouth down in an ice– water bath it violently implodes. I was awed the first time I saw this phenomenon, and to this day I still marvel at the beauty of the interplay between temperature, pressure, and volume that takes place. Because this demonstration can stimulate discussion about states of matter, the kinetic theory of molecules, and the relationship between temperature, pressure, and volume of a gas, it is ubiquitous in physics and chemistry classrooms. The utility and interest in this demo is evidenced by the fact that discussion of this activity in science education journals goes back almost 30 years (1), and numerous variations on the experiment have subsequently been reported (2–6). Traditionally, I have used the imploding can in my general chemistry courses as an introduction to the discussion of gas laws and the relationship between temperature, pressure, and volume of gases. I have done the demonstration and then asked the students to work in small groups in an effort to provide an explanation for the implosion. The activity has been very successful in providing a springboard into the study of gases and the gas laws and provides an active learning environment for the students. However, over the years I have begun to wonder why I do not let the students actually do the experiment themselves? If I enjoy this demonstration so much after having done it dozens of times, imagine how much more the students would enjoy it getting to perform it as novices. List 1. Examples of Student Hypotheses • A change in temperature is directly proportional to changes in pressure and volume in the aluminum can. • If a can is placed over a Bunsen burner, the temperature of the liquid inside will increase. As the water boils, the pressure of the vapor in the can increases. When the hot can is immediately flipped into an ice bath and the mouth of the can is submerged, the sudden temperature drop causes a decrease in pressure. Because the outside pressure is constant, the outside pressure causes the can to crush. • When the can is heated, the water inside the can becomes a gas which increases the volume, temperature, and pressure. When the can is submerged in the ice, the temperature decreases rapidly which changes the water vapor back to a liquid. This will cause a substantial decrease in volume and pressure inside the can. Since the pressure inside the can decreases and the atmospheric pressure outside the can remains constant, the atmospheric pressure pushes the walls of the can inward and essentially crushes the can. • When water is boiled it changes from a liquid to a gas. When a gas is rapidly cooled it condenses, causing a drastic decrease in volume. The decrease in volume is accompanied by a decrease in the internal pressure within the can. The external or atmospheric pressure overcomes the internal pressure with in the can, and causes the can to implode.
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With this in mind, I sought to convert the imploding can demonstration into a guided-inquiry lab activity. A search of the literature yielded two reports of imploding can experiments involving higher levels of student involvement than a traditional demonstration. However, one of these described a lab in which students performed a variation of the experiment (students placed a warm, sealed plastic bottle in ice water) that only called on them to predict the outcome, perform the experiment, and then record the results (7). The other report documented an inquiry process, but the students suggested alternative experiments that were then done by the instructor (8). In an effort to combine the hands-on experience and guided-inquiry aspects of the aforementioned articles, this article describes an exercise in which the students imploded 12 oz aluminum cans and generated hypotheses to explain the phenomenon. Then, with limited guidance from the instructor, the students designed and conducted a series of additional experiments that allowed them to quantify the relationship between the temperature of the water in the soda can and the intensity of the implosion. These data were then used to evaluate the original hypotheses. The format of the class and laboratory activities, examples of student protocols, and a summary of representative results are described herein. Class Format This activity was done in an introductory chemistry course for non-science majors and was taught in a studio lecture format.1 Therefore, the pre-lab discussion was carried out at the beginning of a class period, and then the students immediately began work on the actual laboratory exercise. The students worked in groups of 4 and were given the remainder of this class as well as the next class meeting to complete the lab. Groups that completed the actual data collection early began working on the data analysis and formal reports, whereas slower groups finished the data collection on the second day and completed the data analysis and formal report outside of class. Once the lab activity and associated discussion was completed, more in-depth analyses of the gas laws were completed in subsequent lectures. This activity could be adapted to a traditional lecture–lab format by conducting the discussion in the lecture that precedes the lab and then allowing the students to generate their hypotheses and collect data during the lab period. Pre-Lab Discussion This lab activity was done at the beginning of the unit that discussed the properties of gases and gas laws. Since the liquid and solid states of matter were covered in a previous unit, the students were familiar with the kinetic theory of matter and how the states of matter differ on a molecular level. Prior to conducting the lab, these ideas were reviewed and the kinetic theory of matter was extended to describe gases. The relationship between the kinetic theory of gases and temperature was also discussed,
Journal of Chemical Education • Vol. 86 No. 4 April 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
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Temperature of Water in Can / pC Figure 1. Volume of the can after implosion vs the temperature of water in can before implosion (20 mL of water was placed in the can for each trial).
Figure 2. Aluminum cans after being placed, mouth down, in an ice-water bath. The temperature of the water in the can prior to implosion: (A) 30 °C; (B) 60 °C; (C) 80 °C; and (D) 100 °C (20 mL of water was placed in the can for each trial).
and then the concepts of gas pressure and atmospheric pressure were introduced. Finally, there was a discussion about the relationship between liquid vapor pressure and temperature, and the physical nature of the vapor-to-liquid phase change. Before moving on to the lab activity the instructor made sure the students understood that higher temperatures result in larger vapor pressure and that the decrease in volume that occurs during condensation is quite dramatic (especially when compared to the volume change that occurs when a gas is cooled, but not condensed).
each time; Figure 3A). Some groups also did an entire series of control experiments to supplement the above experiments (e.g., observing the change in can volume versus temperature for cans with no water placed in them-- (Figure 4), and after observing that the cans sucked up some of the water from the submersion bath, a few groups measured the volume of water taken in by the cans as a function of changing water temperature in the can prior to submersion (Figure 3B). A mL
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Laboratory Activity When the students began the lab activity, they were given instructions that described how to perform the imploding can experiment. They placed between 10 and 30 mL of room temperature water in the can, heated it over a Bunsen burner flame or hot plate until the water began to boil and then quickly submerged the can opening into a large ice-water bath. After doing this initial experiment, each group was asked to use the concepts from the class discussion to generate a hypothesis that explained why the can imploded (List 1). All of the groups arrived at some variation of the idea that the pressure of the water vapor inside the can was lowered when the can was placed in the ice bath and that this lower pressure allowed the constant external atmospheric pressure to crush the can. Most of the groups stated that the pressure changes in the can were due to temperature changes, and some even mentioned that the drastic change in volume was caused by the condensation of the water vapor. After the groups decided on their final hypotheses, they were asked to design a set of experiments that would provide data to help them confirm or deny the hypothesis. During this experimental design stage, the groups had the opportunity to consult the instructor for advice. Though the students did get feedback and suggestions from the instructor, every effort was made to ensure the students arrived independently at their final procedure. The type of experiments performed by the students generally fell into two categories: (i) keep all of the variables constant except for the temperature of water in the can prior to submerging it in the ice water and measuring the volume of the can after implosion (Figures 1 and 2); and (ii) keep all of the variables constant except for the temperature of the water bath in which the can was submerged (a can with 90–100 °C water was used
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Temperature of Water in Can / pC Figure 3. (A) the volume of the can after implosion vs the temperature of the submersion bath (30 mL of water was placed in the can and then heated to 98°C for each trial); (B) the volume of water sucked up by the can after implosion vs the temperature of the water in the can prior to submersion (10 mL of water was placed in the can for each trial and the can was generally submerged in the water bath for 20–60 s prior to making subsequent measurements; error bars represent the standard deviation for the two trials conducted at each temperature).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 4 April 2009 • Journal of Chemical Education
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Hazards If Bunsen burners are used to heat the aluminum cans, the necessary precautions for working with an open flame should be taken. Additionally, students should be warned of the potential burn hazards associated with the boiling water and water vapor in the can, whether hot plates or Bunsen burners are used.
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Student Results and Analyses In general, the experiments conducted by the students indicated that the pressure of the vapor in the can prior to submersion in ice water or the temperature of the water bath did seem to directly affect the type of implosion that occurred. As seen in Figures 1, 2, and 3A, if the water in the can was at a higher temperature or if the can with boiling water was placed in a colder water bath, the can imploded more violently, resulting in a smaller final volume. These data confirm the general hypothesis that a larger decrease in vapor pressure inside the can correlates to a more violent implosion. The idea that decreasing the temperature of water vapor in the can caused a pressure drop in the can was further corroborated by the fact that as the temperature of the water vapor prior to submersion in ice water increased, the quantity of water pulled into the can after submersion also increased (i.e., a larger temperature decrease results in a larger vapor pressure decrease inside the can; Figure 3B). In addition, most of the students concluded that the gas– liquid phase change was necessary for a violent implosion to occur. Since the change in vapor pressure of the water molecules during condensation is so drastic, the large and rapid decrease of pressure in the can upon being placed in a colder environment, as well as a constant external atmospheric pressure, is the root cause of the implosion.2 Groups that did control experiments using no water in the can also pointed this out by discussing how these trials lacked a condensation process, and therefore did not implode (Figure 4). The requirement for a gas–liquid phase change was also confirmed by the experiments in which the cans with boiling water were placed in warm water baths (in Figure 3A, when the can was submerged in the 45 °C water bath, no implosion occurred). These results confirmed the more specific hypothesis that the implosion only occurs when a drastic pressure decrease, resulting from condensation, takes place in the can. Final Remarks This laboratory exercise was successful in a variety of ways. As evidenced by the type of experiments performed, the formal reports, and post-lab discussion, the students obtained a solid understanding of the qualitative relationship between the temperature and vapor pressure of water. The students achieved this by being asked to observe and describe the correlation between the implosion of the can and the large pressure decrease upon condensation of the water vapor. My experience suggests this activity is effective in preparing students for the subsequent discussion of gas laws. In the course of learning these concepts in the lab, the students were given the opportunity to develop their own laboratory procedures and experimentally determine the relationship between two variables. These are activities that cannot be done by passively observing an instructor-led demonstration. Perhaps most importantly, the students were able to experience the excitement of using simple temperature changes to implode alumi474
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Temperature of Air in Can p C Before Immersion Figure 4. Volume of the can after immersion vs the temperature of air in can before implosion (the can was empty for each trial).
num soda cans, which led to a much higher level of enthusiasm than observed in previous demonstrations. For these reasons, instructors of introductory chemistry and physics courses at all levels would be well served to incorporate a guided-inquiry activity such as the one described here to introduce the concepts of kinetic molecular theory of matter and gas laws. Notes 1. This class was populated by 18 students; the studio lecture meets for 2 hours each class period, 3 days per week. 2. Battino and Brown provide the vapor pressure of water at varying temperatures (5). The large pressure difference between water vapor and the atmosphere that arises when steam is condensed is adequate to crush a variety of containers (at 0 °C, water vapor has a pressure of 613 Pa).
Literature Cited
1. 2. 3. 4. 5. 6. 7. 8.
Visscher, P. B. Am. J. Phys.1979, 47, 1005. Taylor, T. The Physics Teacher 1982, 20, 458. Sands, R. D.; Blackman, D. J. Chem. Educ. 1982, 59, 866. Steward, J. E. The Physics Teacher 1991, 29, 144. Brown, J. L.; Battino, R. J. Chem. Educ. 1994, 71, 514–516. Gratten, L. M.; Oss, S. The Physics Teacher 2006, 44, 269–271. Shipman, H. L. J. Coll. Sci. Teach. 2001, 30, 318–321. Bauer, C. The Science Teacher 2006, 73, 62–63.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Apr/abs472.html Abstract and keywords Full text (PDF) Links to cited JCE articles
Figure 2 in color
Supplement Student handouts and grading rubric
Instructor notes
Sample lab report
QuickTime movie of imploding soda can (by James H. Maynard)
JCE Cover for April 2009 This article is featured on the cover of this issue. See p 403 of the table of contents for a detailed description of the cover.
Journal of Chemical Education • Vol. 86 No. 4 April 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
