Activity pubs.acs.org/jchemeduc
Visualizing Separations: How Shopping Can Be Useful for Introducing Chromatography Megan L. Nagel* Department of Chemistry, Penn State Greater Allegheny, McKeesport, Pennsylvania 15132, United States S Supporting Information *
ABSTRACT: A trip to the mall is used as a classroom demonstration to illustrate the fundamentals of separations without the need for chemicals or any chemistry background. Student volunteers are the “mixture”, and depending on the shopping list they have been given, they spend varying amounts of time in the “stores” versus moving through and out of the “mall”, as the instructor reads all of the possible shopping stops aloud. In less than 10 minutes, the students observe the separation of the “mixture” into distinct groups, see how this can be represented graphically, and may then apply the knowledge to chemical separations.
KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Elementary/Middle School Science, Demonstrations, Physical Chemistry, Analogies/Transfer, Chromatography, Nonmajor Courses, Student-Centered Learning
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ally, performing these demonstrations requires the appropriate chemicals, glassware, and other equipment. Herein, we provide a quick and simple classroom demonstration that uses student volunteers to illustrate the foundation of chromatographic separations occurring in an adsorption process by drawing an analogy to shopping at the mall. A group of student volunteers behave as the “mixture” that has entered the mall at the same time. As they stop to shop (as directed by the activity), the students begin to separate into distinct groups based on their time interacting with the stores versus moving toward the exit. Because students are a readily available resource in any classroom, using the directed movement of students to illustrate the chromatographic process is an attractive technique.8 Also, because the fundamental theory of separation is the same for paper and thin-layer chromatography as it is for adsorption column chromatography, the analogy holds for a large number of ways in which chromatography may be introduced. The demonstration can be completed in any type of classroom with a small or large number of students.
he basic concept of chromatography, or the separation of a mixture into its individual components, is a widely introduced chemical concept. In fact, there are a number of activities suitable for students of all ages and skill levels that range from separations involving candy to magic markers to leaf pigments.1−5 Generally, chromatographic separations are the result of partitioning the components of a mixture between two phases, a stationary phase and a mobile phase. In this dynamic process, the stationary phase is kept in a fixed position while the mobile phase moves in a set direction. The properties of the stationary and mobile phases determine the extent of interaction of the two, and thus serve as the basis for separation. Depending on the type of chromatography being performed, components may be separated based on a number of different properties, including size, shape, polarity, or charge. The oldest and most common type of chromatographic separation is adsorption chromatography. In this case, the components of the mixture are adsorbed onto a solid stationary phase with varying strengths and frequencies related to their respective intermolecular attractions. In a traditional chemistry course, a discussion of molecular geometry, polarity, and intermolecular forces would serve as the basis for understanding the adsorption process and why certain components elute more quickly, whereas others spend more time partitioned in the stationary phase. To help reinforce the theoretical basis for separations, a number of classroom demonstrations that help visualize the separation process as it occurs have been reported.6,7 Although impressive, understanding why the components separate in these demonstrations requires a working knowledge of general chemistry. Addition© XXXX American Chemical Society and Division of Chemical Education, Inc.
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AUDIENCE Although this demonstration is appropriate for any course that introduces chromatography, it can be especially useful for elementary, middle school, or high school students, as well as nonmajor chemistry courses at the university level, where molecular shapes, polarity, and intermolecular attractions are not necessarily part of the required curriculum. For example, it
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dx.doi.org/10.1021/ed2008685 | J. Chem. Educ. XXXX, XXX, XXX−XXX
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moving through the mall. Those shoppers with long lists spend a greater amount of time in the stores, much like a molecule with a strong affinity for the stationary phase. Students should be reminded that no two shopping lists were the same, yet groups of shoppers exited in distinct groups, emphasizing the point that time in the stores, regardless of which stores, dictated the movement through the mall. At this point in the exercise it may also be useful to point out that the provided shopping lists indicate that each shopper had only one item per store, and each item took an equivalent amount of time to purchase. An important point to make is that two shoppers could spend an equivalent amount of time in the stores if the lists were constructed differently. For example, one student’s list may have four items to get at a single store, but another student may have a list with four separate stores with one item each. Although the profile of how these two students spend their time in the mall is quite different, they will both exit at the same time. If appropriate for the course, this may lead to a more detailed discussion of how and why different components interact with the stationary phase. Because the students observing the demonstration see the “shoppers” all begin side by side, but eventually end up separated into distinct groups, another useful aspect of this illustration is the introduction of resolution at different points in the separation process. In the context of column chromatography, this could lead to a discussion regarding how column length might affect resolution, time it takes for components to elute, and so forth. Furthermore, the small differences in the size of the steps that individual students take as they move through the mall may also provide an introduction to zone broadening and peak width. After completing the activity, the instructor can introduce a graphical representation of the demonstration (Figure 2). The
has been used successfully in a forensic science course for nonscience majors, an introductory chemistry course for nonscience majors, and during an introduction to chromatography for a quantitative analysis class. In the forensic science class, the demonstration was completed at the beginning of a unit on chemical methods of analysis. It not only provided a visual illustration for the students, but it also allowed students to gain comfort with the general nature and vocabulary of chromatography without being burdened by the specific chemistry of the process. In this case, the activity provided a foundation for the introduction of thin-layer chromatography, liquid chromatography, and gas chromatography and was frequently referenced by both the instructor and students throughout the chapter.
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ACTIVITY To visually illustrate what happens to the components of a mixture during the separation process, students role-play a group of shoppers at a mall. To begin, 8−12 volunteers from the class are selected and given a “shopping list”. These lists are of varying lengths and include different stops the shopper must make, with no two lists being identical. However, the lists are constructed such that each shopper has exactly two, four, six, or nine stops to make. Sample lists are shown in Figure 1
Figure 1. Sample shopping lists.
(additional shopping lists are included in the Supporting Information). The students are told they will be going shopping at a mall with one long walkway. The volunteers line up at the front of one side of the classroom, as though they all enter the “mall” at the same time, while the rest of the class observes. Depending on the size of the class, a hallway also works well. The teacher then slowly reads a list of all possible stops in the mall. If the stop is listed on the shopping list, the student must stay where they are, but if it is not on the list, they take one step forward. Much like an actual chromatographic separation, all of the different shoppers start out together and early in the interaction with the stationary phase, or stops at the mall in this case, there is little apparent separation. However, slowly after many of the stops at the mall have been read by the instructor, distinct groups of shoppers begin to emerge based on the length of their shopping list. It then becomes obvious after all of the stops have been read that those students with more on their list take longer to exit the mall than those with a shorter list.
Figure 2. Graphical representation of directed student movement during separation demonstration.
graph has been constructed to closely resemble a typical chromatogram based on 12 different shopping lists distributed to the student volunteers. Using observations along with the graph, students can answer a series of questions, a sample of which are provided in Figure 3. The fundamental understanding of separations as illustrated in the shopping example can later be extended to chromatograms resulting from a chemical separation. Finally, the instructor using this activity should be aware that this exercise is meant merely as a preliminary introduction to separations or as an initial groundwork to begin a more
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POST-ACTIVITY DISCUSSION Once the activity has been completed, the instructor can then use the activity to draw connections to the chromatographic process, likening the stores to the stationary phase. If shoppers have no reason to enter a particular store, they will keep B
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(2) Wigman, L. S.; Kelsch, C. T. J. Chem. Educ. 1992, 69 (12), 991− 992. (3) Kandel, M. J. Chem. Educ. 1992, 69 (12), 988−989. (4) Kimbrough, D. R. J. Chem. Educ. 1992, 69 (12), 987−988. (5) J. Chem. Educ., 2000, 77 (2), 176A − 176B. (6) Heumann, L. V.; Blanchard, D. E. J. Chem. Educ. 2008, 85 (4), 524−526. (7) Bricker, C. E.; Taylor, M. A.; Kolb, K. E. J. Chem. Educ. 1981, 58 (1), 41. (8) Bauer, C. F. J. Chem. Educ. 1982, 59 (10), 846.
Figure 3. Shoppers’ data analysis questions.
thorough discussion. Therefore, to avoid introducing potential misconceptions, students should be reminded that shopping is not, in fact, equivalent to chromatographic separations. Although, the analogy can be helpful, a few important distinctions between the two should be considered when using this activity. First, although the reasons that a person may stop to shop at a store may include desire, low prices, or certain inventory, the interactions of a component with the stationary phase are determined not by some “decision” by the molecule, but by the unique properties of the molecule and the stationary phase. Second, the demonstration suggests that during the chromatographic process, the components of a mixture move in a straight line, from one end to the other of the stationary phase. In reality, although there is general movement in a set direction, even identical components will experience multiple paths and longitudinal diffusion while interacting with the stationary phase. Finally, the mall analogy, one long walkway lined with stores, suggests that the mobile phase is an open space through the center with the stationary phase positioned on either side. Although this may be a more relevant description for partition gas chromatography, adsorption chromatography occurs with the mobile and stationary phases in contact on all sides, more like a mall filled with kiosks, instead of stores. With careful execution, this brief demonstration can successfully provide an initial introduction to chromatographic separations for a wide variety of learners using the resources available in any classroom.
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ASSOCIATED CONTENT
S Supporting Information *
Sample shopping lists. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I would like to thank Bethel University and Matthew Neibergall for his assistance in testing this activity. REFERENCES
(1) Ondrus, M. G.; Brice, L. K. J. Chem. Educ. 1985, 62, 798−799. C
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