In the Laboratory
Making a Chemical Rainbow Marcus Angelin* and Olof Ramström Department of Chemistry, KTH-The Royal Institute of Technology, Teknikringen 30, S-10044, Stockholm, Sweden *
[email protected] During high school chemistry education, a range of new chemical concepts and theories are introduced and old concepts are revisited and further expanded. In the laboratory, there is a clear transition toward more “realistic” chemistry experiments. The students are introduced to basic chemical instrumentation and are allowed to perform simple synthetic experiments or titrations. During this transition, there is an increasing emphasis on laboratory safety. As students gradually begin to handle more hazardous chemicals and organic solvents, it is important to explain what protective gear to use and how to work properly in well-ventilated areas such as fume hoods. It is also important for students to focus on a more applicable and deeper understanding of the chemical concepts. One way to accomplish this is by varying the pedagogy, through, for example, the use of problem-based learning (PBL) activities (1). In PBL, learning is built around a problem that the participants are challenged to solve. There are no “step-by-step” descriptions, rather students work in teams and use critical thinking and available learning sources to find the solution to the problem. This approach was originally used in medical education, where its performance has been analyzed and reviewed (2). More recently, the concept has been expanded to other areas, demonstrated, for example, by earlier publications in this Journal (3). In this laboratory experiment, the students are challenged to make a layered liquid rainbow in a test tube. Students are required to apply their knowledge about aqueous and nonaqueous solutions, densities, and “like dissolves like” principles to solve this PBL-based experiment. There are examples of experiments featuring the construction of rainbow-like mixtures published in the literature (4, 5), and on the Web (6, 7). These are, however, normally based on layering aqueous (and sometimes ethanolic) solutions and result in unstable systems. In our case, six unknown colorless solutions (3 aqueous and 3 nonaqueous) of different densities are used with six vials of uniquely colored pigments (3 aqueous, 3 nonaqueous) ranging through the visible spectrum from violet to red. Without further information, the students are expected to devise an effective strategy to identify which solutions belong to the same solubility category and identify the density order. This experiment is a timely way to introduce the students to working with chemicals and solvents in well-ventilated areas such as fume hoods. It also can initiate discussions about solvent extraction and other important principles involving multiphasic systems. Finally, the student subgroups check their results by coloring the solutions with the pigments, and carefully add each solution sequentially to a test tube, slowly creating a beautiful and stable six-layered chemical rainbow (Figure 1). 504
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Figure 1. Schematic of making a chemical rainbow in a test tube (left) and photo of the test tube containing the colored solutions (right).
Figure 2. The experimental setup including “unknown” liquids and pigments. (Note that in the experiment the pigment vials contain the sponges from the colored pens.)
Experimental Procedure Preparations The six unknown solutions were prepared by the instructor prior to the experiment. The aqueous solutions included calcium chloride, CaCl2 (two solutions of 0.7 g/mL and 0.35 g/mL) and deionized water.1 The nonaqueous liquids were 1,2-dichlorobenzene,2 1,2-dichlorobenzene/ethyl acetate (1:1 mixture), and ethyl acetate. The water-soluble pigments (violet, green, orange) were taken from water-based pens and the other three (blue, yellow, red) from permanent overhead markers.3 The solutions and pigments were collected in vials and the solutions were marked with numbers ranging from 1-6 (Figure 2). Experiment After a suitable introduction to the experiment and general work in fume hoods, each student group (2-3 students) received the six marked vials (1-6) containing the solutions and the vials containing pigments together with the instructions (see supporting information). Each group also received a set of test tubes
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In the Laboratory
Figure 3. Flowchart illustrating one possible strategy to solve the problem. In the first step, liquids are compared to form two groups based on solubility (red and black). (Comparison between liquid 1 and 6 is in the present case not necessary.) In the second step, densities are compared between the groups until the final order is established. In the final step, the liquids are colored and transferred to the larger test tube (= indicate that two liquids are miscible; > and < indicate the relative density of two immiscible liquids).
(2.5 mL) and Pasteur pipets. A larger test tube (20 mL) was provided for the final rainbow construction. Hazards The experiment should be carried out in a well-ventilated area (e.g., a fume hood) and students should wear standard protective clothing and eye protection. Calcium chloride is an irritant and dichlorobenzene is harmful and dangerous to the environment. Ethyl acetate is flammable and an irritant. Results and Discussion The experiment was tested in a high school class. The class was divided into two lab groups, each with approximately 10-15 students. These groups performed the experiment at separate times, both having 90 min dedicated to the experiment. At the beginning of the session, a short introduction was made focusing on the principles and concepts involved in the experiment. The safety aspect was also emphasized, especially regarding experimenting with unknown chemicals and how to correctly work in a fume hood. This was followed by dividing the students into subgroups (2-3 students), each subgroup receiving a copy of the student manual (see supporting information). Because the students had a relatively short time to complete the experiment, a few hints were given on how to come up with possible strategies to solve the problem. The student subgroups then tried to come up with an effective strategy without input from the instructor. Some subgroups started experimenting almost immediately, whereas other subgroups spent a longer time forming a strategy. In a few cases, the subgroup's process was slow and required the instructor to give a hint by asking a leading question. The problem only has one solution, but there are many ways to get there. One way to start is to systematically test the solubility of liquid 1 against the other liquids. This enables the student to separate the vials into two groups based on solubility.
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Next, the members of the different solubility groups are compared until enough information is collected to establish the density order. Subsequent coloring and careful addition to the large test tube gives the students the beautiful final result (see Figure 3 for a flowchart solving this example). Generally, the students used a strategy similar to the one described. However, the strategies were not always completely structured. Instead, more testing was performed than required to solve the problem, and the transformation from test results to a working hypothesis was not clear in all cases. After some individual discussions, however, all subgroups managed to find the correct solution. With the solution in hand, the final step was to color the liquids and layer them in the larger test tube. This posed problems for some of the subgroups, either because of using too much pigment (pigments from pens and overhead markers are concentrated) or because of not layering the solutions carefully enough. The former problem made the colors very dark and it became difficult to see the phase separation, whereas the latter problem resulted, for example, in that liquid 3 was pushed through liquid 2, thereby ending up being mixed with liquid 1. Normally, this problem could easily be solved by repeating this process, but owing to the time constraints, this was not always possible. The majority of the subgroups, however, achieved the expected result. The students submitted a written experimental report describing their logic and how they solved the problem. There were also five questions in the student manual that the students answered individually (see supporting information). The students completed an evaluation after finishing the experiment. The results were positive; all students liked the experiment and 80% graded it with the highest possible mark. More than 80% of the students wanted more problem-based experiments similar to this one and over 90% felt that the experiment had increased their understanding of the solubility and density concepts.
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Conclusions Overall, the experiences from this laboratory experiment were positive. Students were motivated and driven toward finding the solution and creating the “chemical rainbow”. The 90 min time limit during testing, however, was too short and required the instructor to give hints too quickly, inhibiting the PBL strategy. In addition, the allowed time was not enough for some subgroups to repeat the final rainbow construction and achieve a satisfactory result. Consequently, a time frame of 120150 min is recommended for this experiment, allowing more time for the students to think and more time to correct possible mistakes during the final stages. The evaluation was positive, with all students giving high marks to the exercise and its affect on their learning. It also displayed a clear desire from the students to introduce more problem-based learning into the curriculum. The experiment can also easily be modified for use as a demonstration. In this case, the instructor can, in a colorful way, demonstrate the “like dissolves like” principle and how density forces liquids to change positions in a test tube. Ultimately, the principles can be used to construct the liquid rainbow. Because of the system of layering alternating aqueous and nonaqueous solutions following the density scheme, the final rainbow is remarkably stable and can be used for exhibition purposes up to months afterward. Notes 1. The solutions, ranked according to decreasing density, are (1) 0.7 g/mL CaCl2(aq), (2) 1,2-dichlorobenzene, (3) 0.35 g/mL CaCl2(aq), (4) 1,2-dichlorobenzene/ethyl acetate (1:1 mixture), (5) deionized water, and (6) ethyl acetate. Approximate densities are provided in the supporting information. 2. Dichlorobenzene could be substituted with the more common solvent dichloromethane. Dichloromethane, however, is a carcinogen, whereas 1,2-dichlorobenzene is not. A good summary
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is available at the Carcinogenic Potency Database Project Home Page. http://potency.berkeley.edu/ (accessed Feb 2010). 3. The pigments were extracted by breaking the pens using a pair of scissors. The pigments (sponge-like) were taken out and put into vials. Actual color was extracted from the “sponge” using a Pasteur pipet.
Literature Cited 1. Problem-Based Learning: A Research Perspective on Learning Interactions, 1st ed.; Evensen, D. H., Hmelo, C. E., Eds.; Lawrence Erlbaum Associates, Inc.: Mahwah, NJ, 2000. 2. Koh, G. C-H.; Khoo, H. E.; Wong, M. L.; Koh, D. Can. Med. Assoc. J. 2008, 178, 34–41. 3. (a) Coppola, B. P.; Gottfried, A. C.; Gdula, R. L.; Kiste, A. L.; Ockwig, N. W. J. Chem. Educ. 2006, 83, 600–603. (b) Cancilla, D. A. J. Chem. Educ. 2001, 78, 1652–1660. (c) Ram, P. J. Chem. Educ. 1999, 76, 1122–1126. (d) Cannon, K. J. J. Chem. Educ. 1998, 75, 1259–1260. (e) Dods, R. F. J. Chem. Educ. 1996, 73, 225–228. (f) De Jesus, K. J. Chem. Educ. 1995, 72, 224–226. 4. Davis, M.; Henry, C. J. Chem. Educ. 2008, 85, 1088A. 5. Foote, C. J.; Vermette, P. J.; Battaglia, C. Constructivist Strategies: Meeting Standards and Engaging Adolescent Minds. Eye On Education, Inc. Larchmont, NY, 2001; pp 114-115. 6. Mid-continent Research for Education and Learning. http://www. mcrel.org/whelmers/whelm64.asp (accessed Feb 2010). 7. Anne Marie Helmenstine. About.com:Chemistry. http://chemistry. about.com/od/chemistrydemonstrations/ht/rainbowinaglass.htm (accessed Feb 2010).
Supporting Information Available Notes for the instructor; student handout. This material is available via the Internet at http://pubs.acs.org.
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