Estudiante

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 *angelin@kth.se

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 cleartransition 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 protectivegear 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 arechallenged 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 earlierpublications 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 mixturespublished 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 fromviolet 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 andother 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).
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Figure 1. Schematic of making a chemical rainbow in a test tube (left) and photo of the test tubecontaining 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 (twosolutions 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...