Demonstration pubs.acs.org/jchemeduc
A Comprehensive General Chemistry Demonstration Ryan D. Sweeder* and Kathleen A. Jeffery STEM Learning Laboratory, Lyman Briggs College, Michigan State University, East Lansing, Michigan 48895, United States S Supporting Information *
ABSTRACT: This article describes the use of a comprehensive demonstration suitable for a high school or first-year undergraduate introductory chemistry class. The demonstration involves placing a burning candle in a container adjacent to a beaker containing a basic solution with indicator. After adding a lid, the candle will extinguish and the produced carbon dioxide will react with the water to yield enough carbonic acid to neutralize the basic solution resulting in a change to the indicator. This demonstration includes aspects of most of the topics discussed in a typical general chemistry course. Two different methods are described to use this demonstration in a class: as a recurrent demonstration revisited regularly or as the basis for a cumulative oral final exam.
KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Demonstrations, Physical Chemistry, Inquiry-Based/Discovery Learning, Testing Assessment, Acids/Bases, Dyes/Pigments, Gases, Thermodynamics
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n the classroom, demonstrations present valuable opportunities for students to engage with chemical concepts and develop a link between abstract concepts and concrete applications. Demonstrations not only make the material personally relevant, but also serve as a springboard for student-initiated inquiry1 and a method by which instructors can model complex problem solving to support students’ learning.1,2 Therefore, it is unsurprising that this Journal contains an assortment of recently published demonstrations that can be utilized to enhance student learning about a variety of chemical principles.3−7 However, most demonstrations8 are utilized once to highlight a single chemical principle before they are returned to the shelf never to be seen again. In this article, we describe a new chemical demonstration that consists of a candle burning in a closed container next to a stirred, basic solution containing phenolphthalein. Despite its seeming simplicity (as a similar demonstration is recommended for middle schoolers9), this demonstration intentionally creates a complex system that contains nearly all of the major chemical concepts typically discussed in a general chemistry course. Students at all levels can be required to provide explanations with sufficient depth to be appropriate for their specific course. A comprehensive demonstration such as this is advantageous in the chemistry classroom because it can be revisited every several weeks, serving as a means by which students can connect new concepts to prior learning with a familiar concrete example.1 Because the demonstration contains familiar objects (such as a candle), students connect these seemingly disparate concepts of a general chemistry course to a real-world and familiar experience. © 2012 American Chemical Society and Division of Chemical Education, Inc.
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MATERIALS 0.001 M NaOH (M = 40.01 g/mol) solution Phenolphthalein indicator Votive candle Magnetic stirbar Large (0.5−1 gal) glass container with lid (Suitable glass jars are available from most online retailers selling kitchenware.)
PROCEDURE Approximately 200 mL of NaOH solution, a few drops of phenolphthalein indicator, and a magnetic stirbar were added to a 500 mL beaker. The beaker was placed inside the large container and magnetic stirring was begun at a medium speed. The candle was lit and placed inside the large container next to the beaker containing NaOH or a long lighter used to light the candle already inside the container. The lid was placed on the glass container (see Supporting Information for video of the demonstration) (Figure 1). After the demonstration was complete, the solution was recharged by the addition of approximately 0.5 mL of 0.1 M NaOH solution. However, recharging the solution many times created a buffered solution, which elongated the time between the candle extinguishing and the solution turning colorless.
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HAZARDS Phenolphthalein is a suspected carcinogen. It is still safe to use as an indicator in this demonstration if used prudently. Avoid exposure to skin. If it contacts skin or eyes, wash with copious Published: November 8, 2012 96
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the candle release heat during combustion?”).15 Both methods allow students to verbalize their understanding and allow the instructor to provide feedback. For example, when learning about heat and enthalpy, students can discuss the energy associated with the bond breaking and forming around the combustion and with a few assumptions even calculate the temperature change of the gas or the liquid due to the exothermic reaction. Similarly, when discussing intermolecular forces, the students can focus on the phase changes associated with wax, the condensation of the water, or the dissolving of the CO2 or O2 in the solution. Broadening the interpretation of what encompasses the demonstration, one can also consider the solids (the amorphous glass, molecular polymeric stirbar covering, or metal in the stirbar), the magnetic stirrer, and even the light and retinal that allows us to see the demonstration. (A more complete description of many of the applicable concepts can be found in the Supporting Information.) The demonstration provides clear macroscopic observations that can be linked to the underlying microscopic principles. The students can also use symbolic representations of chemistry to fully round out their explanations. Students' learning can be further enhanced by using this opportunity to create a concept map16 that shows how each of the concepts learned are reflected in the demonstration and how they are linked together. Because of the comprehensive nature of the demonstration, its usage in a classroom could take many forms. Some possible examples include (i) having the students make a short video explaining the process to a 11 year old similar to the “Flame Challenge”;17 (ii) having students complete a written test question or 1-min assignment during each unit based on the demonstration;18 (iii) having student groups prepare short videos or presentations explaining the system as a final project or having each group focusing on a specific aspect; and (iv) having students jigsaw19 to talk about the individual concepts in the demonstration at the end of the year as prep for a final.
Figure 1. Schematic of the setup for the demonstration.
amounts of water. The sodium hydroxide solution should be handled with care and appropriate eye protection and gloves are recommended. The use of a match and burning candle necessitate the presence of a fire extinguisher close at hand.
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DISCUSSION Students often have difficulty understanding how different concepts described in a typical general chemistry class link together.10,11 Although they learn and may be proficient at applying individual ideas presented in a given class, rarely do they get the opportunity to explicitly observe the interplay that exists between many concepts. Thus, it is advantageous to have a demonstration that can highlight the interconnected nature of many concepts in a concrete, real-world example. This demonstration involves a burning candle in a sealed system that consumes oxygen until the rate of the reaction is too slow to sustain the combustion12 and the candle goes out. The carbon dioxide diffuses from the flame to the solution where it dissolves in an NaOH solution creating carbonic acid that neutralizes the strong base. The phenolphthalein indicator shows the change by losing its characteristic pink color and turning colorless in the less basic solution. Additionally, the production of products from the combustion reaction is illustrated by the water condensing on the inside of the container. This demonstration allows students to address the three primary levels of chemistry representation: macroscopic, microscopic, and symbolic (including drawings, equations, and mathematical expressions).13,14 For example, when discussing the melting of the wax, the students might point out that the heat melts the wax (macroscopic). They may further explain that the energy acts to break a portion of the London dispersion forces holding the molecules together (microscopic) as they illustrate, via a drawing, the difference between the solid and liquid states (symbolic). Two methods are described to effectively use this demonstration: as a course focal point and as the basis for an oral final exam.
Oral Final Exam
Oral final exams have been shown to be an excellent learning exercise,20,21 leading to better learning22 and better attitudes.23 Because of the comprehensive nature of this demonstration, it provides a suitable substrate on which to base an oral final exam. During the spring 2012 semester, this demonstration was used as the basis for an oral final exam for each of the 66 students enrolled in the general chemistry course. The students were given up to 45 min to talk about the chemistry within the demonstration. They were instructed that they had to address the three primary levels at which we consider chemistry: the macroscopic, microscopic, and symbolic. The students also randomly selected two content areas of chemistry that they had to explicitly address (by drawing cards). These content areas included acid−base, thermodynamics and kinetics, equilibrium, intermolecular forces, gases, energy, and light. The students were presented with an online video showing the demonstration approximately halfway through the semester and given the expectations (see Supporting Information for the video). The video remained available to the students via an online classroom management system. The students were required to undertake a “mock final” using the actual demonstration setup with one of our undergraduate learning assistants and most undertook an optional second practice session. These sessions were designed to provide formative feedback to the students about their understanding and give them practice verbalizing about chemical concepts. The students also were provided with
Recurrent Demonstration
The demonstration can act as a focal point for a class to apply their newly learned knowledge. The instructor can bring out the demonstration on a regular basis (approximately every other week) and give the students an opportunity to discuss how recent course content is reflected within the demonstration. Alternatively, the demonstration can be used to elicit prior student understanding of a chemical concept (“why does 97
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are present in the demonstration. This material is available via the Internet at http://pubs.acs.org.
a list of general chemistry terms, and a recitation session (50 min) was spent with the students working on concept maps showing how the concepts linked together around the demonstration. Independently, the students also convened their own study session to discuss the demonstration, which was attended by approximately half the class. The oral final and the preparation for it was a powerful tool in helping the students develop a greater understanding of all the different chemical principles that they had discussed throughout the year. On an end of the semester evaluation, 90% of the students indicated preparation for the oral final was either a “great help” or “much help” with the other 10% indicating that it was “moderate help” or “little help” in contributing to their learning. Similarly, open-ended comments included statements such as Studying for the final oral exam was when everything started to come together for me. When first looking at the demonstration, I thought it would be impossible to talk for 30 min about that. After looking over the list of chemistry terms and applying all of my chemistry knowledge to the demonstration, I was able to talk for around 40 min. When making the web diagrams to prepare for this exam, we were able to see how everything connects. By trying to connect these concepts to each other, we realized how much we learned throughout the year. These results show the important role that demonstrations1 and understanding application11 play in students developing a deep and useful grasp of chemical concepts. Alternatively, the demonstration could be used as an optional oral final exam in which the potential results are a 100% score or a nonpass with eligibility to retake the oral final or to take the standard written final.22 If the final is used in this manner, the instructor can more quickly move the discussion to more advanced questions by asking probing questions about aspects most likely to cause confusion. The instructor also can pose hypothetical questions, such as “Assuming the lid sealed perfectly, how do you think the pressure would compare now after the solution has changed color to when the lid was first placed on the system?” or “Explain how the demonstration might act differently if we placed a layer of oil atop the basic solution? Why?” Through this process, the instructor can better determine if the students are learning “how to think like a scientist”.24 The student practice sessions provide an important step in the preparation process. In settings where learning assistants are not available, an alternate system should be in place to support students. One option would be to utilize peer evaluation of practice trials, although this method would require explicit instruction for the peer evaluators about the level of depth expected, as students tend to focus and evaluate answers nearly exclusively on the macroscopic level. Alternatively, the instructor can provide a small number of public practice sessions in front of the entire class to help clarify the level of depth expected. A third system could involve the instructor providing taped sample student answers with commentary clearly explaining the appropriateness and accuracy of the answer provided.
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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 We would like to thank our many students involved in the class for sharing their experiences regarding the oral final and Doug Luckie for inspiration in developing a comprehensive demonstration. This material is based upon work supported by the National Science Foundation under Grant Nos. DUE1022754 and DUE-0849911.
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ASSOCIATED CONTENT
S Supporting Information *
A video of the demonstration, diagram of the demonstration setup, sample grading sheet for the oral final exam, and a brief description of how many standard general chemistry concepts 98
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