Laboratory Experiment pubs.acs.org/jchemeduc
Thermometry as a Teaching Tool for Graphing: A First-Day Introductory Chemistry Laboratory Experiment Lea W. Padgett and Catherine E. MacGowan* Department of Chemistry and Physics, Armstrong Atlantic State University, Savannah, Georgia 31419, United States S Supporting Information *
ABSTRACT: Preparing a meaningful laboratory exercise for introductory or general chemistry students to perform, especially on the first day of class, is challenging. Introductory or general chemistry students range from being skilled in laboratory techniques and equipment usage to never having been in a laboratory setting. Subject content can be taught in the laboratory in advance of an associated lecture, but this too is not always popular or practicable. A laboratory exercise is presented that can be taught at the very beginning of the semester. The primary focus is on graphing to investigate the thermal expansion of a liquid, but students can be instructed on recording data precisely from a measuring device, be introduced to the basic physical properties of liquids, practice using a spreadsheet software program for data analysis, and learn good laboratory note-taking. The experiment is appropriate for a first-semester general chemistry course, physical science, or high-school chemistry.
KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulative, Problem-Solving/Decision Making, Physical Properties, Thermodynamics
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of the enclosed liquid as its temperature changes. Students may have common knowledge on the use of liquid-filled thermometers but seldom understand the concepts behind how a thermometer works. However, they may be more familiar with the property of thermal expansivity as it relates to tire overinflation and the subsequent blowouts that can occur as tires warm up. The volumetric thermal expansion coefficient of a material, α (eq 1), can be experimentally obtained by dividing the change in volume (ΔV) by the change in temperature (ΔT) at constant pressure. Students graph their observed changes in volume (ΔV) versus changes in temperature (ΔT) to obtain a trend line where the y intercept can be set to zero. The slope of this line will be the thermal expansion coefficient multiplied by the volume of the flask, ve.
raphing is a fundamental method that integrates mathematical skills with science content to illustrate conceptual relationships.1−6 To use graphical data correctly, students must utilize cognitive memory (recall or concrete learning), convergent thinking (application and analysis), and evaluative thinking (synthesis and evaluation). Graphing not only employs and fosters higher-order thinking skills, it also helps students develop problem-solving strategies.5−13 Students enrolled in an introductory science course often do not possess a sound understanding of the fundamentals of graph construction and have difficulty with the interpretation of graphical data.8,10−15 As a consequence, laboratory activities requiring the use of graphing techniques for data analysis can lead to poor student comprehension of the concepts and principles being investigated. Evaluations of past laboratory reports, laboratory observation, and additional data collected from the administration of the ACS Toledo exam indicated that many of our students display this weakness and associated level of frustration. To address these concerns a hands-on, first-day laboratory experiment was designed that (i) introduces students to the fundamentals of graphing and (ii) allows us to assess early their graphing and laboratory skills.4−6,16,17 The laboratory experiment is relatively simple to perform, is broadbased in content, and uses minimal equipment.
ΔV = νeαΔT
For a general chemistry laboratory the focus can be placed on observation of the general trend and comparing the effects of two or more different liquids to demonstrate that one is more sensitive to changes in temperature and would therefore make a better liquid for use in a thermometer.
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EXPERIMENTAL PROCEDURE Immediately before the experiment, a 30−45 min prelab session is used to discuss the scientific principles of how a liquid-filled thermometer works, to review the basics of graphing theory, to demonstrate the proper reading and recording of glassware precision, and to instruct on how to
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BACKGROUND AND EXPERIMENTAL OVERVIEW Students are exposed to graphing by investigating the physical property of thermal expansivity, employed in thermometers, which is the tendency of materials to increase in volume as temperature increases.18,19 Thermometers indicate temperature on a graduated scale that is based on the predictable expansion © XXXX American Chemical Society and Division of Chemical Education, Inc.
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and temperature are recorded. This procedure is repeated an additional three times for a total of five data points.
set up a laboratory notebook. In the lab, the students work in pairs to take measurements for one liquid. The data for the different liquids are shared among all pairs of students. The data collection requires 1.5 h of lab time. If time remains after the experiment has been completed, students can go back to the prelab classroom to work on graphing the data. This experiment requires a modest amount of equipment (Figure 1). A 25 mL Erlenmeyer flask equipped with a small stir
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HAZARDS Eye protection should be worn at all times while in the laboratory and care taken when handling laboratory glassware and disposal of chemicals. The chemicals used in this experiment are common household and store bought items (water, acetone, 95% ethanol or isopropyl alcohol). Ethanol is flammable and harmful if inhaled or absorbed through skin. Acetone is highly flammable and irritating to the eyes, repeated exposure may cause skin cracking or dryness, and vapors may cause drowsiness and dizziness. Flammability of organic solvents is not an issue in this laboratory because hot plates are not used to heat water as the hot water came from the tap. For this laboratory exercise, the proper technique of inserting a pipet into the rubber stopper was demonstrated. A small dab of grease can help facilitate the insertion of the pipet into the rubber stopper.
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EXPERIMENTAL RESULTS For data analysis, the lowest temperature reading and its corresponding volume measurement are designated TL and VL, respectively. For graphing purposes, ΔT and ΔV need to be computed. The ΔT values are calculated by subtracting TL from each of the remaining recorded temperature values (e.g., T − TL = ΔT), and ΔV values are computed by subtracting VL from each of the recorded volume values (e.g., V − VL = ΔV). Sample data from the students are given in Table 1. A graph of Table 1. Student-Generated Experimental Data for Temperature and Associated Volumes of Tested Liquids Figure 1. Experimental setup. Red food dye was added to better show the liquid in flask.
bar is filled to its brim with the liquid to be investigated. To enhance the readability of the liquid in the disposable pipet, a drop of food coloring can be added. A 1 mL disposable serological pipet (Sterlin) is inserted into a number 0 rubber stopper far enough to just protrude out from the other side so that as much of the scale is made visible as possible. Glass 1 mL pipets can be used and are required when using acetone as a possible test liquid, but for a lower safety risk, disposable plastic pipets are preferred. The stopper−pipet assembly is inserted into the Erlenmeyer flask such that the test liquid is forced approximately two-thirds of the way up the pipet. The Erlenmeyer is placed into a 600 mL beaker that has been filled with enough warm tap water to cover the flask, stopper, and part of the pipet (Figure 1). A thermometer or temperature probe is placed into the beaker to record the temperature of the water bath. As a timesaver and for safety issues regarding the use of flammable liquids in this experiment, students fill the water bath beaker with hot or warm water from the faucet rather than heat the water on a hot plate. With the stir plate on, 5 min is allowed to pass before taking any readings to allow for temperature equilibration. The initial pipet volume and its associated temperature are recorded. To lower the temperature of the water bath, small quantities of ice are added to the beaker and, using the thermometer, stirred until melted. Once the volume in the pipet has stopped changing (approximately 5 min after ice addition), the volume
Liquid
Temperature/°C
Volume/mL
ΔT/°C
ΔV/mL
Ethanol
25.9 23.2 20.5 18.3 15.5 40.5 36.9 33.0 28.2 24.5 19.0 17.2 15.8 13.9 12.0
0.996 0.905 0.803 0.698 0.599 1.003 0.949 0.882 0.820 0.761 0.998 0.902 0.801 0.698 0.600
10.4 7.7 5.0 2.8 0.0 16.0 12.4 8.5 3.7 0.0 7.0 5.2 3.8 1.9 0.0
0.397 0.306 0.204 0.099 0.000 0.242 0.188 0.121 0.059 0.000 0.398 0.302 0.201 0.098 0.000
Water
Acetone
ΔT versus ΔV is plotted with a trend line in Figure 2. The thermal expansivity coefficient (α) can also be determined by dividing the slope of the trend line by the volume of the flask (ve), which is determined by obtaining the mass of the flask empty, filling the 25 mL flask to the brim with water, and then obtaining a new mass. Density is used to convert the mass of water to volume.
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DISCUSSION The primary focus of this experiment was to introduce and review graphing techniques using experimental data. Using their graphical data, students were asked to evaluate the effectiveness of a liquid as a possible choice for a liquid-filled thermometer. B
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In this exercise an experimentally collected and calculated (0,0) data set is plotted; therefore this data manipulation arranges for the trend line to pass through zero. During the prelab session, the difference between manipulated data (e.g., ΔV and ΔT) and raw experimental data points is discussed. Because the data used to create the graph has a manipulated (0,0) data set, having the software program force the trend line through (0,0) would be creating a set of imaginary data points, which is unethical. This presents an instructional opportunity to discuss the ethical use of data as well as experimental error mathematically.
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CONCLUSION A simple, inexpensive, guided laboratory experiment for use at the beginning of a general chemistry or physical science sequence is described. This experiment could easily be adapted into a guided-inquiry format depending on the knowledge level and laboratory skills of the students. The focus of this exercise was on graphing experimental data and its interpretation as it relates to the thermal expansion of a liquid. The exercise also provides the opportunity for students to be instructed on obtaining the proper precision from a measuring device such as the pipet, learning and using basic spreadsheet functions of a software program, and taking good laboratory notes. The exercise is broad-based in content knowledge requirements thus meeting our needs to introduce students to the use of basic laboratory equipment and techniques within the first days of class. We believe that any exercise that requires graphing and interpretation of data will strengthen student-graphing skills. More importantly it gets students into the laboratory setting and familiarizes them with the equipment and laboratory techniques.
Figure 2. Change in liquid volume with varying temperature for three liquids.
To accomplish this task, a 25 mL Erlenmeyer flask of the tested liquid is fitted with a 1.0 mL serological pipet−stopper assembly to imitate the bulb and long glass capillary found in a thermometer. Several different sized flasks were evaluated, but the 25 mL Erlenmeyer flask gave the best results, combining a small size with enough volume of liquid to obtain an easily observable change. A small volume of the test liquid is required to facilitate good heat transfer from the water bath to the liquid inside of the flask. The temperature of the water bath was lowered rather than increased for simplicity: it is easier to control temperature changes by cooling with ice chips than by heating the water bath using a hot plate slowly to allow good heat exchange. Lowering the temperature of the water bath with the addition of ice chips results in the test liquid inside the Erlenmeyer flask−pipet assembly cooling and contracting, thereby causing the tested liquid’s level in the pipet to lower. Thermal expansivity, the property that is being investigated, is linear in nature; therefore a specific starting temperature is not a factor in this experiment. Also, because a liquid’s thermal expansion is temperature dependent, the use of hot tap water for the bath keeps the collected data within the scope of experiment. As the thermal expansion of a liquid changes with changing temperature, it is necessary to keep the experimental conditions to a relatively narrow range of temperatures. Using hot tap water as an upper boundary temperature and ice water as a lower boundary temperature returns data that have a constant slope. Better data can be obtained when students are careful to ensure the pipet volumes have stabilized (e.g., approximately 5 min between temperature readings). The basics of graphing theory were discussed, and tutorial handouts on how to enter data and make a graph using a spreadsheet program (e.g., Microsoft Excel) were made available. By plotting two or more of the test liquids on the same graph, it is easier for students to observe from the steepness of the slopes that not all materials exhibit the same thermal expansion properties. When students opted to plot each liquid separately, they had difficulty relating the slope values to determine which liquid would be most sensitive in a thermometer. This problem likely illustrates a lack of understanding graphing basics. In addition, many of our students had difficulty using the spreadsheet program to graph multiple series on a single plot, which was most likely due to their unfamiliarity with the software.
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ASSOCIATED CONTENT
S Supporting Information *
Student laboratory experiment; instructor preparation notes and information. 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 The authors gratefully acknowledge the students in their General Chemistry I laboratory courses for conducting the laboratory experiment and their peers for providing critical feedback of the laboratory exercise and procedure.
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REFERENCES
(1) Trinajstic, N. In Mathematics and chemistry: The unlikely partner in concepts in chemistry.A contemporary challenge; Rouvary, D. H., Ed.; John Wiley & Sons: New York, 1996; Chapter 2. (2) Vasquez-Mireles, S.; West, S. Sci. Teach. 2007, 74, 47. (3) Graham, D. M. J. Chem. Educ. 1996, 73, 211. (4) Connery, K. F. Sci. Teach. 2007, 74, 42. (5) Bowen, G. M.; Wolf-Michael, R. Res. Sci. Educ. 1998, 28, 77. (6) Cortes-Figueroa, J. E.; Moore-Russo, D. A. J. Chem. Educ. 2004, 81, 69. (7) Domin, D. S. J. Chem. Educ. 1999, 76, 109. C
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(8) Puskin, D. B. J. Chem. Educ. 1998, 75, 809. (9) Riendeau, D. Sci. Teach. 2007, 74, 52. (10) Berg, C. A.; Philips, D. G. J. Sci. Res. Teach. 1994, 31, 323. (11) Berg, C. A.; Philips, D. G. Sci. Educ. 1994, 78, 527. (12) Hanson, R. M.; Riley, P.; Schwinefus, J.; Fischer, P. J. J. Chem. Educ. 2008, 85, 1142. (13) Prilliman, S. J. Chem. Educ. 2012, 89, 1305. (14) Bopegedera, A. M. R. P. J. Chem. Educ. 2011, 88, 443. (15) Black, S. L. J. Chem. Educ. 1996, 73, 776. (16) Henderson, L. L; Mairafzal, G. A. J. Chem. Educ. 1999, 76, 1221. (17) Bent, H. A. J. Chem. Educ. 1986, 63, 890. (18) Barnett, M. K. J. Chem. Educ. 1941, 18, 358. (19) Jasien, P. G.; Oberem, G. E. J. Chem. Educ. 2002, 79, 889.
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