In the Classroom
A First-Class-Meeting Exercise for General Chemistry: Introduction to Chemistry through an Experimental Tour LaRhee L. Henderson* and Gholam A. Mirafzal Chemistry Department, Drake University, Des Moines, IA 50311; *
[email protected] We have developed an approach to the first day of general chemistry class that utilizes principles of constructivism (1, 2), collaborative and cooperative groups (3–8), and studentcentered learning (9). A review of current literature suggests that a number of educators are developing case-study-type materials and have found success with their use (10–17 ). Accordingly, we have written activities that consist of a series of short experiments set within the context of a case study, which we call experimental tours. The case-study setting helps students build connections between chemistry concepts and everyday issues. The experiments allow students to learn chemistry by doing chemistry. You will notice that the experiments are comparable to those often performed by instructors with the students as observers. Our approach differs in that the students themselves perform the experiments. Their hands-on participation encourages them to become actively engaged in the process of scientific discovery. Further, the experiments are placed in a context by use of the case study and the students’ observations are focused through guided questions. Although we typically use a laboratory space for the experimental tour, use of appropriate precautions allows this activity to be conducted in other spaces, as well. For example, we have conducted this activity in a large lecture auditorium by spreading experiments over the front demonstration bench. In another instance, we used a well-ventilated smaller lecture room that was furnished with movable tables and chairs. Student groups conduct the experiments and answer guided questions as they proceed. We have found that groups of about four students are optimal. It has also been our experience that a typical 50-minute class period is sufficient for a brief introduction, group assignments to their experiments, and then the experiments themselves. A subsequent class period is used for presentations and discussion. The experiments in this tour guide the students to discover principles related to states of matter, classification of matter, density and unit conversion problems, and chemical and physical properties and changes. A copy of the tour follows. An Experimental Tour
The Case Susan was cleaning out her kitchen cabinets when she found several interesting samples. •
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Under her sink, in the cleaning supplies, a bottle of household ammonia had been stored near a drain cleaner whose cap had not been appropriately tightened. The outside surfaces of several containers in their vicinity had become coated with a white powdery substance. She was afraid to touch it, so she put on gloves and discarded the containers and their contents. In her pantry, she found a box with unknown contents. Her children had been playing “house” and had
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destroyed the label sufficiently to make it unreadable. It contained a white crystalline powder that was odorless. She knew it was either powdered sugar or corn starch. Not being certain, she discarded it, as well. In her laundry cabinet, she found an old shirt that she then washed only to find that an ink pen had been in its pocket. Though the pen had black ink, the shirt pocket now had several colors of stain.
After working for a few hours, Susan decided to relax with a can of soda. She usually drank diet soda, but she only had regular soda in stock so she settled for it instead. While reflecting on her day’s activities, she pondered whether she should be storing all the cleaning agents together under the sink. Where had that white powder come from? Thinking of white powders, she considered the unlabeled box. Her imagination started churning out scenarios whereby its content was neither sugar nor corn starch, but some potentially toxic substance! It scared her to think that her children had been playing with it and she really wasn’t 100% certain that it was safe. She wondered how she could have deciphered its contents. She wondered how the shirt had become stained with so many colors when only black ink had been in the pen. Were there other pens in the pocket that got lost in the machine and would show up on the next wash day? She wondered about diet and regular soda, how much sugar was she drinking in that 12-oz. can? Is the sugar the reason that her mind was now racing with all these questions? She decided her “rest” period had not turned out to be that restful, and went back to work, resolved to be more careful about storage of household materials and access to them.
Experiments and Questions (Be sure to wear your safety glasses and follow the guidelines for caution at each experimental station.) Experiment 1 Describe everything you can about following samples. I. Experimental. Use samples (a small quantity for each test) of sugar, corn starch, benzoic acid, and carbon to conduct the following: 1. Examine each for its physical properties. 2. Test the solubility of each in water and in hexane. 3. Burn the sugar in a crucible over a Bunsen burner and describe the product(s). 4. Put a few drops of concentrated sulfuric acid on each and describe the results.
II. Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of these experiments to illustrate the following: • • • •
Mixtures, compounds, and elements Heterogeneous and homogenous mixtures Solutions Chemical and physical changes
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In the Classroom
III. Susan found a box with unknown contents in her pantry, which she knew was either powdered sugar or corn starch. Investigate the chemical and physical properties of sugar and corn starch. Suggest how Susan might have differentiated between the two. Experiment 2 I. Experimental 1. Put cotton swabs on each end of a glass tube (18–24 inches long). 2. Carefully, under the hood, wet one with concentrated ammonium hydroxide and one with concentrated hydrochloric acid. 3. Cover each end with a rubber septum. 4. Observe (it will take a few minutes before you see a change).
II. Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of your experiment to illustrate: 1. Molecules are always in motion (and the amount of motion varies with state of matter). 2. NH3(g) + HCl(g) → NH4Cl(s) 3. States of matter 4. A chemical reaction is a rearrangement of atoms. 5. Conservation laws
III. Susan observed that in her cleaning supplies, a bottle of household ammonia had been stored near a drain cleaner whose cap had not been appropriately tightened. The outside surfaces of several containers in their vicinity had become coated with a white powdery substance. Investigate possible chemicals in drain cleaners and suggest an explanation for Susan’s observations. Experiment 3 I. Experimental. Use a can of diet soda and regular soda in the following experiment. 1. Find volume of a can of soda: measure radius and height of a can and use the formula (vol = 3.14 r 2h) to calculate its volume (note that 1 cm3 = 1 cc = 1 mL) 2. Weigh each can. 3. Find the density (in g/mL) of each can using the formula D = mass/volume. 4. Compare the densities of each to that of water: a. Put each can in tub of water and observe. b. What does this suggest about densities and floating (Density of water = 1 g/mL)? 5. Determine the sugar and Calorie content: Assuming that the difference in masses of the two cans is due to sugar content, determine the mass of sugar in the can of regular soda. If sugar contains about 4 Calories/ gram, use your calculations to determine the number of Calories Susan ingested in the can of regular soda. Compare your calculated Calorie value to that listed on the nutritional information on the can’s label. 6. Units of measurement: a. How many cups are in a can of soda? (30 mL = 1 fluid oz., 1 c = 8 fluid oz.) b. How many quarts are in a can of soda? (1 qt = 4 c) c. How many liters are in a can of soda? (1000 mL = 1 L)
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II. Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of your experiment to illustrate: 1. Calculations of volume are related to distance measurements. 2. Use of dimensional analysis helps you solve many chemistry problems. 3. Density is related to mass and volume. 4. Compare conversions when using the metric system with those using the English system of measurements.
III. Susan relaxed with a can of soda. Investigate the terms heat, temperature, calories and Calories. Investigate artificial sweeteners in terms of their chemical constitution and Calorie content. Utilize these in your discussion of Susan’s choice of beverage. Experiment 4 I. Experimental 1. Obtain two TLC strips (e.g., Eastman Kodak TLC silica-coated film) that measure about 2.5 × 10 cm each. 2. Spot water-soluble* inks on a pencil line about 1 cm from the bottom of two strips. The spots should be small, about 2 mm in diameter. 3. Set the TLC strips into a medium-sized beaker (approximately 200 mL) containing about 10 mL of solvent,** making certain that the sampling line is above the level of the liquid solvent. 4. Cover the beaker with aluminum foil and observe changes on the TLC plate. 5. When the solvent has ascended to within about 1.0 cm of the top of the plate, remove the plate from the beaker and mark the upper limit of the solvent front with a pencil. Allow the plate to dry and then circle each of the spots. 6. Calculate the R f value for each spot. A sample is shown below to illustrate the measurements. R f = sample migration distance/solvent migration distance
Solvent front
Spot a hsolv ha Spot b hb
Sampling line
Rf (a) =
ha (mm) hsolv (mm)
Rf (b) =
hb (mm) hsolv (mm)
7. Compare R f values of comparable spots. *Water soluble inks from Pilot Razor point pens or Flair ultra fine pens work well. **A solvent that works well is a mixture of 5:5:5:2 parts 1butanol, ethyl acetate, absolute ethanol, and water, by volume.
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
In the Classroom
II. Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of your experiment to: 1. Differentiate between solutions, mixtures and compounds 2. Differentiate between precision and accuracy
III. When Susan washed the shirt with a black ink pen, she produced a stain with several colors. Suggest an explanation why the stain apparently changed from black to multicolored. After completion of the tour, and during the next class period, groups are asked to present their observations to the rest of the class, leading to class discussion of the basic concepts related to the experiments.
vations, and their conclusions. Thus they develop oral communication skills and become not only learners, but teachers as well. Further, they are introduced to college as a community of learners where students participate actively in the class and the instructor acts as a guide rather than the sole authority in the course, exemplifying student-centered learning (9). We have periodically compared performance on conceptstyle questions by students in course sections taught largely by the traditional lecture style with that of students in sections that incorporate experiential-style approaches utilizing activities as described above. It has been our experience that students in experiential sections earn scores about 30 percentage points higher on this type of question. Literature Cited
Results Through this exercise, students build chemistry concepts onto their knowledge of everyday processes, exemplifying the constructivist approach (1, 2). They work in small groups through these exercises. This allows students with different majors and backgrounds to contribute to the discussion, enriching the constructivism by bringing a greater breadth of backgrounds onto which the new knowledge can be built. Such exercises also acquaint the students with collaborative– cooperative work (3–8) and reinforce the propriety of respecting the authority of peers. Accordingly, this exercise exemplifies collaborative–cooperative work and student-centered learning (9), which we incorporate throughout the rest of the course. Following the small-group exercises, groups instruct the rest of the class on their results and conclusions. This aids development of communication skills. Practicing scientists need to be able to communicate their thoughts and results with other scientists and their communities. Such exercises as this require the students to articulate their work, their obser-
1. Dittmer, A.; Fischetti, J. K.; Wells, D. Equity Excellence Ed. 1993, 26, 40–45. 2. Louden, W.; Wallace J. Int. J. Sci. Ed. 1994, 16, 649–651. 3. Bosworth, K. New Directions Teach. Learn. 1994, 59, 25–30. 4. Bruffee, K. Change 1995, 27(1), 12–18. 5. Gokhale, A. A. J. Technol. Ed. 1995, 7, 22–27. 6. Felder, R. M. J. Chem. Educ. 1996, 73, 832. 7. Kogut, L. S. J. Chem. Educ. 1997, 74, 720. 8. Steiner, R. J. Chem. Educ. 1980, 57, 433. 9. Barr, R. B; Tagg, J. Change 1995, 27 (6 ), 13–25. 10. Smith, R. A.; Murphy, S. K. Am. Biol. Teach. 1998, 60, 265. 11. Cheng, V. K. W. J. Chem. Educ. 1995, 72, 525. 12. Brink, C. P.; Goodney, D. E.; Silverstein, T. P. J. Chem. Educ. 1995, 72, 528. 13. Anthony, S.; Mernitz, H.; Molinaro, M. J. Chem. Educ. 1998, 75, 322. 14. Swan, J. A.; Spiro, T. G. J. Chem. Educ. 1995, 72, 967. 15. Schwartz, A. T.; Bunce, D. M.; Silberman, R. G.; Stanitski, C.; Stratton, W. J.; Zipp, A. P. J. Chem. Educ. 1994, 71, 1041. 16. Nakhleh, M. B. J. Chem. Educ. 1994, 71, 495. 17. Mahaffy, P. G. J. Chem. Educ. 1992, 69, 52.
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