In the Laboratory
Like Dissolves Like: A Classroom Demonstration and a Guided-Inquiry Experiment for Organic Chemistry
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Ingrid Montes,* Chunqiu Lai, and David Sanabria Department of Chemistry, University of Puerto Rico, Río Piedras Campus, P.O. Box 23346, Río Piedras, PR 00931-3346; *
[email protected] Many students have difficulty understanding and applying concepts and principles that are based on abstract knowledge. To overcome this problem, new learning strategies have been proposed to help students visualize in a concrete way the empirical basis of these concepts (1–11). A commonly used method to achieve this goal is the guided-inquiry approach. This approach provides students with the opportunity to conduct experiments aimed at solving a specific problem, analyzing the results, and reaching their own conclusions based on empirical data. Although many guided-inquiry laboratory experiences designed for developing laboratory skills have been proposed, they have not dealt exclusively with the concept of “like dissolves like”. The solubility concept, one of the most fundamental principles for the organic chemist, is usually introduced when describing solvent selection during a recrystallization process (12), although it is presented as an experiment in the Pavia text (13). This principle, which all students are taught but most fail to comprehend, is essential for understanding separation techniques and other solvent-dependent processes such as reaction-solvent selection, discussed throughout the organic chemistry course and laboratory sessions. To ensure student comprehension of the factors that determine the solubility of organic compounds, we have designed a classroom demonstration, as well as a guided-inquiry experience, based on the aforementioned principle. The experiment is intended to be the initial laboratory experience for students enrolled in an organic chemistry course. Classroom Demonstration The instructor could choose to cover this topic either in the lecture or as a short, prelaboratory demonstration (choosing only some of the alcohols) to introduce the concept to the students. This demonstration should follow a discussion on dipole moments, polarity of molecules, and intermolecular forces. Its purpose is to allow students to visualize the interactions that result from mixing miscible and immiscible liquids and to allow them to reach conclusions regarding the effect of functional groups and structural properties of organic compounds on their solubility in different solvents. The demonstration requires an overhead projector and screen, Petri dishes, droppers, water, and methylene bluetinted alcohols (such as methanol, ethanol, n- and tert-butanol, and others) tinted with a 0.01% solution of methylene blue, weight-to-volume. The instructor also has the option of conducting the demonstration using one of the alcohols with solvents other than water, such as acetone or hexane. For the demonstration, a Petri dish is placed on the overhead projector and four drops of water are added to the same spot inside the dish. Four drops of the methylene blue-tinted
alcohol are then placed near the water, taking care that no mixing occurs between the two liquids. Using a toothpick and a back-and-forth linear motion, one liquid is moved into the other, allowing them to interact. In this demonstration, the methylene blue dye is used to enhance the observation process. For liquids that are miscible, the blue tint promptly disperses throughout both liquids upon mixing; however, for liquids that are immiscible, the dye can only be observed in part of the mixture, usually in the form of blue globules. The instructor can show an example of a “positive” solubility test by an initial test in which both of the puddles are water. For this test, one of the puddles would contain methylene blue and the other would not, and they would be mixed accordingly. Students should be asked to observe the projected image in order to determine whether the liquids dissolved in each other or not. During this interactive demonstration the instructor can guide the discussion by asking students to compare the solubility behavior of alcohols with different numbers of carbons or alcohols with the same number of carbons but different structural arrangements in a given solvent. The instructor could also inquire about the solubility of a given alcohol with respect to different solvents. Guided-Inquiry Laboratory Experience The solubility principle “like dissolves like” can also be taught through a guided-inquiry laboratory experience. This experience requires the students to conduct an experimental procedure aimed at determining the effects of the number of carbons, the structural arrangement, and the presence of different functional groups on the solubility of organic compounds in polar and nonpolar solvents. Students reach their own conclusions based on the experimental results.
Procedure For the experiment, students work in pairs. Test tubes and small dropper-capped bottles containing a series of solvents and organic compounds (Table I) are distributed to each group. After the group discusses and considers the variables to be studied, each group performs an experimental procedure to determine the effects of increasing the number of carbons, structure branching, or (if desired) changing the nature of the functional group on the solubility of organic compounds in various solvents. For this experiment, students record their results in a table listing the compounds to be tested versus observed solubility in different solvents. For each trial, ten drops of solvent are placed in the tube. Then the organic compound is added dropwise to the tube, stirring carefully after each addition, until a total of five drops have been added. The students should record all observations in the table. The presence
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In the Laboratory Table 1. Solubility Results of Mixing Organic Compounds in Various Solvents Solvent (10 drops)
Organic Compound (1–5 drops)
Water
CH3OH
soluble
Hexane
Acetone
1–2 drops: soluble
soluble
3 drops: less soluble 5 drops: insoluble CH3CH2OH
soluble
soluble
soluble
CH3CH2CH2OH
soluble
soluble
soluble
CH3CH(OH)CH3
soluble
soluble
soluble
CH3CH2CH2CH2OH
1–2 drops: soluble
soluble
soluble
3 drops: less soluble 4 drops: insoluble CH3CH2CH(OH)CH3
insoluble
soluble
soluble
(CH3)3COH
soluble
soluble
soluble
CH3CH2CH2CH2CH2OH
insoluble
soluble
soluble
(CH3)2CHCH2CH2OH
insoluble
soluble
soluble
CH3CH2CH(OH)CH2CH3
1 drop: soluble
soluble
soluble
CH3CH2C(CH3)2OH
less than 3 drops: soluble
soluble
soluble
2 drops: less soluble more than 4 drops: less soluble ethylene glycol
soluble
insoluble
less soluble
sucrose
soluble
insoluble
insoluble
of turbidity or two phases indicates that the compound is not soluble.
Hazards The alcohols are flammable liquids. Students must review the Material Safety Data Sheet (MSDS) for each reagent before starting the experiment. Before proceeding with the experiment, general instructions about handling hazardous wastes are given, and students are advised to use all chemicals with caution. Appropriate eye protection must be worn. In addition, students are instructed not to leave any unknown material in the waste disposal area.
results demonstrate that acetone is intermediate in its solubility properties between water and hexane. The students also observe that sucrose is completely soluble in water, yet insoluble in hexane or acetone, results that contrast with the trend observed for the series of alcohols. Based on these results and observations the students are asked, as a group, to orally define the rules of solubility of organic compounds in polar and nonpolar solvents, and to explain the meaning of the “like dissolves like” principle.
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What trend can you find in the solubility of alcohols when the number of carbons is increased? in water in acetone in hexane
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What trend can you find in water solubility when the studied alcohols contain branched carbon chains? Considering the differences in structure, discuss the results obtained with: the C4H10O isomers: CH3CH2CH2CH2OH, CH3CH2CH(OH)CH3, (CH3)3COH in water, acetone, or hexane.
Discussion Students should be encouraged to analyze and discuss their results among the groups in order to increase their comprehension of the chemical concepts involved in the experiment. This is achieved by an oral group discussion answering the questions that appear in Figure 1. The students discover that the longer the carbon chain in the alcohol, the more soluble it is in a nonpolar organic solvent like hexane, and the less soluble it is in water. Branching, however, increases the solubility of the alcohol in water as a result of the reduction in the size of the hydrophobic portion with respect to the hydrophilic one. All of the alcohols are soluble in acetone, so the students can learn that acetone is a good solvent for a wide variety of substances. They discover that ethylene glycol is completely soluble in water, partially soluble in acetone, and insoluble in hexane. These 448
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ethylene glycol in water, acetone, or hexane. sucrose in water, acetone, or hexane. •
After analyzing all the data obtained, discuss the meaning of the principle “like dissolves like”.
Figure 1. Discussion questions.
Journal of Chemical Education • Vol. 80 No. 4 April 2003 • JChemEd.chem.wisc.edu
In the Laboratory
“It increased my motivation for the laboratory period.” “All the laboratory experiences should be guidedinquiry.” “I can have more interactions with my peers.” “I feel that I am doing research because the material is not explicit. I have to create procedures and work out conclusions.” “It makes me think harder about the chemical basis of the observations.” “It allows me to apply in the lab what I learned in class and in class what I learned in the laboratory.” Figure 2. Selected student comments from the questionnaire given at the end of the academic year with respect to the guided-inquiry approach.
We found that the student comprehension of the solubility of organic compounds is improved by this experience because they can answer specific questions quite well in the written exams regarding this topic, as well as when they work other separation techniques or choose the best solvent during the recrystallization process. When questioned about the value of the guided-inquiry experience, students provided meaningful feedback. The results on a questionnaire given at the end of the academic year indicated that 71% of the students considered that the learning process using guided-inquiry laboratory experiences was very helpful, 27% considered that it was helpful, and only 2% considered that it was not relevant or helpful to them and that they prefer traditional laboratory experiences. Refer to Figure 2 for selected student comments from the questionnaire. Conclusion The guided-inquiry organic chemistry laboratory developed at the University of Puerto Rico–Río Piedras campus has brought about changes in our students’ understanding of the solubility principle “like dissolves like” and on the factors governing this phenomenon. By using solubility tests, the students explore many of the factors affecting solubility, in the context of one particular functional group class (alcohols). Compounds containing different functional groups can be used as well, if the instructor so desires. The principle of “like dissolves like” is introduced and mastered by students
by involving them in an active experience that allows them to develop analytical skills. In this experience, the instructor or the laboratory manual is no longer the source of knowledge, but a guiding force in the learning process. Acknowledgments We gratefully acknowledge support from the National Science Foundation grant DUE-9354432. We are also grateful to Lillian Bird, Patricia González, and to the 1997–2001 majors organic chemistry laboratory students at UPR–Río Piedras for their participation in the implementation of this experiment. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Shiland, T. J. Chem. Educ. 1999, 76, 107. 2. Spencer, J. J. Chem. Educ. 1999, 76, 566. 3. Venkatachelam, C.; Rudolph, R. W. J. Chem. Educ. 1974, 51, 479–482. 4. New Directions for General Chemistry. A Resource for Curricular Change from the Task Force on the General Chemistry Curriculum, Lloyd, B. W., Ed.; Division of Chemical Education, American Chemical Society, 1994; pp 4–36. 5. Wilson, H. J. Chem. Educ. 1987, 64, 895–896. 6. Gallet, C. J. Chem. Educ. 1998, 75, 72–77. 7. Pickering, M. J. Chem. Educ. 1991, 68, 232–234. 8. Alty, L. T. J. Chem. Educ. 1993, 70, 663–665. 9. Ruttledge, T. R. J. Chem. Educ. 1998, 75, 1575–1577. 10. Ditzler, M. A.; Ricci, R. W. J. Chem. Educ. 1994, 71, 685. 11. Ricci, R. W.; Ditzler, M. A. J. Chem. Educ. 1991, 68, 228. 12. Mohrig, J.; Hammond, C.; Morrill, T.; Neckers, D. Experimental Organic Chemistry. A Balanced Approach: Microscale and Macroscale; W.H. Freeman and Co.: New York, 1998; pp 749– 754; Williamson, K. L. Macroscale and Microsacle Organic Exeriments; Heath: Lexington, MA, 1989; pp 30–32.; Eaton, D. Laboratory Investigations in Organic Chemistry; McGrawHill, Inc.: New York, 1989; pp 80–84. 13. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques: A Microscale Approach, 3rd ed.; Saunders: Philadelphia, 1999.
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