© Division of Chemical Education  • www.JCE.DivCHED.org  •  Vol. 86 No. 7 July 2009  •  Journal of Chemical Education 845

In the Laboratory

Solubility is addressed in introductory chemistry courses owing to its importance in chemistry and related fields. Most laboratory exercises that accompany the study of solubility focus on inorganic salts (1–4). Studies utilizing organic compounds tend to be more advanced, covering partitioning (5) or computa-tions (6) beyond the focus of an introductory course. Baer and Adamus combined the traditional solubility study of inorganic salts in water followed by a short study of organic liquids (7). Because of its limited coverage of organic compounds and no further study of structural relationship to solubility, I decided to expand on this work.

This simple, qualitative experiment focuses on organic compounds and leads to discussions of the relationship between solubility and molecular structure. It also serves as a springboard to discussions on the building blocks of biomolecules and how they assemble. I use the experiment in a college general educa-tion biochemistry course for nonmajors with no chemistry prerequisite, but the experiment could also be used in advanced high school chemistry courses, general chemistry courses for college science majors, or for applied fields such as pharmacol-ogy and environmental studies. It is a guided-inquiry laboratory that quickly transforms the class into a community of learners. No prior knowledge of solubility or the principles regarding miscibility is necessary.

Laboratory

The first part of the laboratory, completed within one hour, involves mixing organic compounds with water in a test tube and simply recording whether they are miscible or im-miscible. The compounds used are acetic acid, acetone, amyl ac-etate, cyclohexane, diethyl ether, ethanol, ethyl acetate, hexane, hexanol, methanol, t-butyl methyl ether, mineral oil, pentanol, propanol, and tetrahydrofuran. This sample of organic com-pounds includes hydrocarbons, alcohols, ketones, ethers, esters, and acids. It includes cyclic and acyclic compounds and brief series of compounds with the same functional group but dif-fering number of carbons. After mixing the compounds the students are asked whether they can formulate a theory of what makes an organic compound soluble in water or not soluble in water. Students hand in a worksheet with their data and a one-page description of their theory of solubility of organic compounds in water. They are given one week to think about the data.

In the second part of the laboratory exercise (completed in a two-hour lab a week later) students predict the solubility of organic compounds in other organic compounds based on their observations of the behavior of the compounds in water. Different theories and predictions are suggested. Students with previous chemical knowledge tend to base their theories on polarity; polar molecules will dissolve other polar molecules and

nonpolar molecules will dissolve other nonpolar molecules. Stu-dents new to chemistry will generalize, predicting that organic compounds that dissolved in water will dissolve in each other and compounds that did not dissolve in water will dissolve in each other. Some students conclude that all organic compounds are miscible with each other because they are organic in nature. Other students will notice the presence of oxygen in molecules that dissolved in water and base their theories of organic solu-bility on the presence or absence of oxygen. Still other students notice the structural features and formulate a theory that acyclic compounds will dissolve in each other, cyclic in each other, those with many carbons in each other, and so forth.

After formulating a hypothesis, students test it by mixing representative organic compounds in a test tube. I suggest pairs that usually contradict1 students’ theories. I provide dipole moments to students who based their theories on polarity (polar compounds mix and nonpolar compounds mix) and ask them to mix moderately polar ethyl acetate with nonpolar hexane. I ask students who clumped all water-soluble organics in one group and water-insoluble organics in another group to mix groups (such as tetrahydrofuran with diethyl ether). For those students who see all organics as miscible in each other I ask them to mix hexane and methanol. Students review their hypotheses and mix more pairs to test the new hypotheses. Sometimes students get frustrated or disillusioned and start interacting with other students to compare or find an answer. The lab becomes full of chemistry chatter and I fade into the background. Eventually the students test all possibilities and come to the conclusion that there are several factors that influ-ence solubility. When they write the lab report a week later, they all have a comprehensive theory on solubility that involves (albeit in less technical and accurate terms) dipoles, hydrogen bonding, number of carbons (hydrophobicity), overall shape of a molecule, and functional groups. Some students disregard the behavior of organic compounds in water but others recognize that what was happening in water relates to what is happening within the organic compounds.

We discuss results in the lecture immediately following the laboratory. I lecture on how water solvates ions and how the size of these ions affects solubility. This leads to later discussion about how ions do not cross the membrane of cells unless they are transported. We talk about why some alcohols dissolved in water and return to discussions about hydrogen bonding when we talk about sugars and amino acids. We talk about the effect of carbon chain length in the solubility of alcohols in water, or among themselves, and again when I talk about fatty acids forming an oil, wax, or membrane. When examining cholesterol in membranes I remind them of why hexane and hexanol mix. The comparisons and analogies abound and are limited only by what you need to stress within the context of the biochemistry lecture.

Solubility Studies of Organic Compounds for Nonscience MajorsMariella PassarelliDepartment of Natural Sciences, University of Maine at Farmington, Farmington, ME 04938; mariella@maine.edu

846 Journal of Chemical Education  •  Vol. 86 No. 7 July 2009  • www.JCE.DivCHED.org  •  © Division of Chemical Education

In the Laboratory

Hazards

Many compounds are used in this experiment. The most important hazards, in addition to flammability, are summarized here. Acetic acid is corrosive and a lachrymator. Methanol is toxic if ingested and damaging to the optic nerve, skin, and respiratory tract. Drowsiness is caused by inhaling the vapor of the following compounds: acetone, diethyl ether, hexane, and propanol. The following compounds are irritants to the respira-tory tract, the eyes, and skin: amyl acetate, cyclohexane, ethanol, ethyl acetate, hexanol, methyl-t-butyl ether, pentanol, propanol, and tetrahydrofuran. Glycerol and mineral oil pose no hazard. An extensive compilation of hazards, exposure limits, symptoms of overexposure, and protective measures is provided in the on-line material. In addition to the standard protective measures of gloves, goggles, and working under a hood, the online material also has suggestions as to how to set-up the lab and conduct the experiment to minimize exposure. The exercise can be done successfully and safely.

Conclusion

In conclusion, this is a technically simple lab that allows students to learn about solubility and scientific thinking. The lab offers a way to quickly familiarize students with the structural complexity of organic compounds and how this complexity influences chemical behavior. The discussion that follows the lab provides a solid base of knowledge by which biomolecu-lar assemblies and interactions can be explained, and guides students into higher levels of cognitive thinking by analyzing many aspects of solubility and synthesizing the outcomes into a comprehensive theory. The students follow the scientific method by observing experiments, formulating a hypothesis, testing the hypothesis, and revising the hypothesis. And finally, the lab cre-ates a collaborative, active classroom environment. It is a simple experiment that achieves many educational goals.

Note

1. An excellent article by Ashkenazi and Weaver (8) describes how lecture demonstrations followed by class discussions in a general

chemistry college course help teach solvent miscibility and refine stu-dent knowledge on the subject. The focus of the article is primarily on how examples of miscibility are presented, the background information given by the instructors prior to the demonstration, and the resulting student predictions because of these two presentations. The pedagogical analysis of how the particular examples contradict or reinforce previ-ous student knowledge could prove invaluable for the instructor when walking around the lab during this experiment orienting the students’ development.

Literature Cited

1. Stanitski, C. L.; Eubanks, L. P.; Middlecamp, C. H.; Pienta, N. J. Chemistry in Context: Applying Chemistry to Society, 3rd ed.; McGraw-Hill: New York, 2003.

2. Harle, H. D.; Ingram, J. S.; Leber, P. A.; Hess, K. R.; Yoder, C. H. J. Chem. Educ. 2003, 80, 560.

3. Stevens, K. E. J. Chem. Educ. 2000, 77, 327–328. 4. Lercher, T. M.; Battino, R. J. Chem. Educ. 2001, 78, 103–111. 5. Goss, K-U.; Schwarzenbach, R. P. J. Chem. Educ. 2003, 80,

450–455. 6. Hessley, R. K. J. Chem. Educ. 2000, 77, 203–205. 7. Baer, C.; Adamus, S. M. J. Chem. Educ. 1999, 76, 1540–1541. 8. Ashkenazi, G.; Weaver, G. Chem. Educ. Res. Pract. 2007, 8,

186–196.

Supporting JCE Online Materialhttp://www.jce.divched.org/Journal/Issues/2009/Jul/abs845.html

Abstract and keywords

Full text (PDF) Links to cited JCE articles

Supplement Student handout, including structures of the compounds and a

data sheet Instructor notes, including contradicting pairs of compounds, a

solubility table, and a compilation of the compounds’ hazards