In the Laboratory
What Is That Colorless Solution? A Qualitative Analysis Laboratory for General Chemistry William R. Furlong, Michael R. Quackenbush, and Ramee Indralingam* Department of Chemistry, Stetson University, DeLand, FL 32723; *
[email protected] There are various formats of qualitative chemical analysis laboratory experiments for the general chemistry curriculum. Most of them take the form of following a scheme for the detection of cations or anions (1–4). A few of them make the problem interesting by introducing the laboratory in the form of a puzzle or mystery (5, 6). Some take the innovative approach of the “nine bottle problem” (7), in which students are given a certain number of unknown solutions. They are then required to identify the compounds in the solutions by mixing the unknowns, two at a time, and observing the reactions, if any (8–11). General chemistry texts emphasize the classification of chemical reactions, such as acid–base neutralizations, reactions that produce precipitates, those that produce gases, those that illustrate displacement of metals from their salts by other metals, and oxidation–reduction reactions (12, 13). Tan et al. (14) have found that many students in Singapore do not seem to be able to make the correlation between good understanding of the classification of reactions and the ability to identify unknown inorganic salts in a qualitative chemical analysis laboratory. We have found that several of our students, too, have the same difficulty. As a way of helping students to bridge the gap, we have developed a laboratory experiment that combines qualitative analysis with classification of reactions. This experiment is suitable for use in high schools as well as in the first-semester general chemistry class in college. It can be completed in a laboratory period of two hours in length. Description of the Experiment We have chosen barium chloride, sodium sulfate, potassium iodide, lead(II) nitrate, sodium carbonate, sodium sulfide, and ammonium chloride as the unknowns because they all undergo characteristic reactions that can be classified and used for identification of the compounds. In addition they all form colorless solutions so that students cannot identify the unknowns by visual inspection. There is one reaction scheme that will allow the students to identify the compounds efficiently by a process of elimination. If the theory has been covered in a class prior to the laboratory, the students may be asked to develop the scheme and have it verified by the instructor before carrying out the experiment. Conversely, the students may be given the scheme and asked various post-laboratory questions that probe their understanding of the reaction scheme and the reason for the order of tests. For the sake of safety and efficiency, the test for sulfide should be carried out first. This will avoid generating noxious hydrogen sulfide vapors when later tests are carried out using dilute acids. A solution of copper(II) nitrate is added to all of the unknowns. A black precipitate forms with sodium sulfide, which serves to identify the sodium sulfide. A pale blue precipitate of copper(II) carbonate forms with sodium carbonate. A confirmatory test for sodium carbonate is carried out by generating carbon dioxide with dilute hydro-
chloric acid and passing the CO2 through a saturated solution of calcium hydroxide to form the characteristic milky calcium carbonate precipitate (Figure 1). An interesting reaction occurs when copper(II) nitrate is added to potassium iodide. A redox reaction occurs: Cu2+ oxidizes I‒, yielding I2, which forms a reddish solution (15). The reduced Cu+ forms a yellow precipitate of CuI. The reaction therefore yields a brownish mixture. This reaction serves to emphasize redox reactions and the use of oxidation numbers to identify oxidizing and reducing agents. The confirmatory test for potassium iodide is carried out by adding acidified silver nitrate to the unknown that produced the brown mixture with copper(II) nitrate. The yellow curdy precipitate of silver iodide confirms potassium iodide. The four remaining unknowns are identified systematically. The addition of dilute HCl produces a precipitate with only lead(II) nitrate. This unknown is confirmed by carrying out a single-replacement (also redox) reaction with a magnesium strip. Ammonium chloride is identified by the addition of aqueous sodium hydroxide: ammonia vapor is given off and recognized by the basic reaction to litmus. Barium chloride and sodium sulfate are identified by the white precipitate of barium sulfate formed when aqueous sodium sulfate and barium chloride, respectively, are added. In the four years that we have carried out this experiment, we have found that our students, while feeling daunted upon initially confronting the procedure, have negotiated their way through the questions and gained a greater understanding of qualitative chemical analysis and the classification of reactions,
Beral pipet containing dilute HCl two-hole stopper (size 00)
13 × 100 mm test tube
poly vinyl tubing
dip end of tubing into saturated calcium hydroxide solution unknown solution
Figure 1. Experimental setup for the sodium carbonate test. Students inject dilute HCl into the unknown solution. Carbon dioxide gas is generated and made to bubble into a solution of calcium hydroxide, forming a milky-white calcium carbonate precipitate.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 8 August 2009 • Journal of Chemical Education
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In the Laboratory
especially because the introduction portion of the student handout provides a tutorial on the concepts needed. We have the students do the experiment in pairs so that they have someone to discuss observations and possible conclusions while they fill in the data sheet. This seems to make the experiment more enjoyable and comparable to solving a puzzle. We also maintain the concentrations of the unknown solutions and the reagents at such a level that the volumes needed for each test are small, and waste is kept to a minimum. White porcelain, 12-well plates are used for all the tests except for the generation of CO2 and the production of the characteristic calcium carbonate precipitate with saturated calcium hydroxide (Figure 1). Details of the student procedure complete with introduction, post-laboratory questions, and notes for the instructor are given in the online material. Hazards The Material Safety Data Sheets on the chemicals used in this experiment emphasize caution with respect to ingestion, inhalation, and skin and eye contact (16). The exceptions are sodium carbonate and sodium sulfate, which are only slightly toxic upon ingestion. Furthermore, Na2SO4 is not expected to have adverse effects owing to skin or eye contact. However, we advocate the preparation of all reagents and unknown solutions in the chemical hood by laboratory staff wearing gloves. Also, care must be taken by students handling barium chloride and lead(II) nitrate, especially as both are unknowns and the students do not know which of the vials contains them. Students should be advised to wear gloves while carrying out the experiment. Literature Cited 1. Slowinski, E. J.; Wolsey, W. C.; Masterton, W. L. Chemical Principles in the Laboratory, 6th ed.; Saunders: New York, 2006; pp 285–313. 2. Postma, J. M.; Roberts, J. L., Jr.; Hollenberg, J. L. Chemistry in the Laboratory, 6th ed.; Freeman: New York, 2004; pp 31-1–32-14. 3. Randall, J. Advanced Chemistry with Vernier; Vernier Software and Technology: Beaverton, OR, 2004; pp 14A-1–14B-5. 4. Oliver-Hoyo, M.; Allen, D.; Solomon, S.; Brook, B.; Ciraolo, J.; Daly, S.; Jackson, L. J. Chem. Educ. 2001, 78, 1475–1478.
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5. Spencer, H. E.; Kusdra, L. J. Chem. Educ. 1998, 75, 487–488. 6. Rybolt, T. R .; Waddell, T. G. J. Chem. Educ. 1999, 76, 489–493. 7. Macwood, G. E.; Lassettre, E. N.; Breen, G. J. Chem. Educ. 1940, 17, 520–521. 8. Marsden, S. Chemistry Resources for Students and Teachers. http:// www.chemtopics.com/intsess/9bottle.pdf (accessed Mar 2009). 9. Postma, J. M.; Roberts, J. L., Jr.; Hollenberg, J. L. Chemistry in the Laboratory, 6th ed.; Freeman: New York, 2004; pp 33-1–33-9. 10. Slowinski, E. J.; Wolsey, W. C.; Masterton, W. L. Chemical Principles in the Laboratory, 6th ed.; Saunders: New York, 2006; pp 315–321. 11. Tan, Y. S. S.; Tan, B. H. I.; Lee, H. K.; Yan, Y. K.; Hor, T. S. A. J. Chem. Educ. 1998, 75, 456–458. 12. Chang, R. Chemistry, 9th ed.; McGraw-Hill: New York, 2007; pp 122–142. 13. Zumdahl, S. S. Chemical Principles, 5th ed.; Houghton Mifflin: Boston, 2005; pp 97–130. 14. Tan, K. C. D.; Goh, N. K.; Chia, L. S.; Treagust, D. F. J. Chem. Educ. 2004, 81, 725–732. 15. Jacobsen, J. J.; Bain, G.; Bruce, K.; Moore, J. W. J. Chem. Educ. 2000, 77, 799–800. 16. Chemical Laboratory Information Profiles, CLIPs, are also available for all of the unknowns: (a) Young, Jay A. Barium Chloride Dihydrate. J. Chem. Educ. 2002, 79, 554. (b) Young, Jay A. Sodium Sulfate. J. Chem. Educ. 2007, 84, 1272. (c) Young, Jay A. Potassium Iodide. J. Chem. Educ. 2006, 83, 1286. (d) Young, Jay A. Lead(II) Nitrate. J. Chem. Educ. 2004, 81, 1709. (e) Young, Jay A. Sodium Carbonate. J. Chem. Educ. 2002, 79, 1315. (f ) Young, Jay A. Sodium Sulfide. J. Chem. Educ. 2009, 86, 919 (g) Young, Jay A. Ammonium Chloride. J. Chem. Educ. 2005, 82, 1618.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Aug/abs953.html Abstract and keywords Full text (PDF) with links to cited URL and JCE articles Supplement
Student procedure including an introduction and post-laboratory questions
Notes for the instructor
Journal of Chemical Education • Vol. 86 No. 8 August 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
