In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Flame Tests Using Improvised Alcohol Burners submitted by:
Veljko Dragojlovic† Department of Chemistry, Northwest Community College, 5331 McConnell Avenue, Terrace, BC V8G 4C2, Canada;
[email protected] checked by:
Richard F. Jones Department of Chemistry, Sinclair Community College, Dayton, OH 45402
Flame tests are inexpensive and spectacular classroom demonstrations. Numerous variations of different levels of complexity and elaboration have been described in this Journal (1–11). In the demonstration presented in this paper an improvised alcohol burner, with a methanol solution of a salt as fuel, produces a long-lasting brightly colored flame. A disadvantage when using a regular alcohol burner is that the burner has to be cleaned and a wick replaced, before a solution of a different salt can be used. The wicks of many alcohol burners have insufficient draw to burn a methanol solution of a metal salt. Moreover, as methanol burns, salt deposits on the wick and the flame is extinguished after a short time, while the wick is contaminated and cannot be used again. For our demonstration, alcohol burners are made from small (5-mL) glass vials. The vials are filled with a methanol solution of the desired salt and a wick, made of single-ply toilet tissue, is added. Thus, a small amount of solvent (5 mL or less) provides a colored flame, which lasts for several minutes (Fig. 1). Vials and paper wicks can be reused. A list of some methanol solutions suitable for this demonstration is given in Table 1. The demonstration works best if one uses a wick made of an absorbant paper. Paper tissue cut to size and twisted into a wick worked well. A toilet tissue works very well and is the right size for burners made of small (5-mL) vials. The draw of such a wick is sufficiently high to allow for the clean and sustained burning of methanol, and salt deposits are not a problem. In fact, as more salt is deposited, the color of the flame becomes more intense. A small amount (5 mL) of the methanol solution (Table 1) of the desired salt is placed in a vial and a wick is added. After waiting for a few seconds for the wick to absorb methanol, the burner can be ignited. For the best results, the wick should be straight and short. The size of the flame can be adjusted by adjusting the length of the wick that is outside the vial. If necessary, its length can be increased, using tweezers, while it is burning. When using a 5-mL vial, the size of the flame can be adjusted to be anywhere from 2 to 5 cm. Thus, the demonstration is most suitable for a class of up to 30 students. Use of this alcohol burner in a flame test demonstration has several advantages. The burner is inexpensive, a number of tests can be run simultaneously, and stock solutions of † Present
address: Department of Math, Science and Technology, Nova Southeastern University, 3301 College Ave., Ft. Lauderdale, FL 33314.
Figure 1. Methanol solutions, from left to right: 5% CuCl2?2H2O, 5% LiCl, 5% KI. (This figure appears in color on p 875.)
Table 1. Methanol Solutions Used to Create Colored Flames Salt
Concentration (%)
LiCl
1
Color Red
CsF
5
Faint blue
KI
5
Violet
CuSO4?5H2O
2
Green
CuCl2?2H2O
5
Blue
H3BO3
Saturated solution
CaCl2
5
Green Orange
SrCl2
5
Red
NaBr
5
Yellow
NaI
5
Yellow
BaO
5
Yellow-green
metal salts can be prepared in advance and stored for future demonstrations. CAUTION: Once all the methanol has burned, the paper wick will start to burn. Therefore, one should not allow the burner to run to dryness. After the demonstration the flame can be extinguished by covering the vial with a glass beaker. Care should be exercised with salts that are oxidizing agents. A wick impregnated with such a salt will continue to burn on its own, producing a large amount of smoke, even after the burner has been covered with a beaker. One way to extinguish such a flame is to (using tweezers) pull the wick out of the vial and immerse the burning wick in a beaker filled with water.
JChemEd.chem.wisc.edu • Vol. 76 No. 7 July 1999 • Journal of Chemical Education
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In the Classroom
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6. Mattson, B. M.; Snipp, R. L.; Michels, G. D. J. Chem Educ. 1990, 67, 791. 7. Ager, D. J.; East, M. B.; Miller, R. A. J. Chem Educ. 1988, 65, 545. 8. Gouge, E. M. J. Chem Educ. 1988, 65, 544. 9. Peyser, J. R.; Luoma, J. R. J. Chem Educ. 1988, 65, 452. 10. Boucher, J. H. J. Chem Educ. 1986, 63, 158. 11. Pearson, R. S. J. Chem Educ. 1985, 62, 622.
Journal of Chemical Education • Vol. 76 No. 7 July 1999 • JChemEd.chem.wisc.edu
