In the Classroom
Chemical Principles Revisited
The Conductivity of Molten Materials Monica E. Thomas W. E. Stebbins High School, Dayton, OH 45424 Audrey A. Cleveland, Rubin Battino,* David A. Dolson, and Michael R. Hall Department of Chemistry, Wright State University, Dayton, OH 45435; *
[email protected] When the topic of ionic bonds is discussed, the point is always made that ionically bonded substances do not conduct in the solid state, but do conduct in the molten state. Even though there are no molecules in the solid and crystals are composed of arrays of ions, there is no electrical conductivity because the ions are bound into fixed positions. However, in the molten state the ions are mobile and are therefore able to conduct electricity. Program 8 on Chemical Bonds of The World of Chemistry videotape series (1) shows a demonstration of this effect using solid sucrose and NaCl, the molten materials, and aqueous solutions of these materials. Sucrose decomposes upon melting at 170–186 °C, and NaCl melts at 801 °C. Porcelain crucibles are used to contain the molten materials. The traditional indicator, a 110-V light bulb in series with a set of electrodes, is used. In this paper we present our results of testing a variety of materials that melt at temperatures that are lower and therefore safer to use, as well as several detection devices that are safe, inexpensive, and visible in large classrooms. Demonstrations of the conductivity of molten salts are not new. Cornog tested KNO3 (2). Castka tested K2Cr2O7, NaOH, AgNO3, and NaCl (3). Stone used KNO3 (4 ); Damerel, NaCl (5); Weaver, glass tubing (6 ); and Humphreys, LiCl (7). Safety measures for working with molten salts are given by Lovering and Gale (8) and Gale and Lovering (9). Apparatus We wished to use small amounts of material and settled on the use of 20- or 25- by 150-mm borosilicate test tubes (softening point 821 °C). These test tubes would not soften for any of the test substances we suggest using. On the other hand, if you insisted on completely fusing NaCl, it would require a 90% silica tube (softening point 1530 °C) or one of pure quartz (1580 °C). Test tubes are easy to handle and can be conveniently clamped. For electrodes we used appropriate lengths of stainless steel welding rod (without flux, TIG rods), obtainable from any welding equipment supplier. These stainless steel electrodes are forced through a cork that is grooved for venting. (We tried “tungsten” welding rods, but these are an alloy and appear to catalyze reactions in some of the molten materials; they also reacted with the salts.) We tried a variety of pencil electrodes (10). The “harder” the pencil, the sturdier the lead appeared to be towards breaking. Number 3 or 4 pencils worked well, but so did No. 2 pencils and the thicker-lead electronic scorer and drawing pencils. The first electrode of a pair is made by cutting off the eraser end and carefully baring 2–3 cm at one end and 1 cm at the other end using a penknife. 1052
The second electrode is prepared in the same way, except that the total length is 1.5 cm shorter. The staggered 1-cm bared ends minimize the possibility that the alligator clips used for connection to the detector will touch. The materials are heated with a Bunsen burner, a butane burner, or a propane torch. A hot-air gun may be used on the materials that melt below 150 °C. Detection Devices For safety reasons we do not recommend using a 110-V light bulb. One safe device that is visible to an entire class uses a 15-W, 12-V light bulb (obtainable from RV suppliers) in series with a 12–16-V ac isolation transformer (11). Another consists of a jumbo LED (Radio Shack #276-086) connected in series with a 9-V battery and a 300-Ω resistor (to prevent burnout from shorts) and can be seen in a darkened lecture hall by everyone, since it is very bright and has an integral lens. The LED is directional and you just point it around the class. We made a compact package for this detector by using a 9-V battery holder and appropriately soldering the LED and resistor to it. For a third device, we modified a High-Gain 500 Advanced Technology 6-V lantern with a krypton bulb (see Fig. 1a for wiring diagram) to indicate elec-
lamp
a resistor 1000 ohms battery 6 volts
+ −
probes
transistor NTE261
transistor TIP127
b
battery 3 volts
resistor + 1000 ohms
lamp
− probes
Figure 1. (a) Six-volt lantern circuit. (b) Three-volt flashlight circuit.
Journal of Chemical Education • Vol. 78 No. 8 August 2001 • JChemEd.chem.wisc.edu
In the Classroom Table 1. Melting Points of Selected Materials mp/°C
Substance Ca(NO3)2 (hydrated) (13) Na2S2O3 (hydrated) Rochelle’s salt (KNa tartrate, hydrated) Alum (hydrated) Sucrose
42.7 45 ~70–80 92 170–186 dec
Ascorbic acid
192
LiNO3 (anhyd)
255
NaNO3 (anhyd)
307
NaOH (anhyd)
318.4
NaC2H3O2 (anhyd)
324
KNO3 (anhyd)
334
K2Cr2O7 (anhyd)
398
LiCl (anhyd)
610
NaCl (anhyd)
801
Na2CO3 (anhyd)
851
Carveth’s mixture 30 wt % LiNO3 (anhyd) 56 wt % KNO3 (anhyd) 14 wt % NaNO3 (anhyd)
120
Mixture A 59% LiCl (anhyd) 41 wt % KCl (anhyd)
352
Mixture B 68 wt % LiBr (anhyd) 32 wt % KBr (anhyd)
310
trical conductivity. This lantern throws a focused or diffused beam that is readily visible. For a fourth device, Figure 1b shows the wiring diagram for an ordinary 3-V flashlight with a krypton bulb. All these inexpensive and safe detectors will light in tap water and the molten salts, but not in distilled water or nonelectrolytic solutions. In our testing we found that the jumbo LED gave a smooth response to conductivity (we used a resistance substitution box). The LED may be coupled via a short length of opaque tubing to a photoresistor (Radio Shack #276-116) to give more quantitative results, by reading resistance on a solid-state multimeter. If the electrodes are maintained in a fixed geometry, the use of standard KCl solutions will yield a cell constant for roughly quantitative results.
pencils will char, and caution must be used. It is best to use a fresh batch of material for each demonstration. All of the ionic materials successfully lit the detectors. Sucrose does caramelize, but does not conduct. Ascorbic acid (vitamin C) decomposes as it melts, but does not conduct in the molten state; an aqueous solution does conduct. Since it is easy to use this apparatus to determine the conductivity of materials, we also filled test tubes to the height of about 3 cm with the following common liquids or solutions, which did conduct: distilled water plus NaNO3, distilled water plus LiNO3, distilled water plus NaCl, tap water, root beer, orange drink, vinegar, and distilled water plus Alka Seltzer. The following did not conduct: distilled water, distilled water plus sucrose, methanol, isopropyl (rubbing) alcohol, and charcoal lighter fluid. There are obviously many substances that can be tested. An interesting demonstration described by Weaver shows that soft molten glass will conduct electricity (6 ). This may be demonstrated by clamping a length of glass rod or tubing between two ring stands and directly over a burner. As the glass is heated, test its conductivity with the stainless steel electrode device. The conductivity increases as the glass softens and decreases as it cools. Hazards With the low-voltage detection devices described in this paper there is no possibility of electric shock. Molten materials are hot and can cause burns. Do not reuse materials, but start with a new batch each time. Do not run current through the molten salt mixtures for more than 2–3 minutes because they do electrolyze (yielding noxious products). Used salts may be discarded in normal trash or flushed (except for K2Cr2O7 which is a carcinogen) with plenty of water. Always wear safety goggles. Conclusions The simple apparatus and detectors described in this paper should make this fundamental demonstration safely available at all levels. We had fun testing “supermarket” materials as well as the molten materials. Acknowledgments MET and AAC acknowledge support from NIH and NSF for an apprenticeship program in science for minority and women high school students and precollege teachers. We thank the checker and reviewers for many helpful suggestions.
Selection of Material Table 1 presents a variety of materials that can be used. Use of hydrated salts such as alum or photographer’s hypo (Na2S2O3) yields very low melting points and good conductivities, but this is “cheating” because an aqueous solution is formed. On the other hand, an ordinary hair dryer will melt these hydrated salts. Carveth’s mixture (12), which has a melting point of 120 °C and a working range up to 500 °C, is particularly useful as a salt mixture. For the higher-melting individual salts, KNO3, NaNO3, and LiNO3, there is a possibility that the wood in the lead
Literature Cited 1. Chemical Bonds, Program 8; The World of Chemistry [Videotape Series]; The Annenberg/CPB Project, 1990. 2. Cornog, J. J. Chem. Educ. 1928, 5, 99. 3. Castka, J. F. J. Chem. Educ. 1940, 17, 487. 4. Stone, C. H. J. Chem. Educ. 1942, 19, 598. 5. Damerel, C. I. J. Chem. Educ. 1952, 29, 296. 6. Weaver, E. C. J. Chem. Educ. 1940, 17, 346. 7. Humphreys, D. A. Demonstrating Chemistry; Chemistry Department, McMaster University: Hamilton, ON, 1983.
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In the Classroom 8. Molten Salt Techniques, Vol. 1; Lovering, D. G.; Gale, R. J., Eds.; Plenum, New York, 1983, pp 14–15. 9. Molten Salt Techniques, Vol. 2; Gale, R. J.; Lovering, D. G., Eds.; Plenum: New York, 1984; pp 4–7.
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10. 11. 12. 13.
Arena, J. V.; Mekies, G. J. Chem. Educ. 1993, 70, 946. Battino, R. J. Chem. Educ. 1991, 68, 79. Carveth, H. R. J. Phys. Chem. 1898, 2, 209. Moynihan, C. T. J. Chem. Educ. 1967, 44, 531.
Journal of Chemical Education • Vol. 78 No. 8 August 2001 • JChemEd.chem.wisc.edu
