A Melting Point Depression Activity Using Two Inorganic Substances Stephen DeMeo Barnard College, Columbia University, New York, NY 10027 Melting point depression activities can be found in many laboratory manual; and have been described in this ~ o u m - 1 (13).Unfortunately, these activities are more appropriate for students enrolled & omanic chemistrv coursesbe&use the activities overwhelmingly use organic compounds. This paver describes an activitv that uses two inormnic wmwunds. iodine and sulfur, to demonstrate meltingpoint depression: Because instruction in first-vear chemistrv courses redom minately focuses on the and inter&ions of horganic substances, iodine and sulfur would be appropriate chemicals for student work in the laboratory. These substances are ideal for several reasons. Iodine and sulfw are studied extensively due to their status as elements, they do not read with each other (41, and they are readily available, inexpensive, and easily disposed of. Most importantly, both have individual melting points below 120 "C, which allows students to take rapid melting point measurements using inexpensive equipment, such as a Thiele tube.
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Experimental Materials
Procedure Slightly crush a few iodine crystals until the pieces are just small enough to go into a capillary tube. If necessary, crush a small amount of sulfur. Separately, fill a capillary tube to about 1cm from the bottom with each solid. Thoroughly mix a small amount of iodine with sulfur, and fill a capillary tube with this mixture. If possible, determine the melting point ranges of the three samples side by side. Relevant Observations W h e n iodine is heated, a small amount sublimes. It rises a few inches up the capillary tube and remains suspended. When the iodine melts, it turns into a black liquid. When the sulfur melts, it turns all at once into a yellow/redbrawn liouid. When iodine and sulfur were mlxed on the watch glass, the mixrurc was lighr-brown. \Vhm the mixture melted. sameof the sohd turned hlnek and some hrown. A few reddish swts also were observed. Disposal
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Iodine crystals P-Sulfur powder: pale yellow, opaque, brittle, monoclinic mortar and pestle or watch glass and metal spatula melting point apparatus (Thiele tube, Hoover, etc.) capillary tubes
Caution:Iodine is a slightly corrosive solid (51,so goggles
should be worn when using iodine. Any iodine stains on the skin can be easily removed with rubbing alcohol and water.
Careful: Consult l a d authorities for exceptions.
Anv c a ~ i l l a r v tubes filled with iodine can be left under a hoodvto iublime, then thrown out in the trash if desired. Unwanted iodine can be reduced with sodium thiosulfate. neutralized, rinsed with water, and disposed of down the drain (7). Capillary tubes filled with sulfur can be thrown out in the garbage. Small amounts of left-over sulfur can be collected and also thrown out in the trash. Discussion
Iodine ran be ground using n mortar and pestle. If n metal spatula is used instwd, it should be quickly wiped free of lodmc to prevent staining. It is not neressary togrind iodine to a fine powder: 11 should be ground only until it just fits into the ravillarv" tube. If t ~ x much ) iodine IS takcn bv thc student and ground too finely, inhalation of any sublimed iodine can create a scmtchv sensation in the back of the throat. Thus. grinding of iodiie and the mixing of iodine and sulfur should be done under a fume hood. Alternatively, students could use a previously ground portion of iodine or be given capillary tubes alreadv filled with iodine. The cavillarv . " tubes wntainine iodine can be flame-sealed. This is recommended because it would minimize the potential release of iodine vapor into the classroom. Flame-sealing does not inhibit the meking point phenomenon.
Melting points and the identification of a substance's purity are of fundamental importance to organic a s well a s inoreanic chemistrv. Knowing that a substance is vure is important becauscit allows Hubsequent cause an2 effect relationshivs to be associated with a sinele substance. This actikty is specifically relevant for discussing the following concepts.
Sample Data Table
Using iodine and sulfur emphasizes the inorganic when demonstrating melting point depression. The addition of this activity fills a small niche that has received little or no attention in this Journal and in commercial laboratory manuals.
Melting Point
Experimental
CRC Handbook (6)
iodine
112-114°C
113.5 OC
sulfur
118-119°C
119.0°C
iodine mixed with sulfur
796
62-68 OC
Journal of Chemical Education
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solids, liquids, and melting points equilibrium and mixtures physical states, crystals, and phase changes purity and impurity narrowness of the melting point range as a criterion for purity depression and broadening of the melting point range as criteria for impurity
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Literature Cited 1.Allen.E. J. Ckm. Educ 1942,19.278-281. 2. Grabar,D. G.;MECrone, W C l Cham Educ lSM),27,MM53.
3.Di Pippo, A G.; Joseph, M. J Chem. Educ 1965,42,A413. 4. The Morck Index, 11th ed.; Budsvati, S., Ed.: Rahway, NJ, 1989. 5. Springer. S. In Impmuing Safety in the Chsmicol L a b o m l o w : A PImLleol Guide; Young, J., Ed.: Wiley: N e w Ymk, 1987.
6. Tha CRC Hczndbwk ofChem&try and Physics, 60th ad.; West, R. C.,Ed.: CRC:Boca Ratan, FL, 1980. 7. Sittig, M. Hondbob of= ond Hosardou Chemimls ond C a r c i n o g p ~2nd , ed.: Noyes: NJ, 1985.
Volume 71
Number 9 September 1994
797
