edited bv GEORGEL. G I L B E R ~ Denison Universily Granville. Ohio 43023
The Effect of Polarity on Solubility Sus~lrrm BY
Brian H. Nordstrom Embry-RiddleAeronautical University PreseM, AZ 86301 Checked by Kenneth H. Lothrop Marshfield H.S. Marshfield.MA This demonstration quite convincingly shows the effect of oolaritv on soluhilitv accordinr to the chemists' adare "like dissolv& like;" therifore, polarsolvents dissolve p ~ l ~ s o l u t e s and nonwlar solvents dissolve nonwlar solutes. Students can ohserveihat iodine (a nonpolar species) dissolves readily in l,l,l-trichloroethane (a nonpolar solvent), but does not dissolve in water (a polar solvent). Immiscibility of liquids with different polarities can also he demonstrated by the failure of the trichloroethane and water to mix. Demonstrations comparing the soluhility of iodine crystals in water and in nonpolar solvents such as carbon tetrachloride are fairly commonl.2. This experiment, however, has been received by the author's students as being more dramatic in its effects, and it is readily scaled up for viewing by large classes. Materials and Equlpment 250-ml stoppered Erlenmeyer flask containing Iz crystals twd.5-g samples of solid Iz 0.5 e KI ~~two---500-mlErlenmeyer flask~ 2 magnetic stirrers, stir bars 3W ml Hz0 300 ml l,l,l,-trichloroethane 250-d separatory funnel (pear-shaped) 2 signs--"I2 in Hz0" and "I2 in TCE" 0
Procedure This is a eood oooortunitv to discuss with students some of the prop&ies o'fiodine. 1;ass around the room for examination the s t o.o.~ e r e dflask containine a few cmstals of solid Iz. Discuss sublimation, pointing out to students the presence of iodine vaDor in the flask. Explain &e operation of the magnetic stirrers, and set a stir bar s ~ i n n i n on e to^ of one stirrer (much to the fascination of beginning studenis). Then pour 300 ml of water into one 500-ml flask, drop in a stir bar, set the flask on a stirrer, and place next to i t the sign reading "Iz in HzO!' Repeat this operation using the 300 ml of l,l,l-trichloroethane (TCE), using the other stirrer and the sign reading "12 in TCE!' Explain to students that 12 and TCE are nonpolar and Hz0 is polar. (l,l,l-trichloroethane is chosen for this demonstration hecause i t is less toxic than other commonly used solvents such as carbon tetrachloride and benzene. Nevertheless, the vapor is irritating. AVOID CONTACT w i t h skin, eyes, a n d
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clothing. Also keep trichloroethane away from contact with active metals.3 The same orecautions anolv .. " to contact with iodine crystals and iodinekapor.) Start the stir bar in the water spinning and drop in 0.5 g iodine crytals. Discuss why very little dissolution occurs. Repeat with 0.5 g iodine in the TCE and discuss the difference observed in soluhility. After the two solutions have mixed for 2-3 min, turn off the stirrers. Pour 100 ml of the iodine-containing water into the separatory funnel and slowly add 100 ml of the iodine-containing TCE, observing the immiscihilitv of the two liquids. Stopper the separator; funnel, shake it vigorously, and allow the layers to separate again. For additional confirmation that "like dissolves like." add 0.5 g KI t o the remaining 200 ml H20 (still containing undissolved iodine) and stir. The Iz and I- combine t o form the thus the iodine goes into solution. triiodide ion, 4-,
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Wheaton, R., "Tested Demonstrations in Chemistry," 6th ed., J. Chem. Educ., Easton, PA, 1965, p. 211. Summerlin, L. R.. "Polar Properties and Solubility." in "Testad Demonstrations in Chemistw." .. 6th ed.. J. Chem. Educ.. Easton.. PA.. 1965, p. 223. Muir, G. D., "Hazards in the Chemical Laboratory." 2nd ad.. The Chemical Society, London, 1977, p. 415.
The Insolubility ot "Soluble" Barium Nitrate SUBMlTTm BY Jesse Reinstein University of Wisconsin-Piatteville Planeville. WI 53818 CHECKED BY
Margaret Raymond University of Washington Seattle. WA 98195 This demonstration is based uoon the unexoected formation of a precipitate of barium nitiate when b a r b carbonate or barium o h o s ~ h a t eis treated with dilute nitric acid. 300 ml or Larger tall-form beaker, magnetic stirrer or glass rod. Demonstration. To 50ml of 1M BaClz contained in thebeaker is added, with stirring, either 50 ml of 0.5 M Na~C03or 50 ml of 0.3 M NanPO*. To the beaker containine the nreci~itatedbarium salt are added. aith constant stirrine. ... four 5-ml nortions of 6 M HNOI. Caution!TheCOz produced may causespattering.The pwipitalp is d i s ~ 0 1 ~at4this p < h .Finally, an additional 30 ml of 6 M HNOt is added, with stirring, to the solution. Within a short period of time a precipitate of Ba(NO3)n is formed. (Note: barium salts are poisonous and any barium in solution should be precipitated as the sulfate and should be disposed of in an appropriate manner.) Remarks. This demonstration can he used in coniunction with a discussion of solubility and dissolotiun ~ f ~ ~ r e ~ ~ i p i t a t e n . The term "solut~lr"and the exr~ression"all nitrates are soluble" may he put into proper perspective using the limited soluhility of barium nitrate.
Volume 61 Number 11 November 1984
1009
