edited ~bv .GEORGEL. GILBERT Denison University ~
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Granville. Ohio 43023
The Effect of Polarity on Solubility SUBMI~TED BY Jean-Marle Cllche and Benoit Labbb Udverslt6 de Sherbrwke Sherbrooke, PO, Canada JiK 2 R i Kenneth H. Lothrop Marshtieid High School Marshllcld. MA 02050 Nordstrom and Lothrop' published a tested demonstration showine the effects of nolaritv on solubilitv usine water and 1.l.l-tr~chloroethane(YCE) is immisciblesoh.e&, solid 1>,which is soluble in TCE but not in water. and also KI. which acts as a solubilizing agent of the undissolved 1; present in the water laver . bv. reaction of I- with I? to form the water-soluble 13- species. Through the use of (wo cylinders containing three solvents floating in distinct lavers (diethyl ether, water, and carbon tetracLloride), a demonstration comparing the solubility of two different solids, CuSOa and 12, has also been puhlished.2 In order to show convincingly that like dissolves like, during a course on chemistry for teachers of welding, we developeda similar demonstration, which we used to increase comprehension of concepts like densitv. r molecular and ionic .. nonnolar and ~ o l a solvents. compounds, solubility, and effects of'polarity on solubility and also miscibilitv. In the nresent demonstration. instead of diethyl ether, pelroleurn e;hw of hoiling point between 60 and 90 " C was ~reierredbecause this solvent is less soluble in water and has a higher boiling point3; instead of CuSOa, sodium dichromate, Na2Cr207.2H20was used because this solid is more readily soluble in water, also giving a more intense color; carbon tetrachloride is substituted by 1,1,2trichloro-1,2,2-trifluoroethane(TTE), which is less toxic.4 Furthermore, the end of this experiment seems to be new, never having been published before. Into a l - L separatory funnel5are slowly poured 200 mL of each one of the followine: first. T T E (the densest of the three), then water, and filally p & ~ ~ eether ~ d (the lightest of the solvents). Liquids form three layers: a t the top, ligroin (fraction of boiling point 60-90 "C with a density, D = 0.67), floating on water ( D = 1.00). and TTE (D = 1.56) a t the bottom. The nonpolar solveits, ligroin and TTE, are both immiscible with water. This is the first occasion to discuss solubility, polarity, and also the concept of density. Directly into the top of the separatory funnel, some crystals of sodium dichromate (D = 2.52) are added with a spatula. The ionic solid passes through the ligroin's layer without coloring it but dissolves rapidly into the polar layer of water, which becomes yellow. Everybody is amazed to observe the middle vellow laver of water. whereas the two othersare not (dored. Di~cussionsshould lead to the conclusion that sodium dichromate is more like water than lieroin and TTE, and this likeness is visible because only water is colored. In the same separatory funnel containing the yellow-colored water, some small crystals of iodine are added. This solid dissolves first in ether; this layer becomes violet. Bigger crystals of 12, because its density is 4.93, can
travel through the water layer without changing its yellow color. Now 1 2 dissolves slowly in TTE. Gentle swirling of the separatory funnel6 aids to dissolve more of 12 in both nonpolar lavers. which become more and more colored. Now the top a i d tl;e hottom layers are colored by 12 in violet, and the middle laver of water is still vellow. The discuss~onshould lead to t i e conclusion that izis nonpolar like ligroin and T T E and different from dichromate and water. After discussion on "like dissolves like", supplementary questions can be asked to complete the demonstration. If ligroin and carbon tetrachloride are both nonpolar liquids why are these two solvents not mixed together? This is the occasion to discuss nolaritv and densitv once again. Now if everybody understands tiese concepL (polar&, density, solubility, and miscibility) well, each one can predict what will happen when all the solvents are shaken and allowed to seoarate. Which laver will combine with another one? What wih be the totalvoiume of the mixedlayers? What will be the densitv of the mixed lavers? Will mixed lavers go to the bottom or to the top of the non-mixed layer?71f you take the precaution to mark with a black pencil the level of each one bf the three layers before mixing, you will be able to see, afterwards, the total volume of the two mixed layers and to discuss the concept of miscibility.
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' Nordstrom, 8. H.: Lothrop, K. H. J. Chem. Educ 1984, 61, 1009.
Widom. J. M.: Edelstein, S. J. Chemistry,an Introduction to General. Organic and Biological Chemist% W. H. Freeman: New York, 1981;pp. 243-244. Petroleum ether of boilinp Point between 30 and 60 OC should be avoided because evaporation 'is too easy at room temperature and solvents will be mixed uo oressure should build uo more easilv. when -~~~~ at the end of the demo&tration. Hexane may also be used; its bo/ling point is 69 OC, and its density = 0.659. * Munchausen, L.; Moore, A. J. Chem. Educ. 1987, 64,544. In order to mix up all of the three colored layers at the end of the demonstration, a separatory funnel is preferred to a graduated cylinder even though we are able to see better in the cylinder the Sum of the two mixed layers of nonpolar solvents. With a cylinder, it is difficult to mix the three solvents. Do not use oaraffinic Daoer . . to wrao UP the top 01 the cyi nder oeca-se paraii n is soluble In nonpolar solvents and some pressure wlll odlo .p on snakmg, c a a n g some loss of liquid. The form of the separatory funnel is aiso important. A slim separatory funnel is preferred to one of globular or bulbous design because it is easier to seethe equal layer of solvents at the beginning and the total of the two added nonpolar layers at the end and aiso because the slim funnel facilitatesthe swirling without mixing the two nonpolar solvents. A glass rod may also be used to facilitate the dissolution of I, in TTE by swirling. 'Attention should be paid to the density of the different liquids used. For example, after mixing up of the three colored layers, the mixture of ligroin (boiling point 60-90 "C) with TTE formed a denser fraction than water, but the mixture of l,l,l-trichloroethane(TCE)or methyiene chloride Is lighter than water, forming then the top layer. Furthermore, separation of fractions when using methyiene chloride, chloroform,or TCE should be avoided because the dissolution of I, in this Solvent gives a burgundy color,. different from the violet one obtained when 12 is solubilized in ligroin (or other solvents). This differencein coloration is probably due to the presence of inhibitors in TCE (Rothchild,R. J. Chem. Educ. 1983, 60. A228-229). Volume 65 Number 9
September 1988
813
