Appetizing Colloids
Solution B:
J o h n T. Riley Western Kentucky University Bowling Green, KY 42101 Cheeked by: George Virtes Ward Melville High School Setauket, N Y Most chemists will agree on the value of cited practical examples and the use of visual aids and/or lecture demonstrations in the presentation of material in chemistry. The more unusual, or even ridiculous, examoles and demonstra~i.,nsh;tw a greater impact ;and arc mtsr oiten remrmlu.r(rl 11sthe 5 1 ~ d ~ l l fsincr 5 , ~ 0 1 ialwilsi 1 seem Lu have their unihided attention during the presentation. An explanation of why an unusual demonstration works, using the principles that are being discussed, helps the student understand and remember these principles. Two nooular . . examnles of colloidal disnersions that can he demonstrated rather easily in the classroom are the formation of a foam upon mixine a solution of aluminum sulfate (or anv weak acid) kith a sol&on containing egg alhumin and sodiu& hicarhonate, and the formation of a gel upon mixing ethanol with a saturated solution of calcium acetate. The foam is a dispersion of a gas in a liquid and is formed when the acidic aluminum sulfate solution reacts with the bicarhonate ion to release COr, which is trapped by the alhumin solution. The egg alhumin and sodium hicarhonate solution has the appearance of beer, so that when a 50-ml portion is poured into a tall class or mue mntainine 50-ml of the aluminum sulfate solution, students are startled hy the huge amount of "beer foam" that is formed. The eel is a dispersion of a solid. calcium acetate, in the liquid e t h a h w a t e r mixture, which cdagu~ates to form a viscous gelatinous body. Students are fascinated by the instant formation of the slightly opaque solid mass from the two clear solutions. I have combined these two demonstrations into a ridiculous exhibition that always generates plenty of laughs.
Solution C:
Submitted by:
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Reagents
Solution Ai
50 ml of 95%ethanol containing 0.5 ml of 1% phenolphthalein
Solution D: Solution E:
50 ml of 95% ethanol containing 0.5 ml of 1% thymolphthalein add 2 drops of 50% NaOH to 100 ml of saturated calcium acetate while stirring vigorously 20 g Al2(SO4):~ in 100 ml of water 15 g NaHCOs and 1.4 g egg albumin in 100 ml water. Add the egg alhumin and NaHC03 slowly while heating and stirring. Continue heating and stirring until most of the solid has dissolved. Cool and decant the liquid, and discard the undissolved material.
Demonstration
To produce instant "strawberry gelatin" from clear solutions, Dour solution A and 10 ml of solution C simultaneouslv into a'wide mouth shallow container (i sherbet dish works best) in a wav that allows maximum mixine of the two solntions. ~ i k e w i i e"blueberry gelatin" can be by mixing s ~ ~ l u t i oBnwith 10 ml of solution C. For a topping, mix 30 ml of solution D and 30 ml of solution E in a 250-ml heaker. Use a tablespoon or kitchen spatula to dip the thick foam out of the beaker and "plop" it into the two containers of gel to produce attractive dishes of "gelatin dessert topped with whipped cream." Other indicators can be used to give additional "flavors" of gelatin. Examples are hrom-cresol purple, which gives a "grape" color (use 1ml of 0.1% indicator solution) and alizarin vellow R which nives a "lemon" color (use 3 ml of 0.1% indicator solution). The ethanol solutions of these indicators are yellow and the contrast in color chance, from vellow to some &her color, is not as great as it is us& phen&hthalein and thymolphthalein, which changes from colorless to red and blue, respectively. Nevertheless, when gels with each of the four indicators are prepared, using premeasured quantities of the reagents and mixine them ouicklv., and each eel ~is topped with a generous portion of foam, the results look like a caieterin dfadw rilsc.T h i s is a n exrellrnt, i ~ n dilpperi~it~g. drnimitmtion I U periwm nt the very end ilia late morning class. ~d
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~olume57. Number 2, February 1980 / 153
