In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Crystallization from a Supersaturated Solution of Sodium Acetate Jamil Ahmad Department of Chemistry, University of Botswana, Private Bag 0022, Gaborone, Botswana;
[email protected] An eye-catching demonstration involving crystallization from a supersaturated solution uses the sodium acetate–water system (1–4 ). A supersaturated aqueous solution of sodium acetate will keep indefinitely, but when it comes in contact with even a tiny crystal of the salt, it starts crystallizing and soon the whole liquid turns to a solid. There are a few variations of this demonstration all of which involve preparing a solution beforehand and seeding it with a crystal of the salt. The version described here can be done on an overhead projector and is suitable for showing to a large class. In this variation the progress of crystallization can be followed as it takes place. Materials 30 g sodium acetate trihydrate 10 mL distilled water overhead projector transparency 250-mL Erlenmeyer flask 500-mL beaker Bunsen burner Procedure The solution should be prepared beforehand and set aside to cool to room temperature at least half an hour to an hour before the demonstration. Once prepared, the supersaturated solution can be kept for days, provided care has been taken to remove or dissolve all the solid particles. To make the solution, place 30 g of sodium acetate trihydrate and 10 mL of distilled water in a 250-mL Erlenmeyer flask. Using a wet tissue paper, wipe away any crystals of the salt that might have stuck to the inside of the neck of the flask. Meanwhile, heat about 200 mL of water in a 500-mL beaker over a Bunsen burner to over 80 °C, and place the Erlenmeyer flask in it. When the contents of the flask have turned into a liquid, remove it from the hot water and swirl it gently to dissolve all the remaining particles of the salt that may still be sticking to the sides. Cover the mouth of the flask with aluminum foil or an inverted beaker and allow to cool to room temperature. For the demonstration, place a transparency on the stage of the overhead, and carefully pour the solution onto it in the shape of a canal about 1 to 2 cm wide, or in the form of a circular puddle about 10 cm in diameter. The canal can be roughly straight, wavy, or any other shape. Add a small crystal
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of the salt to one end of the canal or to the center of the circular puddle as the case may be. Crystallization starts immediately and is seen on the screen as a moving dark front, which eventually replaces the clear image of the transparent liquid by the dark image of the opaque solid mass. The progress of the crystallization can be clearly followed on the screen. After the crystallization is complete, the transparency may be lifted from the projector and tilted, or even turned upside down, to show that the liquid has solidified and will not flow under gravity. If a wavy canal is used, individual crystals can be seen at the bends. In the case of a circular shape, the crystallization progresses in a circular front centered on the seed crystal, and the formation of individual crystals is quite evident on the screen. The experiment works best with a conventional overhead projector that has its light source below the transparency. If a portable projector is used in which light shines from above, the width of the canal may have to be increased to compensate for the thick dark outlines of the image arising from the convexity of the canal. After the demonstration, the solid can be scraped off the transparency and stored for reuse. Since the supersaturated solution crystallizes so easily, special care must be taken to eliminate all traces of the solid salt from the Erlenmeyer flask. It is strongly recommended that more than a single solution (preferably three or four) be kept ready for the demonstration, in case crystallization starts in one or more solutions before the seed crystal is added—or even while the solution is being poured onto the transparency. Hazards This demonstration poses no significant hazards. Literature Cited 1. Bacon, E. K. J. Chem. Educ. 1948, 25, 251; Reprinted in J. Chem. Educ. 1987, 64, 805. 2. Hiegel, G. A. J. Chem. Educ. 1980, 57, 152. 3. Gilbert, G. L.; Williams, L. G.; Shakhashiri, B. Z.; Dirreen, G. E.; Juergens, F. H. In Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1; Shakhashiri, B. Z., Ed.; University of Wisconsin Press: Madison, 1983; pp 27–30. 4. Fun with Chemistry, Vol. 2; compiled and edited by Sarquis, J.; Sarquis, M.; Institute for Chemical Education, University of Wisconsin: Madison, 1993; pp 287–291.
Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
