Replacement Reactions Using a Dissecting Microscope Chuk-Yin Lam-Erwin Russell Sage College, Troy, NY 12180 Joseph Sprague State University of New York at Cobleskill, Cobleskill, NY 1204 are spectacular, such a s the formation of featherlike silver Microscopic qualitative analysis was developed a long crystals. Apatient student will observe a similar growth of time ago and was modified through the years for practical copper on iron wire if they happen to perform this test. application ( I ) . Although the method has been recently proposed a s a way of developing observational skills with Double Replacement Reactions chemical reactions (21, it has otherwise been generally forgotten. With the growing popularity of microscale experiPrecipitation ments in general chemistry we find it surprising that the I n addition to the solutions listed above, prepare 0.2 M method has not enioved a similar m w t h . We believe that " " solutions of Na2C03, NaOH, and Na3P04. Students are microscale experiments can be made more enjoyable than asked to mix a drop or two of each solution in column A their full-scale countemarts and here propose ~.o l i c a - . the a . with a drop or two of each solution in column B. Microtion of the microscopic method to two common experiments often included in general chemistry laboratory courses: scopes and spot plates are used to observe any reaction. metal reactivity and double replacement reactions. I n traditional experiments with replacement reactions, colorful precipitates are often producid to promote student NaG03 interest. Unfortunately these precioitates commonlv inNaOH volve toxic metal ions k t h high disposal costs. ~ e ~ l a c i n ~ them with less toxic and less costly metals generally reNap04 sults in a series of milky white precipitates likely to be described a s "borinf by the students. This paper presents a . . . solution to this d j i e i m a of student interest versus toxicity and dlsposal cost. The students perform drop-sized cxpcrimenta i n clear glass or plastic spot plates under dissectThey are asked to note the rate of appearance and the ing microscopes. If heavy metal ions are chosen for the rerelative amounts of precipitates,.and write complete and action, the small quantities have low disposal costs; if less net ionic equations consistent with their observations. We toxic and less costly substitutes are chosen, students will choose not to use the more colorful (but usually toxic) salts boring remain sumrisinelv interested because there are no -" commonly employed in this type of experiment because precipitates a t 20x magnification. Even single replacethere is surprising variation in milky-white precipitates ment reactions. which normallv do not involve h e a w metwhen viewed with a microscope. After precipitates of carals, benefit from this approach: bonate, hydroxide, and phosphate are prepared, the stuReactivity of Metals dents are asked to observe the effect of adding 1-2 drops 1 M HC1 to each. Prepare 0.2 M solutions of CaC12,CuSO4, FeS04, MgS04, ZnSO*, andAgN03, and a 1M solution of HCI. The metals Acid-Base Neutralization and the amounts required per student for each experiment are: a small piece of Ca turning, 1cm of copper wire, 1cm Scrape some fine powder from various over-the-counter of magnesium ribbon, 1cm of iron wire, a matchhead size antacid tablets such as Alka-Seltzer (NaHC03 and citric pile of zinc powder and a corn kernel size piece of lead. The acid), or Tums (CaC03) onto a watch glass. Add a couple copper, magnesium, and iron should be cleaned with fine drops of 1 M HC1 to the solid. If Alka-Seltzer is used, stusandpaper to expose a fresh surface. We make the experidents should be asked to test it also with water and explain ment a bit open-ended. Students are first asked to rank the the results. They should eventually conclude that sodium metals in order of reactivity by observing the reaction of citrate is the actual antacid that makes it to the stomach. each metal with a few drops of water using a spot plate and Similar reactions can be carried out using pieces of white microscope. They then test the metals with 1 M HC1 and chalk or limestone. Colored chalks do not work a s well. refme their ranking. They learn more after the second test but still not enough. The last step is for the students to Literature Cited design their own tests, using appropriate metals and salt 1 . Charnot, E. M . ; Masan, C . W . , H o n d h d of ChernlcolMionsmpy; Wilqv: NewYork, solutions to complete the reactivity order. While all of the 1940:Vol 2, p 30 2. Winokur, R.;Monroe, M. J. Chem. Edue 198.5,62,157 tests are interesting when viewed with a microscope, some
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Volume 69 Number 10 October 1992
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