Using a Dissecting Microscope in Teaching Introductory Chemistry Robert Winokur Department of Biological Sciences, University of Nevada, Las Vegas, NV 89154 Manus Monroe Chemistry Department, Indian Valley Colleges, Novato, CA 94947
In an effort to have high school and iunior college students develop observational skills and acquire an excitement about chemistrv. stereoscopic dissecting microscooes (from 7 to 40 power) have been used to ohserveihe physicil characteristics and chemical reactions of several substances, a few of which are described in this paper. The reactions must be viewed using a bright source of direct incident (not transmitted) light. Both lights and microscopes can often he borrowed from a biology department. Depression slides are useful hut not essential. Safety glasses are recommended, especially when working with acids. Reactions with CuSO. Students are instructed to adjust the magnification of their microscooes to between 20 and 30 Dower and to d a c e a d r o ~ or two ofbeionized water into the depression o f s glass side. A few grains of freshly prepared anhydrous CuS04 are placed, using the sharp end of a spatula, extremely close to hut not on the water drop. Students first observe that what appears as a white powder to the naked eye is actually granular. Within 2-5 min, some of the granules gradually change to light blue crystals. After viewing the coior change, students i r e then instructed to push several granules into the water, using either a needle or small elass rod. and to observe the individual hvdrated crystals. gtudents then add additional anhydrois CuSOd to the d r o of ~ water and stir. and the rate a t which the crystais dissolve is noted. The relationship between the rate of dissolution and the concentration of a solution is discussed, as well as the concept of (equilibrium) vapor pressure of water and the use of cowuer(I1) sulfate as an indicator in desic.. cants. With the magnification set at 10 or 15 power, a few particles of 40 mesh magnesium powder are finally added to the solution. The production of copper metal on the surface of the magnesium particles is then observed. The details and possible mechanism for this redox reaction are discussed, as well as the physical contact required between the solid magnesium particles and the aqueous copper(I1) ions in the solution. If the deionized water is fairlv acidic. students mav see the nroduction of gas (Hz) bul;hles on' the surface df the met'allic particles due to a redox reaction between magnesium and hydrogen ions. Reaction between Magnesium Powder and 1 M HCI Another orocedure that is esoeciallv effective for this tvne of magnified observation is the'reaction between magneszm powder (40 mesh) and 1M HCI. The individual particles are animated by the production of hydrogen gas huhhles on their surfaces and make a compelling spectacle when viewed under magnification. Students are instructed to adjust the magnis 1M HCI into fication to 10 or 15 Dower. to dace a few d r o ~of the depression i n t h e slide; and to add few particles of
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maenesium. A maenification of 20 to 30 Dower will allow a close-up view of the animated particles and gas production. At this ~ o i n tif. a ~ ~ r o u r i a twe e . discuss the concept of redox reactions between hydrog& ions and some -elemental metals. Reaction between Na2C03 and HCI Students are instructed to adjust the magnification between 10 and 30 power. Next, they place approximately 1mL of 3 M NazC03 on either a Syracuse dish or a watch glass and slowly add, drop by drop, 1 M HCI to the solution. Students will observe the evolution of gas from the solution. Here, we discuss metathesis reactions (NaCO-(ao) 2HCl(aa) a . HzCOdaq) 2NaCl(aq)) and the de"&position of an acid at room temperature (HzCOdaq) H20 COzt). It may he useful to mention that when-an acid decomposes, one can deduce the formula of the gas by "subtracting" a water molecule from the formula of the acid (HzC03 - Hz0 = Con). Similar reactions can he carried out with small pieces of chalk or crushed sea shells.
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Reaction between Copper Metal and Silver Ions Students are instructed to adiust the maenification to 10 or 15 power, and using a small piece of sandpaper or emery cloth. sand a 1-2-cm piece of 12 or 14 gauge comer wire. This process removes anythin layer of oxides& chiorides and exposes pure copper. They then place a few drops of 0.1 M AgN03 in the depression of the slide and place a 0.5-cm piece of sanded wire into the solution. After students have viewed the growth of needles and feather-like crystals of silver metal, they are instructed to adjust the magnification between 20 and 30 power. With such magnification, they can observe the details of the lengthening of the silver crystals. Again, a discussion of redox reactions between metallic atoms and cations of different elements is appropriate. Dissolving KMnO., in Deionized Water This procedure is designed to allow students to observe crystals dissolving and to note the local coloring of water around a dissolving crystal. Any power between 10 and 30 may he used. Again, students place a few drops of deionized water in the depression of the slide and, using a spatula, place one or two KMn04 crystals in the water. These few exoeriments serve to illustrate how direct observations with i microscope of chemical reactions can be used to teach and dramatize basic chemical principles. Many similar experiments can be developed to suit the needs of individual courses. After all, chemistry is an experimental science, and without excitement in the laboratory and skills of observation. students mav fail to understand and appreciate .. our discipline. Volume 62 Number 2
February 1985
157
