edlted by
J. DUDLEY HERRON Purdue Un#vers~ty West Lafayette, Indiana 47907
"lnvitations to Enquiry": The CalcitelAcid Reaction D. R. Driscoll Cariberra College of Aduanced Education P.O. Box 1 Belconnen A.C.T. 2616, Australia A recent Forum article by Dauphinee1 described bow an apparently paradoxical or unexpected aspect of a simple chemical reaction could be used to good effect in one's teaching. The experiment described below involves the reaction between calcite and hydrochloric and sulfuric acids and has similar potential. The reaction can be carried out on an individual basis, but there are some benefits in performing i t as a projected demonstration, using a filmstrip projector and a Perspexa (acrylic) or glass-and-perspex cell. The internal dimensions are such as to allow the cell to accommodate 2-3 semi-micro test tubes supported by short lengths of rubber tubing in the manner shown in the diagram.
The cell may be made out of 1.5-mm Perspex' (for the front and back) and 3.0-mm Perspexa for the sides and bottom. The Perspexa pieces may he cut readily enough with a hacksaw and may he joined either with a proprietary cement, or with glue made by dissolving Perspex" "sawdust" in chloroform. Although easy to manipulate, Perspex" tends to become scratched, and I prefer to use glass sheet cut to size for the front and back of the cells. Glass may be readily joined to the Perspexa sides and bottom with a silicone rubber cement. Depending on the type of projector used, the cell will usually
672 I Journal of Chemical Education
need to be supported a t an appropriate height on a stand, which may conveniently be made from polystyrene foam or wood. Water should be added to the cell to minimize refraction effects. The demonstration involves the reaction of calcite with acids. Place two semi-micro test tubes in the cell. To one add 2 M HC1, followed by a small, nicely shaped crystal of calcite. One observes that gas is evolved continuously, and that the calcite remains transparent. Discuss in terms of the ionic equation for the reaction 2Htc,r
+ CaCOm = H&II + Ca2+r,,i + COz,,)
This equation suggests that any source of H+ (in sufficient concentration) would lead to a similar reaction. Propose that one repeats the experiment but using 1 M HzS04 instead. (Whv 1M. and not 2 M?). Before one does this. ask the stndents to predict what might happen. On carrying out the experiment, one notes that there are a few bubbles of pas initiallv but the ieaction almost ceases within a few seconds. ~ u r thermore, the calcite crystal hecomes opaque. There is now a wealth of questions to be addressqd. Why did the initial evolution of gas in the second case die away very quickly? What is responsible for the slight residual reaction? Whence the opacity of the calcite? Was the second piece of calcite somehow less reactive than the first? How could one test this hypothesis? The explanation, of course, involves the formation of a protective coating of the slightly soluble CaS04 on the calcite, but before one gets to this point there is a good deal of interesting chemistry to explore. The reaction points up what is something of a potential trap in the use of ionic equations, viz. a tendency for students to overlook the possibility of reaction between the "s~ectator"ions. h l y c w n preterence for carr)ing 0111 the reaction as ;L prtriectrd d~,mmsrratiunis hased larrels on the voint that the magnified image enables one to seereadily certain things that are difficult to see in the ordinaw. wav. even when the test tube is a few centimeters away from one's nose. 'Dauphinee, G . A,, 3. CHEM. EDUC., 56, llfi(1979).
