"Rubberized" Bone: A Demonstratlon of Solubility Principles, Acid-Base Reactions, and Biochemistry Many students have difficulty in working chemistry problems because they fail to see chemical principles working in a concrete situation andlor to combine various aspects of chemistry. Sometimes a short, simple demonstration will help students in perceiving the presence of chemical principles in an everyday object and encourage them to try to make chemistry have a more "real" meaning for themselves-a process essential if they are to apply chemical principles to new situations. With this goal in mind, I have prepared a "rubberized" bone demonstration that is useful in illustrating different chemical principles. This demonstration also has the advantage that it is easily prepared and kept from year to year. The "rubberized" bone is made by simply immersing a suitable hone (e.g., a large turkey bone) in hydrochloric acid overnight. (The exact concentration is not critical-1 M HCI has performed satisfactorily-but the hone should come from cooked meat, as the grease in and on the surface of an uncooked bone hinders the contact between the acid and the phosphate.) Excess acid is then removed by immersing the hone in distilled water. The product resulting from this acid treatment looks as if no change had occurred, but the hone can now be folded backon itself quite easily. When not in use, it can he stored in aplastic bottle in a freezer, and it is easily thawed out (-1 h) when needed. I t also withstands repeated freezing and thawing. Chemlcal Principles Illustrated by t h e Bone One area in which this bone is a useful demonstration is the solubility of ionic compounds. As an introduction to detailed solubility rules, I start with a general framework and then add exceptions to that framework. The first principle of this general framework is that ionic compounds in which both ions are multiply-charged generally do not dissolve, with the intracrystalline attraction between the multiply-charged ions prevailing over hydration. I use Ca3(P0& as a simplified formula for the mineral content of bone and then indicate that this mineral should become progressively more soluble as the charge on the phosphate is reduced as it combines with H+. When I show them the hone treated with acid, it appears that chemistry has failed me-until I bend the hone in half and then Let it spring back into shape. Such a bone is also useful when weak acids and weak bases are introduced. The phosphate in bone is used as an example of one of the many common occurrences of weak bases, and the equations for the reactions of nonhydroxide bases with an acid can take on some concrete meaning for the students. Still later, the equation for the dissolution of Cas(P01)s and the equation for the reaction of the P043- with acid can he used to illustrate Le Chatelier's principle in the context of both solubility equations and acid-base reactions. Although it is surprising to students that so much of hone is not mineral, it can be pointed out that a framework, such as collagen, for bone should be expeded-both to guide the deposition of the mineral to give the bone its proper shape and to allow o .~ e n i n e so s that the red blood cells oraduced in the marrow can leave. In summary, airubberired" hone demonsrrarion will illustratr a number of different aspects ofchemisrry-srparateand ly, or in rombination. In addition, the unexpected rharartrristinoi [he treated bone tend tu ratrh srude~~ts'atrention providr somr nwrivatim fur students ro look for the "rraliry" of rhemiral prinriples in the world around them. ~
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Thomas P. Chlrplch Memphis State University Memphis. TN 38152
Volume 63
Number 6
June 1986
519
