Illustrating the Concept of Sparingly Soluble Salts Using Various Copper Compounds

摘要

Previous articles in this Journal have explored the difficulty many students have understanding solubility qualitatively (1,2). An even larger number of articles have appeared dealing with the approaches and challenges associated with calculations involving slightly soluble salts (3-6). One difficulty beginning students have is conceptualizing the differences in solution species concentrations for solids with varying thermodynamic solubility product constants, Ksp. Although many students can perform the solubility calculation, using a simple algorithm, they do not have a conceptual fiamewoik to recognize the difference between solubilities from 1CT3 to 1CT6 mol/L or even smaller. We have developed a visual demonstration of the different molar solubilities of several copper salts and linked them with the solubility equilibrium computations involving these salts. The goal is to allow students to visually observe differences in molar solubilities of a variety of sparingly soluble salts with solubility product constants spanning many orders of magnitude, thus developing a more concrete understanding of the information conveyed by the magnitude of the thermodynamic solubility product constant. The demonstration may also be used to illustrate the effects of a common ion on the solubility of a salt. In the demonstration very small copper concentrations are "visualized" with color formation by the complexation of copper with bathocuproinedisulfonic acid disodium salt.