Although students of general chemistry often correctly
solve different kinds of numerical problems (in solubility
equilibrium, for example, Ksp and solubility calculations), this
alone does not guarantee a conceptual understanding of the
phenomenon because of misconceptions that persist after
instruction (1–4). The following problem allows us to evaluate
conceptual knowledge about solubility equilibrium and to
diagnose difficulties in relation to previous concepts. It involves
the following topics: dissolution, stoichiometry, chemical
equations, the particulate nature of matter, ionic compounds,
chemical equilibrium characteristics, solubility, the common
ion effect, and Le Châtelier’s principle.
To achieve an adequate conceptual understanding
implies the ability to offer explanations and descriptions at
the macroscopic level (experiments), the microscopic level
(atoms, molecules, ions), and the symbolic level (symbols,
formulas, equations), and the ability to establish appropriate
connections among the three. One barrier to understanding
chemistry is that instruction operates predominantly on the
symbolic level—that is to say, on the most abstract level of
the three (5). For this reason the problem below begins with
a representation using particles; it is similar to methods used
in assessing students’ conceptual knowledge about the kinetic
theory of gases (6 ) and the application of Le Châtelier’s
principle to homogeneous gaseous equilibria (7). The numbering
of the particles (uncommon in this type of diagram)
allows us to assess comprehension of the dynamic aspect of
the equilibrium.