Diagnosing students' misconceptions on solubility and saturation for understanding of phase diagrams

Students in introductory chemistry classes often harbor or develop misconceptions about solubility concepts of solutions that inhibit their ability to understand of the nature of solution behavior. Subsequently, when these prior misconceptions are carried over into introductory materials engineering classes, they also inhibit students' ability to understand the liquid and solid solution concepts necessary to effectively understand and use phase diagrams. The end result is that students may never fully comprehend and appreciate the relationships between phase diagrams, materials' microstructures and associated correlations to materials' properties. This research, which is associated with misconceptions revealed from a Materials Concept Inventory (MCI), has investigated students' understanding of concepts of solubility and saturation that are associated with liquid and solid solution behavior necessary to understand phase diagrams. Student knowledge of solution behavior was studied with two-tiered questions. For the first tier-question students chose a schematic diagram that represented the solution characteristics that portrays their mental model about some aspect of solution behavior. In the second-tier response students then described the reason for their choice of a given diagram for the first-tier question. Misconceptions about liquid-solid solution behavior in chemistry classes have been moderately well studied. Those related to the concepts of solubility and unsaturated, saturated, and supersaturated solutions are also pertinent to the study of solid solution behavior. In fact, the prevalence and persistence of solution-related misconceptions is demonstrated by the fact that changes of student understanding on a solution-concept related item on a previously administered MCI only moved from 39% to 67% correct on pre and post class administration of the MCI. For example, in a salt-saturated water solution, with water-saturated salt resting at the bottom of a beaker, students often believed that putting a small additional amount of salt in the beaker would increase the concentration of salt in the solution. One student said, "adding salt to an already saturated salty solvent will increase concentration slowly." The most common misconception in this research was that a supersaturated solution contained both liquid and solid phases instead of a solution with solute concentration above equilibrium. The diagnosis of this and other misconceptions represents identification of faulty mental models of the physical behavior of solutes and solvents and creates the possibility of devising interventions to address the misconceptions. Thus, the misconceptions about solution behavior carried over from chemistry classes need to be first diagnosed and then addressed in introductory materials engineering classes in order to more fully understand and use phase diagrams. Additional details on the nature of the misconceptions and suggestions about possible interventions to address them are discussed.