Student Discovery of the Relationship between Molecular Structure, Solubility, and Intermolecular Forces

General chemistry students need to understand the concept of intermolecular forces and use them to predict the properties of materials. To do this, they must create symbolic representations of molecules, relate these to physical observations, and explain them using submicroscopic theoretical constructs. When these concepts are taught separately, students lack the context to fully understand and retain them. In this inquiry-based laboratory experiment, students explored the interrelated concepts of molecular structure and solubility in an applied context and then explained their individual results and the class results using the concept of intermolecular forces. Specifically, they drew the molecular structures of an analogous series of alcohol molecules and evaluated the relative solubility of the alcohols in polar versus nonpolar solvents. Designing the procedure to avoid student exposure to hazardous reagents facilitated their exploration of the underlying concepts. Using Sharpie pigment in each alcohol as an indicator dye helped the students visually assess their relative solubility in different solvents. Students typically observed that the relative solubility of the alcohols in the solvents could be predicted based on key factors that affect molecular polarity and hydrophobicity, in particular hydrocarbon chain length and surface-to-volume ratios. The students explained the pooled class solubility results based on the types and strengths of intermolecular forces present in each substance. Significance testing performed on our pre- and postlaboratory quiz data indicate that completing this experiment significantly improved students' conceptual understanding of structure–property relationships. Item difficulty and discrimination index scores were used to validate these quiz questions.