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

Abstract

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.

A Colorful Solvent Extraction Demonstration for Teaching the Concept of “Like Dissolves Like”

“…Solutions and solvent extractions involving colorful dyes have been the focus of several recent papers. Baldock et al published a general chemistry laboratory experiment investigating the solubility of various alcohols in water versus hexane using ink from a Sharpie pen to make the layers more visible . Others developed organic laboratory experiments investigating the solubility of dyes in aqueous or organic solvents as a function of solution pH and the resulting dye structure. , Broyer et al also described a classroom demonstration of an acid–base solvent extraction of dyes suitable for organic chemistry students .…”

What Is the Structure of a Polymer in Solution?

“…Connecting the macroscopic properties of polymers to their microscopic structures is important and challenging. Baldock et al have recently noted that “[o]­ne way to help students learn challenging course topics is to engage them in active learning exercises that incorporate multiple representations such as macroscopic observations coupled with submicroscopic level explanations.” Therefore, in order to lead students toward a better understanding of the nature of polymers in solution, we include a significant amount of microscopic model development in the laboratory activity.…”

Intermolecular Forces Dominoes! A Game for Aiding Students in Their Review of Intermolecular Forces

“…In this context, one of the topics that cause most difficulty for students during their undergraduate studies is that of intermolecular forces. − Intermolecular forces are the electrostatic forces of attraction that mediate interaction between molecules and ensure that systems remain cohesive. , Mastery of intermolecular forces enables students to comprehend a varied range of chemical phenomena, varying from certain macroscopic properties, such as melting and boiling points, miscibility/solubility, to the ability of molecules to orient themselves, as well as the manifestation of quantum probabilistic phenomena, such as the instantaneous appearance of dipoles or the interaction between drugs and receptors. − Nevertheless, because this requires a thorough understanding of valence electrons, chemical bonding, Lewis structures, bond polarity, molecular geometry, and molecular polarity, students consider the subject challenging and intrinsically complicated. Even more complex is the progression from the atomic or molecular scale to the multiparticular scale to then comprehend that the macroscopic properties of substances are derived from the behavior of systems at the submicroscopic level. − In this context, students struggle to understand that intermolecular forces can only be explained in terms of an important collectivity of molecules consisting of one or several types, i.e., on a multiparticular scale and thus not only at the molecular level.…”