To See IMFs on a Surface of Glass: A General Chemistry Lab Exploring Intermolecular Forces on Surface Structures through Causal Mechanistic Reasoning

Abstract

Intermolecular forces (IMFs) make up a fundamental concept of chemistry and one that is integral to students’ understanding of the properties and interactions of matter. Despite this, students struggle to apply IMFs to real phenomena in their world. Here we describe a first-semester general chemistry laboratory in which students functionalize the surface of glass slides and observe the interaction of water and heptane drops with the surface, allowing them to integrate IMF, molecular modeling, and causal mechanistic reasoning to explain observable and measurable phenomena. In the activity, students perform and describe a series of simple reactions that covalently bond the silane molecules acetoxypropyltrimethoxysilane and octyltrimethoxysilane to the glass surface. They then characterize the slides by adding drops of water to the modified slide, taking profile pictures with their cell phones, and determining the drop half angles from the pictures using ImageJ software. Students also added drops of heptane to the slides and observed their interactions with the slides, contrasting those with the interactions of the water drops. This lab activity invites students to consider the material of the lab on the macroscopic and submicroscopic levels as they describe the functionalization of glass slides, observe the interaction of the modified and unmodified slides with drops of water and heptane, and then construct explanations that reinforce their learning of IMFs and molecular structures. The experimental procedure and data collection proved to be robust, with most students producing data that were consistent with expectations and supported their claims about the IMFs between water molecules and between the water molecules and the surface.