Mass Is Better than Light: Students’ Perception of Using Spectrophotometry and Gravimetric Analysis to Determine the Formula of a Hydrate in the General Chemistry Lab

Abstract

A key part of the practice of chemistry is the analysis of chemical composition, including through gravimetric analysis and spectrophotometry. However, the complexity of doing multiple calculations to obtain analytical evidence, such as that required to determine an empirical formula, presents a challenge if such analytical methods are to be understood by students and if they support meaningful learning about other chemical concepts and methods. In this study, we investigate student use of spectrophotometry and gravimetric analysis to determine the number of water molecules in hydrates of copper(II) salts, a method previously described by Barlag and Nyasulu. Using phenomenography to analyze students’ reports through the lens of meaningful learning, we identified four distinct perceptions and, within them, information on how students make sense of the complex analytical steps involved in the experiment. We identify how meaningful learning is present where students recognized that spectrophotometry was based on light–matter interactions (cognitive), was faster and more accurate (psychomotor), and allowed students to express confidence in the process and their results (affective). However, it is also the case that meaningful learning was compromised where students had trouble conceptualizing spectrophotometry, saw it as a set of disconnected steps, and saw absorbance as a computer-generated value and not a property of the solution. This led to the perception that gravimetric analysis provided a more direct and understandable technique. We discuss the implications of these findings for chemistry education research (CER) and curriculum development in the undergraduate teaching lab.

Toward a Model of Cognition for UV/Vis Spectroscopy

“…Specifically, this tendency was observed in the absence of an intervention focused on the particulate level. Milligan and Wink (2024) also reported that students more readily used and learned via directly observable properties (e.g., mass) than with indirectly observable properties (e.g., absorbance) . In our findings, there were different ways of reasoning with color and we illustrate when this can be a limitation and when it can be an affordance.…”

“…These findings suggest that for molecular-level light–matter interaction phenomena, intentional support may be needed to develop students’ reasoning at the submicroscopic level . This finding is echoed in recent work comparing gravimetric methods to spectrophotometric methods . Milligan and Wink reported four perspectives of description from students’ written lab reports about using spectrophotometric methods.…”

“…This finding is echoed in recent work comparing gravimetric methods to spectrophotometric methods . Milligan and Wink reported four perspectives of description from students’ written lab reports about using spectrophotometric methods. Students described spectrophotometry as “Too complicated” when they did not express understanding of absorbance or how to utilize the obtained measurements.…”

“…Much of the educational research in the context of spectroscopy has focused on this latter interpretation competency specifically for NMR spectroscopy, a widely used tool for chemical identification. Substantially less has been done to explore the formerhow students reason about light interacting with matterand this has been done in the context of atomic spectra , the photoelectric effect, infrared spectroscopy, , and very recently, UV/vis spectroscopy. , Körhasan and Wang (2016) interviewed undergraduate students about atomic spectra and observed the use of multiple mental models . For example, a primitive scientific model was characterized by recognizing discrete energy levels and an interacting photon but with no connection to the electronic transition.…”