Beyond misconceptions: idealisation boundaries and the persistent difficulty of quantitative chemistry

The persistent difficulties students exhibit with quantitative concepts in chemistry—Avogadro's law, the mole, stoichiometric ratios, and concentration—have been documented across four decades of empirical research, span continents and educational levels, and have resisted the targeted misconception-remediation interventions designed to address them. We argue that this resistance is structural rather than developmental. Drawing on philosophical analyses of idealisation by Cartwright, Woody, Weisberg, and Potochnik, we propose that the fundamental laws of quantitative chemistry are not approximate descriptions of real systems but exact descriptions of idealised ones, whose application requires the systematic suppression of information about molecular identity, position, history, or structure that students are otherwise correctly taught to attend to. We term this systematic suppression identity erasure, and we show that it operates as an unmarked idealisation boundary in standard chemistry instruction. Avogadro's law is examined as the paradigm case; the mole concept, stoichiometric ratios, and concentration are shown to perform analogous acts of erasure. Three pedagogical principles follow: name the act, show the boundary, historicise the choice. The framework complements existing diagnostic frameworks (Tümay's emergence, Talanquer's structure–property progression, Sevian and Talanquer's chemical thinking, Johnstone's triplet) by addressing a dimension they leave underspecified: the epistemological status of the equations of chemistry as theory-licensed instruments rather than transparent descriptions of nature.