A grandeza "quantidade de matéria" e sua unidade "mol" : uma proposta de abordagem histórica no processo de ensino-aprendizagem

Abstract

The chemical quantity "amount of substance" and its unity "mol" are badly understood by teachers and students of all levels, as attested by some research on this subject. Even in textbooks these concepts are introduced in a non-contextualized way, leading to a lack of comprehension. These two ideas are very important since they perform a conceptual bridge between the macro world of perceptible quantities (mass and volume of reactants) and the micro domain of the abstract and conceptual approach (atoms, molecules and number of particles participating in a chemical reaction). In this work we recover the historical evolution of stoichiometry since its origin, with the introduction of equivalent weight, by the end of the XVIII century until the consolidation of the chemical quantity "amount of substance", by the middle of the XX century. These two concepts deals with the same problem, e.g., the ratio among the amount of reactants that perform chemical reactions, but from two different (equivalentist and atomistic) points of view (different paradigms). The combination laws of Richter, Proust and Gay-Lussac, and the Dalton and Avogadro´s hypothesis are discussed, as well as the introduction, by Ostwald, in 1900, of the term "mol" as a unity of mass, in his equivalentist view. We stress that the complete understanding of "amount of substance" and "mol" can be achieved only if one comprehends the historical evolution from the continuous (mass and volume) to discrete (atoms, molecules and number of particles) concepts involved in chemical reactions.