La vecchia e la nuova definizione di mole: le ragioni della metrologia vs. quelle della chimica

In recent years, the definition of mole, the unit of the amount of substance, has been modified in order to express all units of the International System by direct reference to universal constants. The old definition refers explicitly to both mass and elementary units; it suggests the idea that the mole is a bridge between the macroscopic and microscopic registers. On the other hand, the new definition emphasizes the counting aspect, referred to any kind of elementary unit. Paradoxically, this results in the disappearance of substance from the unit of the quantity amount of substance. Either positive or negative remarks to this change of definition have been expressed by several authors that highlight its epistemological, disciplinary, lexical and educational implications. We analyze some of these aspects. More in details, we discuss the thesis that the new definition of mole reflects a view of chemistry according to which the microscopic perspective prevails, and this entails the loss of reference to the macroscopic register, which is nevertheless fundamental to the construction of meanings. This could be related to the profound changes undergone by the cognitive practices of chemistry in this last century.