Processes of Conceptual change in
Learning about The Nature of Matter

Conceptual change is a complex process composed by several learning mechanisms. Radical conceptual change or restructuring is only required when students' previous theories in a given domain are based on incompatible pressupositions (Vosniadu, 1994), ontologies (Chi, 1992) or conceptual structures (Pozo, 1996) with those supporting the scientific theory in this same domain. In this study, after applying several paper and pencil tasks to different groups of adolescents, as well as to adult novices and experts in Chemistry, we analyze the processes of conceptual change in the specific domain of Chemistry, in which it is required to change three main ideas or pressupositions about the nature of matter: conservation of matter, intrinsic motion of particles and the idea of vacuum. According to our results, these three basic concepts would require different kinds of conceptual change. Thus, the conservation of matter involves only the discrimination of different categories of change in matter (physical versus chemical), a process of conceptual revision or tuning. On the other hand, both the assimilation of the intrinsic motion of particles and the idea of vacuum would require a radical conceptual change, because these concepts are not compatible with students' implicit theories of matter, which assume, from the direct appearance of reality, that matter is static and continuous. However, this process of conceptual change should not be directed towards replacing poor organized and implicit, common-sense theories by more complex and explicit, scientific theories. In most cases, conceptual change is a process of contextual differentiation between theories, rather than a matter of replacement (Caravita and Halldn, 1994; Pozo, Sanz and Gomez, in press). Subjects would have different representations for different contexts: Learning Chemistry is not a matter of giving up common sense in everyday settings, but one of learning new, more complex, theories that are useful= for more demanding tasks. But in order to really understand scientific theories, students not only need to multiply and differentiate contextually their representations -they must also metacognitively integrate different hierarchical levels of knowledge. Scientific theories, like Russian dolls, should be able to embed more simple conceptual structures, but not viceversa. Conceptual change in fact involves not only differentiation between models (Pozo, Sanz and Gomez, in press) but also an increase in the explanatory coherence (Thagard, 1992) or consistency of the theories (Pozo and Gomez, in press). Thus, a really difficult requirement for any theory of conceptual change is to predict and explain how subjects activate discriminatively= their knowledge according to certain contextual variables and how the role played by the context changes as subjects become more expert in a given domain.