Learning fundamental concepts in science in order to develop deep understanding of complex scientific concepts is an essential task. However, research that uses rigorous methods to investigate how students' concepts develop over time is limited. In this article, we answer two key questions about learning progressions for the concept of phase transitions: (1) what mental models of matter and phase change do students possess across grades, and (2) what conceptual pathways do students follow for these concepts? We describe the results from a study involving fourth, fifth, and seventh through twelfth grade students (N= 832). Our work led to two main findings. First, students' mental models of the targeted concepts gradually shifted from direct and sensory to synthetic and then to scientific-like. Second, based on our cladistics analysis, we found that some crucial concepts developed before students possessed scientific models. Our results provide a novel perspective from which to revisit learning progressions associated with the development of core scientific knowledge.
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