Understanding the particulate nature of matter (PNM) is vital for participating in many areas of science. We assessed 11 students' atomic/molecular-level explanations of real-world phenomena after their participation in a modelling-based PNM unit. All 11 students offered a scientifically acceptable model regarding atomic/molecular behaviour in non-heated solids. Yet, 10 of 11 students expressed the view that, in response to added heat energy, atoms/molecules in a solid increase in movement to a degree beyond what is scientifically accepted. These students attributed a gas-like model of atomic/molecular movement to situations involving a heated solid. Of the students who held two conflicting models of atomic/molecular movement in solids, almost all provided justification for doing so, indicating their holding of the conflicting models was unproblematic. These findings can be interpreted to mean that students may drop constraints of certain scientific representations and apply, assess, or revise models when explaining unfamiliar phenomena. In fact, we believe students may develop conflicting causal models as a result of misperceptions they acquire, in part, during classroom instruction regarding atomic/molecular movement. However, our findings may also be interpreted as an incidence of student model development that may later aid their understanding of a more complex model, one that involves substantial sub-atomic electron movement to account for heat transfer in solids. Whether or not this is the case remains to be seen. Implications for student learning and instruction are discussed. (Contains 2 notes.)