This article examines the process of students' conceptual changes in respect of air pressure and buoyancy as a result of instructing with the Dual Situated Learning Model. The dual situated learning events of this model were designed according to the students' ontological viewpoint on the science concepts as well as on the nature of these concepts. Results demonstrated that the notion of buoyancy required more dual situated learning events for conceptual change to occur than that for air pressure. Instead of attributing the difficulty involved in conceptual change to the mismatch of ontological category of the concepts, the author proposes that the hierarchical level of the scientific concepts would determine how easy or difficult it is to bring out a conceptual change. Concepts of higher hierarchical level subsume more essential underlying concepts, thus making it more difficult for conceptual changes to occur.