[[abstract]]Although the concept of diffusion is usually classified as the simple concept in science, Marek(1986, 1988, 1991) argued that the ratio of students who could explain diffusion correctly is low and the conceptual change of that is difficult. Based on the theory of emergent and causal schemata proposed by Chi(2000), the purpose of this research is to investigate the understanding of diffusion possessed by eighth and ninth grade student who have learned the concept of particles. In addition, four kinds of instructional methods including text-teaching, POE-personal explanation, POE-group discussion, role-playing, were given respectively in order to make conceptual change happen. The students’ performance on conceptual change was examined via three perspectives: motivation and emotion, ontology, and epistemology, proposed by Tyson, Venville, Harrison, & Treagust (1997). The results are summarized as the following: 1. The misconceptions of particle of substance in liquid state are more in variety and ratio than which in solid or in gaseous state. In addition, cognition conflict for the conception of particle of liquid-state substance happens between daily experience and textual knowledge in textbook. 2. Although the particle view of pure substance and mixture in liquid state are consistent in science, they are not considered to be consistent for the students. And many students change their view of that even when different contexts of problem with the same substance are given. 3. Before instruction, only 4.4% of 181 students could explain diffusion correctly. After instruction, 25.3% students could explain it correctly. 4. Students view the property of diffusion as either the physical reaction or the chemical reaction. There are four kinds of mental models for each kind of property, and 23 submodels were found, corresponding to 44 kinds of pictoral and propostional dual representations. These four mental models are double-movable, single-movable, passive double-movable, and teleological models, each one of those has anchored position according to Vosniadou’s framework theory and Chi’s theory of ontological category. When conceptual change happens in the instruction, a lot of patterns of two-model changes are found frequently. 5. According to the interviews with the students and the instructional results of POE-personal explanation and POE-group discussion, a few students with emergent schemata did not choose to use them because of possessing the similar view of Aristotle about the dynamics of objects and the interference of properties of concrete operator analogied to the particle, and this finding violates the arguments proposed by Chi(2000) and Perkins & Grotzer(2000). 6. Comparing the performances of four different kinds of instruction in this study, role-playing instruction is the best of all. Because it attains to the significant difference of conceptual change, minimizes the misconceptions of substance in the liquid state and in the solid state, motivates students’ interest in learning, and makes the highest ratio of students to explain Brownian movement correctly. The difference between role-playing and POE-either personal explanation or group discussion is that POE couldn’t make students interested in learning. Text-teaching is the useless one if performed individually rather than combined with other kinds of instruction. 7. Comparing with Newton and Bernoulli, the extent of similarity and difference between scientists and students with different mental models is various.
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