This study was part of a 6-year integrated project designed to build a databank of students' science conceptions in Taiwan. The main purpose of this study was to identify the characteristics of students' mental models regarding acids/bases, understand their changes in mental models, and explore sources that might influence students in constructing models. The study involved 38 ninth-grade students, focusing on three high-achieving students as well as three low-achieving students who were interviewed thoroughly. The results show that students mainly adhered to the Phenomenon Model, the Character-Symbol Model, the Inference Model, and the Scientific Model. Also, it is possible to draw a parallel between the characteristics of some models and various models of acidity in the history of science. The results also show differences in mental models and the weights of sources influencing high-achieving and low-achieving students. Evidence for the weight of sources is not consistent with the findings reported in other studies. We also discuss the implications for teaching and curriculum design.
