This study was part of a 6-year integrated project designed to build up a databank for students' science conceptions in Taiwan. This study was based on our previous findings of the characteristic and sources of students' mental models about acids and bases. In this paper we discuss the link between these mental models and the teacher's pedagogical content knowledge. The authors adopted the naturalistic observation methods to observe a teacher's pedagogical content knowledge in a ninth grader classroom. The findings showed that the teacher's instruction nearly followed the teacher's guide, but used a "spiral" method of teaching and emphasized rote learning. In addition, in teaching propositions, the teacher spent the most time supplementing the calculation of pH values, which was covered in previous edition of the textbook. Furthermore, the teacher overemphasized the specific properties of special strong acids, rather than general properties. In teaching representations, the teacher spent most time on elaborating textbook, giving examples and calculating pH values. Everyday experiences, TV media and teaching experiments were important sources of teacher's examples. However, these sources were not based on students' preconceptions of acid and base, therefore her teaching reinforced students' incorrect understanding.
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