This paper discusses seventh-grade students' explanations of dissolution and combustion and also identifies their understanding of the differences between physical and chemical changes. A teaching strategy was initially negotiated within an action research group and this strategy was then employed in teaching seventh-grade students. The teaching approach applied the idea that discrete particle changes can be used to differentiate chemical reactions from simple physical changes. Data were collected by an action research group teacher who conducted interviews with dyads of students from different chemistry classrooms. The interviews were transcribed, subsequently analyzed and evaluated in co-operation with researchers from the university. The main mistakes and alternative conceptions that have been identified are discussed. Further, some implications for developing appropriate teaching strategies and curriculum materials are also summarized.