Abstract
In this study, we investigated elementary student's understanding about dissolving. 21 G5 students and 21 G6 students were interviewed using"Contradiction -Explanation Method." The results were as follows. The onotological presupposition that unsupported objects fall downwards, was crucial base of student's understanding about dissolving. Subjects showed five types of conceptual integration of their ontological reasoning with scientific reasoning. Furthermore, the development trajectory was proposed in knowledge acquisition about the uniformity of solution. Finally, the implications of these results for science education were briefly discussed.
