The aim of this longitudinal research is to examine the development process of scientific and consistent knowledge structures of first, second, and third-grade students in middle school within the concepts of melting and dissolution according to conceptual change theories at the end of the education and training processes carried out in educational institutions. The research is carried out in a district of a metropolitan city in the Marmara Region with the participation of 43 middle school students. Open-ended questions developed for concepts were used to collect data. Friedman test is used to determine whether there is a difference between students' consistent and scientific knowledge structure scores regarding the concepts of melting and dissolution. As there is a significant difference between the students' consistent and scientific knowledge structure scores regarding the concepts of melting and dissolution, the Wilcoxon Signed Rank Test is used for dual comparisons between student levels. In addition, Spearman's correlation coefficient of rank differences is used to determine if there is a relationship between the class levels of the students and the scores of students' consistent and scientific knowledge structure regarding the concepts of melting and dissolution. As a result of the research, it is determined that the students' consistent and scientific knowledge structure scores regarding the concepts of melting and dissolution differ depending on the students' levels. When all the findings obtained from the research are evaluated together, it can be said that the development process of students' knowledge structures regarding the relevant concepts is more compatible with the knowledge in pieces conceptual change theory. Based on the results of the research, some suggestions were made.