The topic of physical and chemical changes is one of the basic and essential issues of both the lowersecondary school science curriculum and the upper-secondary school chemistry curriculum in many countries. The focus of the present study is to investigate the students' cognitive structures on the topic of physical and chemical changes at different grade levels and to compare how the aspects of students' cognitive structures differ at 8th, 9th, and 10th-grade levels. The data were collected from a total of 388 students (126 8th-graders, 129 9th-graders, and 133 10th-graders) through using a word association test (WAT) which contains eight stimulus words. Both the relatedness coefficient (RC) method and the response frequency mapping method were used in the analysis of the data and mapping of the group cognitive structures of the students. The variables related to the concepts or ideas contained, the connections between the concepts, and the information processing skills, which are the three main aspects in defining the cognitive structures, are compared according to the grade level. It has been concluded that the cognitive structure of the 9th-grade students is the most comprehensive, and integrated, and has a higher information retrieval rate. On the other hand, when the data obtained from both the RC values and the response frequency mappings were examined, it was found that there were also several similarities in the cognitive structures of the three grade levels. For instance, the relationship level of the stimulus word 'energy' with other stimulus words is very weak at all grade levels.