This study aimed to investigate the effects of Problem Based Learning on high school students' understanding of ionization of water and acid and base strength. Students' beliefs about Problem Based Learning were also analysed. A quasi-experimental design was conducted in this study. While students in the experimental groups were instructed via Problem Based Learning, teacher-centred approach was used in the control groups. Before the instructions, a prerequisite knowledge test was applied to identify their prerequisite knowledge to learn the topics, and no significantly differences were found between experimental and control groups. After the instructions, a post-test was applied to determine their understanding of the topics. The results indicated that the mean scores of the students in the experimental groups were significantly higher than those in the control groups. Problem Based Learning Assessment Scale results reflected that students' positive beliefs increased after each activity. Based on these results, it can be concluded that PBL instruction is effective in concept learning in chemistry education.