<![CDATA[Previous studies have shown that Problem-Based Learning (PBL) effectively improves students’ critical thinking skills, learning outcomes, and engagement in stoichiometry. However, previous studies have mainly focused on learning outcomes. At the same time, the implementation of PBL through Classroom Action Research (CAR), which emphasizes continuous reflection and instructional improvement across cycles, has received limited attention. This study aimed to improve Grade 10 students’ critical thinking skills and learning activities in stoichiometry by implementing the PBL model. This study employed the Classroom Action Research (CAR) method involving 20 Grade 10 students and was conducted in two cycles, each consisting of planning, implementation, observation, and reflection. The success criteria were determined based on improvements in students’ critical thinking skills, learning activities, and learning mastery from Cycle I to Cycle II. Data were collected using pretests, posttests, student worksheets (LKPD), and observation sheets. The average posttest score increased from 82.50 in Cycle I to 85.25 in Cycle II, while student learning activities improved from 46.00% to 68.06%. The N-gain values were 0.254 in Cycle I and 0.175 in Cycle II, both categorized as low, indicating limited improvement due to students’ relatively high initial achievement. In addition, learning mastery increased from 80% to 85%. These findings indicate that implementing the PBL model through continuous reflection and instructional improvement can enhance students’ critical thinking skills and learning activities. Therefore, PBL can serve as an effective instructional strategy for chemistry teachers to promote active learning and foster students’ critical thinking skills in stoichiometry.]]>
