The teacher’s activity in the classroom relies upon both the determinants of the teaching learning situation (students, content, syllabus, school…) and the beliefs and knowledge of the teacher. This study addresses the activity of two teachers working on the same subject, the spontaneous evolution of chemical systems in a 12th grade form. During a training session aiming at developing the teachers’ professional knowledge (content knowledge, CK and pedagogical content knowledge, PCK) upon this topic, each teacher presented his/her lesson plan and goals before implementation. Then each teacher watched his/her own classroom video to comment and discuss his/her choices and actions and the students’ behaviours. The analysis of the way the classroom situation evolved shows that the classroom management of the experienced teacher gave more possibilities for the students to express their reasoning. Consequently, watching the video, the experienced teacher could reflect on her students’ reasoning and showed various types of knowledge by commenting her actions: pedagogical knowledge, PK, and knowledge of students’ difficulties and knowledge of instructional strategies to overcome these difficulties which are two components of PCK. The beginner teacher expressed limited professional knowledge (whether CK, pedagogical knowledge, PK or PCK). The discussion will propose some reasons.