The knowledge to be taught regarding chemical changes and equilibrium states in the French chemistry syllabus (grade 12) has been categorized in terms of theory, models and reality. The experimental level is split into reality-as-perceived and reality-as-idealised which leads to two descriptions of experiments. The different models that interpret an experimental situation depending on the issues raised are presented. This categorization of knowledge constitutes a framework which is put to the test to analyse students' understanding during teaching sessions. We seek to determine whether this framework enables to characterize the different kinds of knowledge at stake in the classroom and students' reasoning about chemical changes. Two case studies are set out: in the first one the students had to write a chemical equation to predict a chemical change knowing the composition of a mixture whereas in the second one they had to write the chemical equation after having done an experiment. The interactions between the teacher and the students are analysed in terms of concepts attributed to the different levels of the framework. The analyses reveal the students' difficulty to master the chemical equation, to mobilize a kinetic model, to make a proper link between the model level and the reality-as-idealised or the reality-as-perceived.