Our study base on the teaching about gas at the upper school level. Many researches about the students' conceptions on gases have showed the difficulties to learn the concepts associate to gas. Base on these studies and with a socio-constructivist approach, we have designed a teaching sequence on gases in a group of research-development, which is composed by teacher and researcher. We have made the choices to introduce the microscopic model of gas at the beginning of the sequence to facilitate the interpretation of macroscopic variables. This work proposes to follow the student's learning during this teaching sequence. We have adopted two strategies: the first is a quantitative approach which consists in making a survey in several classes (about 100 students) and the second is a qualitative approach which involves a case study of 2 students. The questionnaire is used to assess the impact of teaching on the students' conception in three classes. To studying the evolution of the student's ideas finely, with an in-depth case study. Two fifteen students, who were at the upper secondary school level (grade 10), have been filmed in a real class room during a three weeks teaching sequence on gases. Two methodologies have been used for analysis these videos: (1) reconstructed the student's ideas trough the language with the context in which it has been produced and (2) based on the study of the different student's actions during this teaching on gases. Trough these analysis, we can follow the evolution of student's ideas depending on her activity, and we can see how a new idea becomes stabilised.
