In this paper we discuss the role of language in the process of elaborating chemistry concepts in the classroom. We analyse a teaching episode showing the relationship between scientific language and common language and the contradiction, present in the constructivist classroom, between the convenience of giving voice to the students and the need of following a previous instructional plan. We try to show how this approach fits in a change of perspective in science education research, from an analysis centered towards an analysis of classroom as a socio-cultural space
