Language development is a critical issue of the investigative approach in science. Writings are very important in science teaching. It was interesting to verify if writings influence the change of ideas of pupils and their learnings. Two types of approaches were proposed to the pupils : a transmissive approach with a few writings made by the pupils, and an investigative one with more writings in order to determine their role. The results show that the ideas of pupils change with the two types of approach, but scientific concepts are better acquired when pupils are in a research situation. Writings generated by pupils allow a better knowledge anchoring, permitting them to check information by referring to their previous work and to visualise their line of reasoning. Pupils can discuss about these writings, enabling them to progress in their research. Writing impact on learnings is nevertheless difficult to quantify.