In literature, several processes have been suggested to describe conceptual changes being undertaken. However, a few parts of studies analyse in great detail which students' learning processes are involved in physics classes during teaching, and how they are used. Following a socio-constructivist approach using tools coming from discourse analysis, we suggest studying three processes of students' learning: (1) establishing links between ideas, (2) increasing the domain of applicability of ideas, or (3) decreasing the domain of applicability of ideas. Our database consists of video data and written worksheets of two students at the upper-secondary school level (Grade 10 [15-year-old students]) during a one-month teaching sequence about gas. Based on semiotic resources contained in oral and written language, we reconstruct in great detail all the ideas about gas expressed by both students during the entire teaching sequence. Our analysis deals with (1) how learning processes are identified based on the ideas expressed by students, and (2) how the three learning processes are used by the two students during teaching. Our results show that during the teaching sequence: (1) the emergence of the networks of three ideas is supported by networks of two ideas expressed previously by students; (2) both students express more networks of two ideas than networks of three ideas; (3) the process "increasing the domain of applicability" of an idea or a network of ideas is very often involved; and (4) the process "decreasing the domain of applicability" of an idea or network of ideas is rarely used by them. (Contains 10 figures, 3 tables, and 3 notes.)