This case study of a science lesson, on the topic thermal expansion, examines the language demands on students from an integrated science and language perspective. The data were generated during a sequence of 9 lessons on the topic of "States of Matter" in a Grade 7 classroom (12-13 years old students). We identify the language demands by comparing students' writings with the scientific account of expansion that the teacher intended the students to learn. The comparison involved both content analysis and lexicogrammatical (LG) analysis. The framework of Systemic Functional Linguistics was adopted for the LG analysis. Our analysis reveals differences in the meaning and the way LG resources were employed between the students' writings and the scientific account. From these differences, we found the notion of condition-of-use for LG resources to be a significant aspect of the language that students need to appropriate in order to employ the language of school science appropriately. This notion potentially provides a means by which teachers could concurrently address the conceptual and representational demands of science learning. Finally, we reflect on how the complementary use of content analysis and LG analysis provides a way for integrating the science and language perspectives in order to understand the demands of learning science through language.