This study attends to the possibilities of analyzing students' writings at a lexicogrammatical level in terms of their use of linguistic resources, and the insights afforded by this approach into the challenges students faced when they employed the language of school science related to the phenomenon of expansion. For the purpose of this study, the data consist of Grade 7 students' written assignments generated from a sequence of lessons on the topic of "States of Matter." We analyzed the language by employing the Systemic Functional Linguistics framework. The analysis reveals similarities and differences in the use of linguistic resources among the students, from which we identified several patterns of language use that appeared to be significant for realizing scientific meanings. These patterns of language use suggest that students face challenges not just in learning to use language as both a cognitive and a semiotic tool but also in interpreting the requirements of task when employing the language of school science. The findings demonstrate the productivity of this approach in expanding our understanding of the conceptual and representational challenges that students face when learning the language of school science. (Contains 4 figures and 4 tables.)