In the studies reported here, we investigate the effects of context on students' molecular-level ideas regarding aqueous solutions. During one-on-one interviews, 19 general chemistry students recruited from a two-year community college and a research university in the United States were asked to describe their molecular-level ideas about various aqueous solutions in the contexts of conductivity and boiling-point (BP) elevation. Results indicate that context is important for determining the molecular-level ideas that students express. Specifically, students were significantly more likely to draw pictures of aqueous NaCl as separated ions in the conductivity context compared with the BP elevation context, for which they more often drew "molecular" NaCl. This phenomenon was particularly striking because the students drew molecular-level NaCl(aq) pictures in the BP elevation context just minutes after completing the identical task in the context of conductivity. Additional data from laboratory assignments and course examinations further indicate that, even if students are able to correctly represent the molecular level in some contexts, their knowledge may remain inert in slightly different contexts. The results emphasise the importance of the context dependence of molecular-level ideas and have implications for designing instruction in which students develop robust, coherent understandings that they can apply appropriately in new contexts.