Researchers believe that the way that students talk, specifically the language that they use, can offer a window into their reasoning processes. Yet the connection between what students are saying and what they are actually thinking can be ambiguous. We present the results of an exploratory interview study with 10 participants, designed to investigate the role of language in university physics students' reasoning about heat in thermodynamic processes. The study revealed two key findings: (1) students' approaches to solving certain heat-related problems are related to the way in which they explicitly define the word 'heat' and (2) students' tendency to reason with heat as a state function in inappropriate contexts appears to be connected to a model of heat implicitly encoded in language. This model represents heat or heat energy/thermal energy as a substance that moves from one location to another. In this model, students talk about thermodynamic systems as 'containers' of heat, and temperature is a measure of the amount of heat 'in' an object.