In this article we examine teacher instruction on scientific literacy tasks and teacher expression of ultimate and subordinate purposes during one teaching sequence of a science unit. By using a Practical Epistemology Analysis and Systemic Functional Grammar we can provide a view of the direction learning takes and the consequences for student text production. The material comprises transcribed audio recordings of teacher instruction, students' pair work and written texts. The results show that the students are mainly involved in hands-on activities while aspects of scientific literacy are not foregrounded. Language use is dominantly spoken and, when written text is requested, no explicit instruction on how to write is given, resulting in a variety of texts from 'more-spoken-"like"' to 'more-written-"like"' without adhering to scientific genre. Ultimate purposes are never expressed while subordinate purposes are to some extent made explicit, but obscured by the dominant focus on 'doing', resulting in uncertainty about why the activity is requested. As a result, the learning direction is not always in accordance with teacher intention.