The advocacy for writing in science and STEM-related fields through reasoning-based writing, critical thinking and argumentation has been accentuated as a core competence in curriculum documents. In South Africa, the CAPS document for Physical Sciences acknowledged the importance of writing and consequently, call on teachers to engage in teaching language across the curriculum. This study examined the characteristics of writing prompts in three selected samples of textbooks to understand how writing is addressed in the Physical Sciences textbooks. Specifically, the study examined the frequency and characteristics of writing prompts included in the chemistry component of three selected Physical Sciences textbooks for Grade 12 learners. A total of 533 writing prompts were identified and analyzed using a coding scheme adopted for the study. Findings revealed that learners are prompted to write in variety of ways, with prompts that positioned them to explain why, discuss your ideas, explain how and explain what. While the prompts that ask learners to explain why and discuss your ideas were the most frequently coded, the press to write their reasoning with specific features that are inherent in the respective topics were inadequate. The implications for curriculum developers are the need for explicit instructions on the length of writings required, and for authors to develop adaptable guidelines to enable textbook users understand the writing expectations.