This study assessed the quality of preservice primary teachers' written arguments in an open inquiry laboratory task. An analysis of the features of the laboratory task was also undertaken to ascertain the characteristics of the task that facilitated or constrained the development of participants' written arguments. Australian preservice primary teachers (n = 12) took part in a chemistry content course incorporating an inquiry oriented learning environment, explicit argumentation instruction, and numerous opportunities to engage in argumentation. Data sources included group written artefacts from an open inquiry laboratory task, and results indicated that two of the three groups failed to produce quality arguments in the written task. Data analysis indicates that a myriad of factors may have mediated groups' argument quality in the laboratory task including the adequacy of individual participants' background science conceptual knowledge, a lack of argumentation scaffolds in the task, the non-provision of alternative data, viewing the data as self-evident, a reliance on traditional reporting genres, and the non-inclusion of critical discussions. The laboratory task included a number of cognitive and epistemological features which were not aligned with authentic scientific inquiry including limited opportunities for extensive data transformation, complex theory-data coordination, utilisation of complex methods, multiple types of reasoning, generalisation of results to other contexts, and engaging in the review of "expert research". Implications of these findings highlight the importance of scrutinising the inquiry tasks learners engage with, in the chemistry laboratory context, to ensure they promote authentic scientific reasoning.