Formative assessment is an important component of teaching as it enables teachers to foster student learning by uncovering, interpreting, and advancing student thinking. In this work, we sought to characterize how experienced chemistry teachers notice and interpret student thinking shown in written work, and how they respond to what they learn about it. Drawing on qualitative methods from different educational fields, we analyzed data collected during focus groups of middle and high school teachers. Using a "chemical thinking" lens, teachers' formative assessment practices were characterized as descriptive vs inferential in noticing, evaluative vs sense-making in interpreting, and directive vs responsive in acting. Four major patterns emerged in teachers' interpreting of student thinking and proposed acting. These patterns affected the diversity of ideas that teachers noticed in student work. Ways of using the findings are offered for chemistry teachers wishing to examine and diversify their own noticing practices, and for professional development efforts in this area.