The effective use of formative assessment (FA) has been demonstrated to confer positive impacts on student learning. To understand why and how FA works, it is necessary to characterize teachers' FA practices, but because both teaching practice and learning depend on the nature of the discipline, there are disciplinary aspects to examining this. This study aimed to develop an analysis of chemistry teachers' FA practices through the lens of the chemical thinking framework. Two cohorts of middle and high school science teachers participated in year-long professional development with the goal of improving their FA practices in teaching chemistry. Each teacher submitted FA portfolio chapters throughout the year. To develop an approach to use in ongoing research that will analyze teachers' progress across the year, the final FA portfolio chapters of participants (N = 13) were analyzed to characterize FA task design, the teacher's purpose in implementing the FA, and how the teacher evaluated student work. FA tasks were found to range from revealing students' mastery of concepts to uncovering students' chemical thinking. Teachers also demonstrated a range of purposes behind their use of the FA tasks, and a range of focuses when evaluating student work. Correspondence among the FA task, a teacher's purpose for its use, and the teacher's evaluation approach revealed patterns that echo the broader research in science education, but with instantiation in chemistry. Ways for teachers to assess and diversify their own FA practices based on these findings are presented.