Extant research on the efficacy of drawing for assessing learning in chemistry suggests it may provide richer information on the quality of students' mental models. Here, we examine the relative utility of formative assessments that involve drawing modalities as well as verbal and symbolic modalities. We analyzed the drawings, written explanations, and symbolic notations made by 36 students learning about chemical reactivity from a dynamic visualization. Our analysis demonstrates that drawing affords the emphasis of different conceptual features across different representational modalities. Students represented spatiotemporal structures (i.e., particle composition and location) more frequently in their drawings but represented spatiotemporal transformations (i.e., particle motion and interaction) more frequently in their written explanations. Students represented more conceptual features in both their drawings and written explanations than they did in the symbolic notation commonly used in the chemical sciences. Our findings suggest that assessment items that involve drawing can help evaluators construct richer models of student knowledge, but they should not be used in isolation from verbal or symbolic modalities.