Conceptual understanding in chemistry requires students to connect quantitative calculations, microscopic chemical processes (e.g., atoms and molecules), and macroscopic outcomes (e.g., concentrations, color, temperature). The ChemCollective Virtual Lab's interactive simulation environment fosters the mapping between these representations by providing students with a fully functional online chemistry lab with realistic chemicals, glassware, and equipment. In the ChemVLab+ project (www.chemvlab.org), we enhance the virtual lab by developing intelligent tutoring for this open-ended simulation environment and creating embedded formative assessment activities. Students have opportunities to apply chemistry knowledge to meaningful contexts and receive immediate, individualized feedback as the system estimates their proficiency based on their actions. We report findings from a classroom field study of ChemVlab+ activities. Data from classroom observations, preposttest learning gains, detailed log file analyses, and teacher interviews suggest that students were actively engaged, that students improved their understanding of chemistry, and that teachers found the activities worthwhile.