Argumentation is one of the central practices in science learning and helps deepen students' conceptual understanding. Students should learn how to communicate ideas including procedure tests, data interpretations, and investigation outcomes in verbal and written forms through argument structure. This article presents a negotiation model to show how argument can be a vehicle to drive students to learn core ideas of density. The negotiation model consists of five phases: (1) creating a testable question, (2) constructing an argument in groups, (3) critiquing arguments publicly, (4) advancing students' arguments, and (5) writing and reflecting individually.