This study examines the impact of argumentation on pre-service science teachers' (PST) conceptual understanding of chemical equilibrium. The sample consisted of 57 first-year PSTs enrolled in a teacher education program in Turkey. Thirty two of the 57 PSTs who participated in this study were in the experimental group and 25 in the control group. The experimental group students learned the concept of chemical equilibrium through argumentation; the control group students learned the same concepts through business as usual (i.e. lectures, supported by laboratory experiments). The intervention lasted for 12 instructional hours, of which 4 were spent in the laboratory. The chemical equilibrium concept test was administered to both groups of students one week after the intervention. The results show that the experimental group students performed significantly better than then control group students on the chemical equilibrium concept test. The mean difference between two groups is 14.026. This difference is statistically significant at (*p < 0.001). However, the control group students performed significantly better on the comprehensive course final exam.