Chemical equilibrium is among the most conceptually demanding topics in secondary chemistry, with students frequently developing persistent misconceptions due to its dynamic nature and the prevalence of teacher-centered instruction that limits inquiry and reasoning opportunities. This study examined how a Design-Based Research Inquiry Based Learning (DBR–IBL) approach an instructional framework that integrates iterative design cycles with student-centered inquiry pedagogy affected Grade 11 students’ conceptual understanding of chemical equilibrium and critical thinking skills. Rooted in constructivism and Chemistry Education Research (CER), the study employed a mixed-methods design implemented over six weeks in a public secondary school in Injibara City, Ethiopia. Quantitative data were collected using a Critical Thinking Test and a Chemical Equilibrium Diagnostic Test administered pre- and post-intervention. Qualitative data were gathered through structured classroom observations, semi-structured interviews, reflective discussions, and analysis of student learning artifacts. The intervention was iteratively refined through DBR cycles to address evolving student learning needs. Quantitative results showed significant achievement gains: mean scores rose from 29.30 (SD = 7.84) to 42.63 (SD = 3.48). A paired-samples t-test confirmed statistical significance, t(29) = – 13.75, p < 0.001, with a very large effect size (Cohen's d = 2.51). Qualitative analysis revealed five interconnected themes: conceptual restructuring of chemical equilibrium understanding, growth in critical thinking and scientific reasoning, increased engagement in collaborative inquiry, challenges in adapting to student-centered learning, and positive perceptions of chemistry real-world relevance. Collectively, the findings indicate that the DBR–IBL approach enabled students to actively construct scientific knowledge, confront persistent misconceptions, and engage in evidence based reasoning. The study contributes empirical support for design-oriented inquiry in complex chemistry instruction and recommends broader integration of DBR–IBL alongside further research into its long-term transfer ability across diverse educational settings