Context-based science courses stimulate students to reconstruct the information presented by connecting to their prior knowledge and experiences. However, students need support. Formative assessments inform both teacher and students about students' knowledge deficiencies and misconceptions and how students can be supported. Research on formative assessments suggests a positive impact on students' science achievement, although its success depends on how the formative assessment is implemented in class. The aim of this study was to provide insights into the effects of formative assessments on achievement during a context-based chemistry course on lactic acid. In a classroom action research setting, a pre-test/post-test control group design with switching replications was applied. Student achievement was measured in two pre-tests, two post-tests and a retention test. Participants were Grade 9 students from one secondary school in the Netherlands. Repeated-measures analysis showed a significant effect of formative assessments on students' achievement. During the implementation of the formative assessments, intriguing discussions emerged between students, between students and teacher, and between teachers. Adding formative assessments to context-based approaches reinforces their strength to meet with the current challenges of chemistry education. Formative assessments affect students' achievement positively and stimulate feedback between students and teacher(s).