Background and purpose: Active-learning as a student-centered learning process has begun to take more interest in constructing scientific knowledge. For this reason, this study aimed to investigate the effectiveness of active-learning implementation on high-school students' understanding of "acids and bases". Sample: The sample of this study was 45 high-school students (average age 17 years) from two different classes, which were randomly assigned to the experimental (n = 21) and control groups (n = 25), in a high school in Turkey. Design and methods: A pre-test consisting of 25 items was applied to both experimental and control groups before the treatment in order to identify student prerequisite knowledge about their proficiency for learning "acids and bases". A one-way analysis of variance (ANOVA) was conducted to compare the pre-test scores for groups and no significant difference was found between experimental (M[subscript E] = 40.14) and control groups (M[subscript C] = 41.92) in terms of mean scores (F[subscript 1,43] = 2.66, p greater than 0.05). The experimental group was taught using an active-learning curriculum developed by the authors and the control group was taught using traditional course content based on teacher-centered instruction. After the implementation, "Acids and Bases Achievement Test" scores were collected for both groups. Results: ANOVA results showed that students' "Acids and Bases Achievement Test" post-test scores differed significantly in terms of groups (F[subscript 1,43] = 102.53; p less than 0.05). Additionally, in this study 54 misconceptions, 14 of them not reported in the literature before, were observed in the following terms: "acid and base theories"; "metal and non-metal oxides"; "acid and base strengths"; "neutralization"; "pH and pOH"; "hydrolysis"; "acid-base equilibrium"; "buffers"; "indicators"; and "titration". Based on the achievement test and individual interview results, it was found that high-school students in the experimental group had fewer misconceptions and understood the concepts more meaningfully than students in control group. Conclusion: The study revealed that active-learning implementation is more effective at improving students' learning achievement and preventing misconceptions. (Contains 6 tables and 1 figure.)