The present study describes high school students' conceptions about acids and bases in terms of pH, pOH, microscopic level, strength, and concentration. A total of 27 high school students participated in the study. The data was collected using 3 POE tasks and a semi-structured interview. The data analysis demonstrated that most of the students had poor understanding related to a drawing of weak and strong acids. In addition, the findings revealed that the POE's were effective in terms of gathering students' predictions and reasons for the prediction of outcomes in an open-ended format. The POE tasks also revealed that some of the students had misconceptions regarding pH and pOH. The students believed that pH was a measurement of the acidity, while pOH was a measurement of the basicity. The findings obtained have certain implications for the secondary chemistry program.