Students' evaluation of findings that do not match hypotheses they have established based on their current understanding and at the same time do not expect has an important role in their conceptual changes in inquiry activities. One of the issues that may encounter unexpected findings is the issue of chemical energy. Most students believe that spontaneous reactions will be exothermic, and that endothermic reactions can occur by external heating. In this study, the scientific reasoning of the prospective teachers who participated in an inquirybased experiment about deciding the heat of a spontaneous endothermic reaction as a rare phenomenon in daily life was examined. The study was conducted with 58 prospective teachers in the first year of a faculty of education.The data were collected from prospective teachers' experiment sheets and interviews . When the hypotheses established by the participants and their decisions about the heat of the reaction were examined, it was determined that some of them exhibited consistent scientific reasoning, while many showed confirmation bias; while some of them changed their alternative understanding, most of them did not. Based on these results, it may be suggested to conduct studies examining the scientific reasoning of learners towards different phenomenon.