We examined how first year students (10th grade) of Greek Senior High School could conceptualize the influence of the molecular weight of saturated fatty acids on the melting and the freezing point, during the "change of phase" phenomenon using the Microcomputer-Based Laboratory (MBL) system. Students had to freeze a melted substance, observing at the same time the change of its temperature and trying to explain the temperature vs time curve. They used different saturated fatty acids in order to understand how the molecular weight of each organic acid affects the melting-freezing point. Data about the students' conceptualizations was collected using three methods: videotape recordings, field notes and supplementary semi-structured interviews. The analysis of the students' responses to the same questions before and after the experiments showed statistically significant differences on their responses in every case. After the experiments there was a clear increase of correct responses concerning the freezing point of the saturated fatty acids, the relation of the freezing point to the molecular weight and the description of this relation. (Contains 2 tables.)