The principles of process-oriented guided inquiry learning (POGIL) are applied to a binary solid-liquid mixtures experiment. Over the course of two learning cycles, students predict, measure, and model the phase diagram of a mixture of fatty acids. The enthalpy of fusion of each fatty acid is determined from the results. This guided inquiry experiment, which was developed as part of the POGIL-PCL (Physical Chemistry Laboratory) Project and tested at multiple institutions, relies on "green" materials and can be carried out by students using simple or sophisticated apparatus.