This document discusses using a phase change material (PCM) to improve the performance of a single basin solar still. It defines a PCM as a substance that absorbs and releases a large amount of thermal energy during its phase change from solid to liquid and back again. The document explains that a PCM maintains a regulated temperature by absorbing heat as the external temperature rises until its melt point, then releasing heat as the external temperature falls until it solidifies again. It lists the six main types of phase changes as evaporation, condensation, freezing, melting, sublimation, and deposition and provides examples like freezing and melting ice or boiling water.