Figure above summarizes the basic transitions that a substance may undergo as a result of changes in temperature or pressure. For any phase change, energy will be either released (for exothermic processes) or absorbed from the surroundings (for endothermic processes). Processes in which intermolecular interactions are weakened or broken (melting, vaporization, and sublimation) require an input of energy; they are endothermic. Processes in which attractive interactions are formed (freezing, condensation, and deposition) release energy, so they are exothermic.
Bethany is curious about the changes that occur when water changes states due to temperature changes. To study this, she observes two different units of water. Unit A is frozen and will be heated until melted. Unit B is liquid and will be cooled until frozen.
Which unit of water will be undergoing an exothermic process?