What occurs to the temperature of a substance during a phase change?

During a phase change, the temperature of a substance remains constant because the energy supplied to the substance is used to alter the structure of the molecules rather than to increase the kinetic energy of the molecules, which is what would raise the temperature. For example, during the process of melting, heat is added to ice. Although this heat is absorbed, it does not cause the temperature of the ice to rise until all of the ice has melted into water. Instead, the energy is utilized to break the hydrogen bonds between the ice molecules. Similarly, during boiling, when water transitions from liquid to vapor, the temperature of the water remains constant as it absorbs heat to convert into steam.

This concept is essential in understanding how heat and temperature relate to phase changes, which is critical in HVAC applications where materials change states often, impacting system performance and efficiency.