What is the primary characteristic of a phase change?

A phase change is a process where a substance transitions from one state of matter to another, such as from solid to liquid (melting) or liquid to gas (evaporation). The key characteristic of this process is that it involves energy transfer, typically in the form of heat, without a change in temperature. During a phase change, energy is absorbed or released by the substance as it overcomes intermolecular forces, allowing it to change state. For example, when ice melts into water, it absorbs heat but remains at 0 °C until the entire mass has turned into liquid.

Other options, although related to chemical or physical changes, do not accurately characterize phase changes. While energy conservation is a principle that always applies, it is not specific to phase changes and does not convey the unique aspect of temperature remaining constant during the process. Formation of a new substance and chemical reaction completion refer to chemical changes rather than physical changes like phase transitions, which do not alter the chemical composition of the substance.