What does a phase change refer to?

A phase change refers to the transition of a substance between solid, liquid, and gas phases. This occurs when a substance absorbs or releases energy, primarily in the form of heat, which alters its temperature and structure, allowing it to change from one state to another.

For example, when ice (solid) is heated, it can melt into water (liquid); conversely, when water is cooled, it can freeze into ice. Similarly, water can evaporate into steam (gas) when heated, and steam can condense back into water when cooled. Understanding these transitions is essential in thermodynamics, as they involve changes in energy associated with the interactions between molecules in different states of matter.

While the other options touch on related concepts, they do not encompass the full definition of a phase change. The reaction of solids to heat may imply a change in temperature but does not specify the transition between states. Changing the temperature of a material is a process that can lead to a phase change but is not the phase change itself. The mixing of different states of matter describes a mixture rather than a singular transition of a material from one phase to another.