What is a phase change?

A phase change refers to a transition from one state of matter to another, such as from solid to liquid (melting), liquid to gas (evaporation), gas to liquid (condensation), and so forth. This process involves the absorption or release of energy, typically in the form of heat, but does not result in a change in temperature during the phase change itself. For example, when ice melts into water, the temperature remains constant at 0 degrees Celsius until all the ice has turned into water, despite the addition of heat.

In contrast, changes in temperature, chemical reactions, or pressure changes do not define a phase change. A change in temperature typically involves an increase or decrease in molecular motion but does not alter the state of matter. Similarly, a chemical reaction involves a transformation that changes the chemical composition of a substance rather than merely changing its state. A change in pressure can affect the state of a substance and its phase transitions but is not itself a phase change. Therefore, the accurate definition of a phase change is that it is a transition between different states of matter.