Phase and State Changes refers to the transformation of a thermodynamic system from one phase or state to another. Although the terms ‘phase’ and ‘state’ are used interchangeably, they are in fact different ways to describe a chemical system. A ‘state’ of matter is a more specific term than a ‘phase’ of matter. The state of matter of a particular system is the state of a particular system within that system, while a phase can not only refer to a set of states within such a system, but also the type of molecular motion as well as pinpointing a particular region in three dimensional space.
A phase or state change usually occurs as a result of a change in an external condition, which changes the energy of the whole system. An increase in temperature would increase the energy of the whole system, giving the matter contained within the defined system more internal/heat energy. If enough energy is applied to the system, solids will change to liquids through the process of melting; liquids will change into gases through the process of vaporization; and gases may even change into plasma through the process of ionization.
Although phase changes are frequently seen in nature, for example snow melting into water, chemists and engineers may manipulate external conditions in the laboratory to exploit certain types of phase changes. Thus, understanding the thermodynamic rationale behind phase changes is important for understanding the energetics of a given chemical system.