The phase changes involving the absorption of heat are melting, vaporization, sublimation and ionization. Melting describes the process of a solid turning into a liquid, while vaporization occurs when a liquid changes into a gas. Ionization is the process used to convert a gas into plasma, and the process by which a solid turns directly into a gas is known as sublimation.
All four phase changes require either an increase in temperature, a decrease in pressure or both, and they are referred to as endothermic reactions because they absorb heat. The increase in heat leads to an increase in kinetic energy among the molecules of the substance. The increase in kinetic energy leads to more movement by the molecules, allowing them to spread out further into the lower pressure state of matter. A gas changes into plasma either through a temperature increase, as with the other phases, or through an electromagnetic charge. In this change, the electrons become so energized that they jump between atoms, changing the charge of the molecules. In addition to the traditional four states of matter, other intermediate states may occur due to heat absorption. Additional phases of matter include glass, crystals, superfluids, condensate, degenerate matter and quark-gluon plasma.