Scientists typically refer to a substance changing its physical form as a changing state of matter. These changes to the state of matter are from changes in temperature and air pressure, which affect the density of solids, liquids and gases by either pushing the particles closer together, producing a denser state or pulling them further apart.
The term for when a solid turns into a liquid, such as when ice turns to water, is melting. When the opposite happens, and the liquid solidifies, the process is called freezing. When a liquid turns into vapor, it is changing its state of matter from a liquid to a gas in a chemical change referred to as vaporization, and the reverse change of state, from a gas to a liquid, is called condensation. A solid can also turn directly into a gas through a process called sublimation, such as when dry ice melts. The process called deposition occurs when a gas turns directly into a solid, such as when frost forms on a window.
In addition to the usual solid, liquid and gaseous states, certain elements can also take on a plasma state through an ionization process. Only gases can ionize and become plasma, typically when they undergo a significant increase in temperature.