[Encyclios]

States and phase transitions of matter

The states of aggregation of matter depend both on the nature of the matter and on the temperature and pressure of the environment in which it is located; based on the variations of these two environmental parameters, physical transformations also called state transitions (or phase transitions) take place. Matter can exist in several states, also called phases; the four fundamental states are:

• solid state;
• liquid state;
• gaseous state;
• plasmatic state.

These four descriptions, each implying that the matter has certain physical properties, represent the three phases of matter. A single element or compound of matter might exist in more than one of the three states, depending on the temperature and pressure.

A phase transition is a physical process in which a substance goes from one phase to another. Usually, the transition occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point of the substance.

The nature of the phase change depends on the direction of the heat transfer. Heat going into a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat from a substance changes a gas to a liquid or a liquid to a solid.

• solid to liquid = melting (or fusion)
• solid to gas = sublimation
• liquid to gas = boiling, evaporation, vaporization
• liquid to solid = solidification, freezing
• gas to liquid = condensation
• gas to solid = deposition
• gas to plasma = ionization
• plasma to gas = deionization, recombination

A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium, certain properties of the medium change, often discontinuously, as a result of the change of some external condition, such as temperature, pressure, or others. A phase transition is achieved by changing the thermodynamic parameters to reach a particular limit.