States of Aggregation

of matter, states of the same substance (for example, water, iron, sulfur) the transitions between which are accompanied by discontinuous changes in free energy, entropy, density, and other fundamental physical properties. For example, water under normal pressure of 101,325 newtons per m² (760 mm of mercury) and at 0°C crystallizes into ice, and at 100°C it boils and becomes steam. Consequently, water is capable of existing in a solid, liquid, and gaseous state of aggregation. A fourth state know as plasma is frequently added to these three. The existence of several aggregation states is due to differences in the nature of thermal motion of molecules (or atoms) of matter and their interaction. In gases molecules interact only slightly and move freely, filling up the entire volume in which the gas is confined. In liquids and solids—the condensed-phase systems—the molecules (or atoms) are close to one another and interact very strongly. This causes liquids and solids to assume a definite volume. However, the nature of the motion of molecules in liquids and in solids is different, and this accounts for differences in their structure and properties. In solids in a crystalline state, the atoms execute only small oscillations about points in the crystal lattice; the structure of these bodies is characterized by a high degree of order—long-range order in the arrangement of atoms. The thermal motion of molecules of a liquid consists of a combination of small oscillations about equilibrium positions and frequent jumps from one equilibrium position to another. The jumps are responsible for the existence of only close range order in the arrangement of molecules (or atoms) in liquids and also for the mobility and fluidity characteristic of the liquid state.

Plasma is recognized as a special state of aggregation of matter because the charged particles of the plasma, in contrast to the neutral molecules of an ordinary gas, interact at long distances. This accounts for some of the unusual properties of plasma.

Transition from a structurally more ordered state of aggregation of matter to a less ordered state can occur either instantaneously (at a certain temperature and pressure) or continuously. The possibility of continuous transitions (such as of a liquid into vapor) is indicative of a certain arbitrariness in the definition of states of aggregation of matter. This arbitrariness is evident in existence of solid amorphous substances which retain the structure of liquids; of several forms of the crystalline state in a number of substances; of liquid crystals; of a special highly elastic state in polymers which is intermediate between the vitreous state and the liquid state; and of other phenomena. For these reasons, modern physics replaces the concept of states of aggregation of matter by the broader concept of phase.