liquid
[′lik·wəd]What does it mean when you dream about a liquid?
Because of the perceived “fluid” nature of emotions, liquids can symbolize emotions. Additionally, liquids are sexual symbols. Any liquid can also represent alcohol, as in the expression “liquid refreshment.”
Liquid
the state of aggregation of matter that is intermediate between the solid and gaseous states. Although liquids retain some features of both solids and gases, they also have a number of features, such as fluidity, that are inherent exclusively in them. Like solids, liquids retain their volume, have free surfaces, and have a certain tensile strength upon omnidirectional extension. On the other hand, a liquid in sufficient quantity assumes the shape of the container into which it has been placed. The possibility in principle of a continuous transition from liquid to gas indicates the similarity of the liquid and gaseous states.
A distinction is made among one-component, or pure, liquids and two-component or multicomponent liquid mixtures (solutions), according to chemical composition. According to their physical nature, liquids are divided into normal (common) liquids; liquid crystals, with strongly developed anisotropy (dependence of properties on direction); and quantum liquids, such as liquid 4He, 3He, and their mixtures, which have special quantum properties at extremely low temperatures. Normal pure liquids have only a single liquid phase—that is, only a single type of each normal liquid exists. Helium 4He may exist in two liquid phases, the normal and the superfluid, and the liquid-crystalline materials may exist in the normal phase and one or even several anisotropic phases.
Macroscopic homogeneity and isotropy in the absence of external influences is a common feature of all normal liquids, including mixtures. These properties make liquids akin to gases but differentiate them sharply from anisotropic crystalline solids. From the modern point of view, amorphous solids (for example, glasses) are supercooled liquids and are distinguished from the common liquids only by the numerical values of their kinetic characteristics, such as considerably higher viscosity. The range of existence of the normal liquid phase is limited at the lower end of the temperature range by phase transition to the solid state, or crystallization, or (depending on the magnitude of applied pressure) by phase transition to the superfluid state for 3He and 4He and to the fluid anisotropic state for liquid crystals. At pressures below critical pressure/?,., the normal liquid phase is limited at the higher end of the temperature range by the phase transition to the gaseous state, evaporation. At pressures p > pc, the phase transition is absent and the liquid becomes indistinguishable in its physical properties from a dense gas in this region. The highest temperature Tc at which the liquid-gas transition is still possible is called the critical temperature. The values of pc and Tc determine the critical point of the pure liquid, at which the properties of the liquid and gas become identical. The existence of the critical point for the liquid-gas transition makes possible a continuous transition from the liquid state to the gaseous state, avoiding the region in which the gas and liquid coexist.
Thus, upon heating or a reduction in density, the properties of a liquid (thermal conductivity, viscosity, self-diffusion, and so on) change, as a rule, in such a way as to approach the propertiesof the gas. Near the crystallization temperature, however, most of the properties of normal liquids (such as density, compressibility, specific heat, and electrical conductivity) are close to the corresponding properties of solids. The values for heat capacity at constant pressure (cp) of a number of materials in the solid and liquid states at the crystallization temperature are given in Table 1. The small difference between these values indicates that the thermal motion in solids and liquids near the crystallization temperature has approximately the same nature.
| Table 1. Heat capacity of some substances at crystallization temperature [J/(kg.°K)] | ||||||
|---|---|---|---|---|---|---|
| Na | Hg | Pb | Zn | Cl | NaCl | |
| Cpi Solid.................. | 1,382 | 138 | 146 | 461 | 1,620 | 1,405 |
| cpi liquid.................. | 1,386 | 138 | 155 | 542 | 1,800 | 1,692 |
Molecular theory of liquids. The forces of intermolecular interaction are of identical nature in liquids and crystals and are of approximately the same magnitude. The existence of strong intermolecular interactions in liquids leads, in particular, to the existence of surface tension at the interface between a liquid and any other phase. Because of surface tension, the liquid tends to assume a shape such that its surface is minimal (at a given volume). Small volumes of liquid usually have the characteristic shape of a drop. In the absence of external forces, when only intermolecular forces are acting (for example, under conditions of weightlessness), the liquid assumes a spherical shape. The effect of surface tension on the equilibrium and motion of the free surface of liquids, the boundaries between liquids and solids, or the boundaries between immiscible liquids are part of capillary phenomena.
The phase state of matter depends on the physical conditions in which it exists—mainly on the temperature T and the pressure p. The characteristic governing factor is the ratio ε(T, p) of the mean potential energy of intermolecular interaction to the molecules’ mean kinetic energy. The ratio depends on the temperature and pressure. For solids, ε(T, p) ≫ 1; this means that the forces of intermolecular interaction are large and hold the molecules, atoms, or ions in the vicinity of their equilibrium positions—the crystal lattice points— in spite of the thermal motion of the particles. Thermal motion in solids has the character of collective oscillations of the atoms (ions) about the crystal lattice points.
Gases represent the opposite limiting case, ε(T, p) ≪ 1; the attractive forces between molecules are insufficient to keep them in close proximity to each other, as a result of which the positions and velocities of molecules are distributed almost randomly.
For liquids, ε(T, p) ~ 1; the intensities of the ordered intermolecular interactions and the disordered thermal motion of the molecules have about the same magnitudes, which determines the specific features of the liquid state of matter. The thermal motion of molecules in the nonmetallic liquids consists of a combination of collective oscillations of the same type as in crystals and of jumps of the molecules between temporary equilibrium positions (centers of oscillation) that occur from time to time. Each jump takes place when a molecule receives an activation energy sufficient to break its bonds with the surrounding molecules and for its jump into the environment of other molecules. The result of a large number of such jumps is a more or less rapid mixing (self-diffusion takes place, which may be observed, for example, by the method of tagged atoms). The characteristic frequencies of the jumps are ~1011-1012 sec-1 for the low-molecular-weight liquids; they are much lower for the macromolecular liquids, and in some cases—for example, in highly viscous liquids and glasses—they may be extremely low. In the presence of an external force that retains its direction for a longer time than the intervals between jumps, the molecules are displaced on the average in the direction of the force. Thus, static or low-frequency mechanical actions lead to the manifestation of the fluidity of liquids as the total effect of a large number of molecular transitions between temporary equilibrium positions. If the frequency of actions exceeds the characteristic frequencies of the molecular jumps, elastic effects (for example, shear elasticity) typical of solids are observed in liquids. The molecular theory of liquids explains the homogeneity and isotropicity of normal liquids by the absence in liquids of long-range order in the relative positions and orientations of the molecules. The positions and orientations of two or more molecules located far from each other are found to be statistically independent. As a rule, a “rigid” long-range order in the orientation and location of the molecules, atoms, or ions exists in an ideal crystal. In liquid crystals, long-range order is observed in the orientation of the molecules but not in their location.
Liquids are sometimes divided into nonassociated and associated, according to the simplicity or complexity of their thermodynamic properties. Associated liquids are assumed to contain relatively stable groups of molecules, or complexes, which behave as a unified whole. The existence of such complexes in some solutions is proved by direct physical methods. The presence of stable associations of molecules in one-component liquids has not been conclusively demonstrated.
The basis of modern molecular theories of the liquid state was the experimental discovery in liquids of short-range order, which is the correlation in the relative positions and orientations of closely situated groups consisting of two, three, or more molecules. These statistical correlations, which determine the molecular structure of the liquid, cover a region extending over several interatomic distances and rapidly disappear for particles separated from each other by great distances (absence of long-range order). Structural studies on real liquids, which made possible the establishment of this feature of the liquid state, are performed by methods of X-ray diffraction analysis and neutron-diffraction study.
Liquids are divided into simple and complex categories according to their structure and the methods for their description. One-component atomic liquids belong to the first, relatively small class. The description of such liquids requires only the identification of the relative position of their atoms. This class includes pure liquid metals, liquefied inert gases, and (with some reservations) liquids with symmetrical molecules containing few atoms—for example, CC14. For simple liquids, the results of X-ray or neutron diffraction analysis may be expressed in terms of the so-called radial distribution function g(r), which is shown in Figure 1. The
function characterizes the distribution of particles in the vicinity of an arbitrarily selected particle, since the values of g(r) are proportional to the probability of finding two atoms or molecules at a given distance r from each other. The shape of Ihe g(r) curve graphically shows the existence of a certain degree of orderliness in a simple liquid; on the average, the immediate vicinity of each particle contains a fixed number of particles. The elements of the g(r) function change only insignificantly with temperature and pressure. The distance to the first peak determines the mean interatomic distance, and the mean number of nearest neighbors (mean coordination number) of the atom in the liquid may be determined from the area under the first peak. In most cases, these characteristics near the melting line are close to the smallest interatomic distance and coordination number in the corresponding crystal. However, in contrast to a crystal, the true number of nearest neighbors of a particle and the true interatomic distance in a liquid are random values rather than constant numbers, and only their mean values are determined from the g(r) plot.
Strong heating of liquids and the approach to the gaseous state leads to a gradual flattening of the function g(r) according to the decrease in the degree of short-range order. In a rarefied gas, g(r) ≈ 1.
In complex liquids and in liquid mixtures, interpretation of X-ray patterns is more complex, and in many cases it is impossible. Water and some other low-molecular-weight liquids are exceptions, since fairly complete studies and descriptions of statistical structures exist in such cases.
The theory of kinetic and dynamic properties of liquids (diffusion, viscosity, and so on) is less fully developed than the theory of the equilibrium properties (heat capacity and others). The dynamic theory of the liquid state is quite complex and at present has not been sufficiently developed. Numerical methods, which make possible the calculation of the properties of simple liquids using high-speed computers, have received great development in the theory of liquids. The molecular dynamics method is of the greatest interest because of its capability for direct computer simulation of the simultaneous thermal motion of a large number of molecules with a given interaction relationship and for reproduction of all the necessary statistical data about the system from the observed trajectories of many separate particles. Exact theoretical results concerning the structure and thermodynamic properties of simple nonmetallic liquids have been obtained in this way. A separate and as yet unsolved problem is the question of the structure and properties of simple liquids in the immediate vicinity of the critical point. Some success in this area has been achieved recently by the methods of similarity theory. On the whole, the problem of the critical phenomena in pure liquids and mixtures remains insufficiently elucidated.
A separate problem is the question of the structure and properties of liquid metals, which are considerably influenced by the presence of the collective electrons in them. In spite of some successes, there is no complete electron theory of liquid metals. Considerable and as yet unresolved difficulties were encountered in explaining the properties of liquid semiconductors.
Main trends in research on the liquid state. The numerous macroscopic properties of liquids are being studied and described by methods of various branches of mechanics, physics, and physical chemistry. The equilibrium mechanical and thermal properties of liquids (compressibility, heat capacity, and others) are being studied by thermodynamic methods. A very important task is the determination of the equation of state—the dependence of pressure and energy on density and temperature, and in the case of solutions, also on the concentrations of components. The knowledge of the equation of state makes possible the determination by thermodynamic methods of numerous relationships among the various mechanical and thermal characteristics of liquids. There is also a large number of empirical, semiempirical, and approximate theoretical equations of state for various individual liquids and their groups.
Nonequilibrium thermal and mechanical properties in liquids (diffusion, thermal conductivity, electrical conductivity, and others), particularly in mixtures and in the presence of chemical reactions, are being studied using methods of the thermodynamics of irreversible processes.
The mechanical motions of liquids regarded as continuous mediums are being studied in hydrodynamics. The most important is the Navier-Stokes equation, which describes the flow of viscous liquids. For the so-called Newtonian liquids (water, low-molecular-weight organic liquids, and molten salts), the viscosity is independent of the conditions of flow (under conditions of laminar flow, when the Reynolds number R < Rerit.); in this case the viscosity is a physicochemical constant, which is determined by the molecular nature of the liquid and by its state (temperature and pressure). The viscosity of non-Newtonian (structurally viscous) liquids depends on the conditions of flow even for low Reynolds numbers (liquid polymers, glasses in the softening range, and emulsions). The properties of non-Newtonian liquids are studied in rheology. The specific features of the flow of liquid metals that are associated with the electrical conductivity of metals and their great susceptibility to the action of magnetic fields are being studied in magnetic hydrodynamics. The application of the methods of hydrodynamics to problems of the molecular physics of liquids is studied in physicochemical hydrodynamics.
REFERENCES
Frenkel’, la. I. Sobranie izbrannykh trudov, vol. 3. Moscow, 1959.Fisher, I. Z. Statisticheskaia teoriia zhidkostei. Moscow, 1961.
Landau, L. D., and E. M. Lifshits. Mekhanika sploshnykh sred. Moscow, 1953.
Fabelinskii, I. L. Molekuliarnoe rasseianie sveta. Moscow, 1965.
Skryshevskii, A. F. Rentgenografiia zhidkostei. Kiev, 1966.
Fizika prostykh zhidkostei: Eksperimental’nye issledovaniia. Moscow, 1972 [in press]. (Translated from English.)
I. Z. FISHER