Only two elements are liquid at room temperature and pressure: mercury and bromine. Four more elements have melting points slightly above room temperature: francium, caesium, gallium and rubidium.
Pure substances that are liquid under normal conditions include water, ethanol and many other organic solvents. Liquid water is of primordial importance in chemistry and biology; it is believed to be a necessity for the existence of life.
Important everyday liquids include aquous solutions like household bleach, other solutions (homogeneous mixtures, multiphasic liquids) like mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like milk and blood, and colloids like paint.
Liquid crystals, used in LCD displays, cannot be classified within the classical three states of matter; they possess solid-like and liquid-like properties. The same holds for biological membranes.
Properties
Quantities of liquids are commonly measured in units of volume. These include the SI unit cubic metre (m³) and its divisions, in particular the cubic decimetre, more commonly called the litre (1 dm³ = 1 l = 0.001 m³), and the cubic centimetre, also called millilitre (1 cm³ = 1 ml = 0.001 l = 10-6 m³).
The volume of a quantity of liquid is fixed by its temperature and pressure. Unless this volume exactly matches the volume of the container, one or more surfaces are observed.
In a gravitational field, liquids exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth. If a liquid is at rest in a uniform gravitational field, the pressure, p, at any depth, z, is given by
where:
is the density of the liquid (assumed constant)
is the gravitational acceleration.
Note that this formula assumes that the pressure at the free surface is zero, and that surface tension effects may be neglected.
Objects immersed in liquids are subject to the phenomenon of buoyancy. (Buoyancy is also observed in other fluids, but is especially strong in liquids due to their high density.)
Liquids have little compressibility: water, for example, does not change its density appreciably unless subjected to pressures on the order of 100 bars (equivalent to the pressure 1 km below the surface of the ocean). In the study of fluid dynamics, liquids are often treated as incompressible, especially when studying incompressible flow.
The surface of a liquid behaves like an elastic membrane in which surface tension appears, allowing the formation of drops and bubbles. Capillary action, wetting, and ripples are other consequences of surface tension.
Viscosity measures the resistance of a liquid which is being deformed by either shear stress or extensional stress.
Phase transitions
A typical phase diagram. The dotted line gives the anomalous behaviour of water. The green lines show how the
freezing point can vary with pressure, and the blue line shows how the boiling point can vary with pressure. The red line shows the boundary where sublimation or deposition can occur.
At a temperature below the boiling point, any matter in liquid form will evaporate until the condensation of gas above reach an equilibrium. At this point the gas will condense at the same rate as the liquid evaporates. Thus, a liquid cannot exist permanently if the evaporated liquid is continually removed. A Liquid at its boiling point will evaporate more quickly than the gas can condense at the current pressure. A liquid at or above its boiling point will normally boil, though superheating can prevent this in certain circumstances.
At a temperature below the freezing point, a liquid will tend to crystallize, changing to its solid form. Unlike the transition to gas, there is no equilibrium at this transition under constant pressure, so unless supercooling occurs, the liquid will eventually completely crystallize. Note that this is only true under constant pressure, so e.g. water and ice in a closed, strong container might reach an equilibrium where both phases coexists.
Liquids can display immiscibility. The most familiar mixture of two immiscible liquids in everyday life is the vegetable oil and water in Italian salad dressing. A familiar set of miscible liquids is water and alcohol. Liquid components in a mixture can often be separated from one another via fractional distillation.
Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception.
Structure
Correlations
In a liquid, atoms do not form a crystalline lattice, nor do they show any other form of long-range order. This is evidenced by the absence of Bragg peaks in X-ray and neutron diffraction. Under normal conditions, the diffraction pattern has circular symmetry, expressing the isotropy of the liquid. In radial direction, the diffraction intensity smoothly oscillates. This is usually described by the static structure factor S(q), with wavenumber q=(4π/λ)sinθ given by the wavelength λ of the probe (photon or neutron) and the Bragg angle θ. The oscillations of S(q) express the near order of the liquid, i.e. the correlations between an atom and a few shells of nearest, second nearest, ... neighbors.
A more intuitive description of these correlations is given by the radial distribution function g(r), which is basically the Fourier transform of S(q). It represents a spatial average of a temporal snapshot of pair correlations in the liquid. g(r) is determined by a relatively simple calculation of the average number of particles found within a given volume of shell located at a distance r from the center. The average density of atoms at a given radial distance from the center is given by the formula:
where n(r) is the mean number of atoms in a shell of width Δr at distance r, and ρ is the mean atom density.[1]
g(r) provides a means of comparison between diffraction experiment and computer simulation. It can also be used in conjunction with the interatomic pair potential function in order to calculate such macrospopic thermodynamic parameters as the internal energy, Gibbs free energy, entropy and enthalpy of the disordered system.
Radial distribution function of the
Lennard-Jones model fluid.
A typical plot of g versus r shows a number of important features:
- At short separations (small r), g(r) = 0. This indicates the effective width of the atoms, which ultimately limits their distance of approach.
- A number of obvious peaks appear, at increasingly reduced intensities. The peaks indicate that the atoms pack around each other in 'shells' of nearest neighbors. At very long range, g(r) approaches a limiting value of 1 (or unity), which describes the average density at this range.
- The attenuation of the peaks at increasing radial distances from the center indicates the decreasing degree of order from the center particle. This illustrates vividly the origin of the term "short-range order" in classical liquids and glasses.
Experimental verification of the radial distribution in simple liquids has been obtained by methods relying on the scattering of X-rays, where constructive interference is limited to peaks found within a limited radial distance r. Thus, peaks of decreasing amplitude appear only where the conditions for the constructive interference of X-rays are satisfied. The result is the characteristic periodic arrangement of light and dark bands of local intensity maxima and minima -- analagous known to the diffraction pattern of the X-rays reflected from crystalline planes). [2]
Hidden structure
A number of authors have identified a static "hidden structure" and explored the dynamics of structural transitions in liquids. Utilizing molecular dynamics methods, they have separated the study of the liquid state into two parts:
- Mechanically stable packings of molecules via potential minima;
- Vibrational motion (generally anharmonic) about those mechanically stable points.
All configurations are "quenched" by a steepest-descent construction into a nearby potential minimum. The systems exhibit a "defect softening" phenomenon, or mean attraction between defects, which influences the spectrum of normal mode vibrational frequencies at the local potential minima for liquids that solidify into body centered cubic crystals. Attempts to reconstitute the equilibrium pair correlations functions by thermally broadening the quenched versions, using Einstein or Debye approximations, were clear failures. Evidently, the true phenomena in such systems entail substantial anharmonicity.[3]
The presence of "hidden structure" in supercooled liquids has been supported by the electron microscopic studies, indicating a well-defined "micellar" structure of glass which is interpreted as being the result of a superlattice of paracrystalline domains. The geometrical disorder of glass is therefore only exhibited at length scales above 10 nanometers (approximately the size of the elementary domain). Various degrees of interdomain ordering can therefore be realized.[4]
Dynamics
Atomic vibrations
Andrade focused his studies on the mechanism of structural transformations (or diffusionless transformations) in liquids. He emphasized that the intermolecular forces in the solid and the liquid state must be quite similar, and cited Lindemann's theory of melting, which has been remarkably successful in yielding accurate values for the atomic vibrational frequencies of the normal modes of vibration of simple solids. Lindemann supposes that melting occurs when the amplitude of the vibrations of the atoms about their equilibrium positions becomes a fixed large fraction of the interatomic separation distance.[5][6]
The essential difference between the liquid and solid state is therefore not the magnitude of the intermolecular force under which the molecule vibrates -- but rather the amplitude of the motion. In the liquid state, this is so large that the molecules come into contact quite often. As a result, they are disturbed and the "position of equilibrium", which in a crystalline solid is fixed, is slowly displaced in a liquid. Therefore, a molecule in a liquid can be viewed as vibrating relatively to a slowly displaced equilibrium position. The vibration has the same frequency as (identical) molecules in the solid state.
Frenkel also considered the dynamics of thermal motion of atoms about their static equilibrium positions in the rigid elastic network. The rigidity of crystals is in full agreement with the conception that this 'heat motion' reduces to vibrations of small amplitude about invariable equilibrium positions, while the characteristic fluidity of liquids is due to the fact that the positions of the atoms in a liquid body are not permanent. When the period of atomic or molecular vibration is large compared with the time scale of an applied external force, elastic deformation may occur. If, however, the vibrational period is small compared with the time scale during which the body is acted upon by a force of constant magnitude and direction, it will yield to this force via irreversible plastic deformation. [7]
In the study of the high-frequency dynamics of simple liquids and solids near their melting points, the particular condition of zero vibrational frequency has been referred to as the "thermodynamic limit" (υ → 0). The conclusions of inelastic light scattering studies near the melting point is that there is no discernible difference between the liquid and solid vibrational spectra at sufficiently high frequencies. Thus, on the short time and length scales probed by these experiments, melting causes no discontinuous change in the microscopic dynamics of the substance. The lower the frequency, the larger the discontinuity between liquid and solid behavior -- so that in the thermodynamic limit (zero frequency) the transition is first order. [8]
Effects of association
The mechanisms of atomic/molecular diffusion (or particle displacement) in solids are closely related to the mechanisms of viscous flow and solidification in liquid materials. Descriptions of viscosity in terms of molecular "free space" within the liquid[9] were modified as needed in order to account for liquids whose molecules are known to be "associated" in the liquid state at ordinary temperatures. When various molecules combine together to form an associated molecule, they enclose within a semi-rigid system a certain amount of space which before was available as free space for mobile molecules. Thus, increase in viscosity upon cooling due to the tendency of most substances to become associated on cooling.[10]
Similar arguments could be used to describe the effects of pressure on viscosity, where it may be assumed that the viscosity is chiefly a function of the volume for liquids with a finite compressibility. An increasing viscosity with rise of pressure is therefore expected. In addition, if the volume is expanded by heat but reduced again by pressure, the viscosity remains the same.
The local tendency to orientation of molecules in small groups lends the liquid (as referred to previously) a certain degree of association. This association results in a considerable "internal pressure" within a liquid, which is due almost entirely to those molecules which, on account of their temporary low velocities (following the Maxwell distribution) have coalesced with other molecules. The internal pressure between several such molecules might correspond to that between a group of molecules in the solid form.
Structural relaxation
The mean lifetime of an atom in its equilibrium position has been identified as the relaxation time, as originally described in Maxwell's kinetic theory of gases. In the simplest case of a monatomic liquid, the structural relaxation must reduce to a change of the degree of local order, yielding a more compact arrangement of higher density when the liquid is compressed, or a lower density when expanded. This change in the degree of local order must in general lag with respect to the variation of the volume (or the pressure), since it is connected with a rearrangement and redistribution of mutual orientations. These processes require a certain activation energy, and thus proceeding with a finite velocity. This is the origin of the viscous relaxation due to irreversible plastic deformation in the case of supercooled liquids near the glass transition.
