Liquid is one of the three classical states of matter. Like a gas, a liquid is able to flow and take the shape of a container, but, like a solid, it resists compression. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena.
The density of a liquid is usually close to that of a solid, and much higher than in a gas. Therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, they are both called fluids.
Liquid is one of the three primary states of matter, with the others being solid and gas. A liquid is a fluid. Unlike a solid, the molecules in a liquid have a much greater freedom to move. The forces that bind the molecules together in a solid are only temporary in a liquid, allowing a liquid to flow while a solid remains rigid.
A liquid, like a gas, displays the properties of a fluid. A liquid can flow, assume the shape of a container, and, if placed in a sealed container, will distribute applied pressure evenly to every surface in the container. Unlike a gas, a liquid may not always mix readily with another liquid, will not always fill every space in the container, forming its own surface, and will not compress significantly, except under extremely high pressures. These properties make a liquid suitable for applications such as hydraulics.
Liquid particles are bound firmly but not rigidly. They are able to move around one another freely, resulting in a limited degree of particle mobility. As the temperature increases, the increased vibrations of the molecules causes distances between the molecules to increase. When a liquid reaches its boiling point, the cohesive forces that bind the molecules closely together break, and the liquid changes to its gaseous state (unless superheating occurs). If the temperature is decreased, the distances between the molecules become smaller. When the liquid reaches its freezing point the molecules will usually lock into a very specific order, called crystallizing, and the bonds between them become more rigid, changing the liquid into its solid state (unless supercooling occurs].
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 aqueous 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.
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).
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
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.
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.
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.
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 macroscopic thermodynamic parameters as the internal energy, Gibbs free energy, entropy and enthalpy of the disordered system.
A typical plot of g versus r shows a number of important features:
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—analogous known to the diffraction pattern of the X-rays reflected from crystalline planes. 
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:
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.
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.
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.
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. 
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. 
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 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.
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.
The viscosity of liquids is characterized by a low activation energy. Thermal fluctuations break joining bonds with higher temperatures resulting in higher concentrations of broken bonds (or configurons). According to Ojovan  in the molten state configurons form large clusters which are macroscopically large. Broken bonds facilitate the irreversible plastic deformation (or flow) of the liquid. Also, the viscosity of amorphous solid materials is characterized in the glassy state by a high activation energy. The bond system of an amorphous material changes its Hausdorff dimension from Euclidian 3 below glass transition temperature Tg – where the amorphous material responds mechanically as an elastic solid on most experimental timescales — to fractal 2.55 (± 0.05) above Tg – where the amorphous material exhibits the rheology observed in a classical liquid .
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.   
In tribology, liquids are studied for their properties as lubricants. Lubricants such as oil are chosen for viscosity and flow characteristics that are suitable throughout the operating temperature range of the component. Oils are often used in engines, gear boxes, metalworking, and hydraulic systems for their good lubrication properties.
Many liquids are used as solvents, to dissolve other liquids or solids. Solvents are found in a wide variety of applications, including paints, sealants, and adhesives. Naptha and acetone are used frequently in industry to clean oil, grease, and tar from parts and machinery. Surfactants are commonly found in soaps and detergents. Solvents like alcohol are often used as antimicrobials. They are found in cosmetics, inks, and liquid dye lasers. They are used in the food industry, in processes such as the extraction of vegetable oil.
Liquids tend to have better thermal conductivity than gases, and the ability to flow makes a liquid suitable for removing excess heat from mechanical components. The heat can be removed by channeling the liquid through a heat exchanger, such as a radiator, or the heat can be removed with the liquid during evaporation. Water or glycol coolants are used to keep engines from overheating. The coolants used in nuclear reactors include water or liquid metals, such as sodium or bismuth. Liquid propellant films are used to cool the thrust chambers of rockets. In machining, water and oils are used to remove the excess heat generated, which can quickly ruin both the work piece and the tooling. During perspiration, sweat removes heat from the human body by evaporating. In the heating, ventilation, and air-conditioning industry (HVAC), liquids such as water are used to transfer heat from one area to another.
Liquid is the primary component of hydraulic systems, which take advantage of Pascal's law to provide fluid power. Devices such as pumps and waterwheels have been used to change liquid motion into mechanical work since ancient times. Oils are forced through hydraulic pumps, which transmit this force to hydraulic cylinders. Hydraulics can be found in many applications, such as automotive brakes and transmissions, heavy equipment, and airplane control systems. Various hydraulic presses are used extensively in repair and manufacturing, for lifting, pressing, clamping and forming.
Liquids are sometimes used in measuring devices. A thermometer often uses the thermal expansion of liquids, such as mercury, combined with their ability to flow to indicate temperature. A manometer uses the weight of the liquid to indicate air pressure.
It is difficult to compress a liquid.
where ρ is the density of the liquid (assumed constant) and z is the depth of the point below the surface. Note that this formula assumes that the pressure at the free surface is zero.