Liquid
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. Some liquids resist compression, while others can be compressed. 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.
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[edit] Introduction
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).
[edit] Examples
Only two elements are liquid at room temperature and pressure: mercury and bromine. Five more elements have melting points slightly above room temperature: francium, caesium, gallium, rubidium and iodine.[1] Metal alloys that are liquid at room temperature include NaK, a sodium-potassium metal alloy, galinstan, a fusible alloy liquid, and some amalgams (alloys involving mercury).
Pure substances that are liquid under normal conditions include water, ethanol and many other organic solvents. Liquid water is of vital importance in chemistry and biology; it is believed to be a necessity for the existence of life.[citation needed]
Important everyday liquids include aqueous solutions like household bleach, other mixtures of different substances such as mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like blood, and colloids like paint and milk.
Many gases can be liquefied by cooling, producing liquids such as liquid oxygen, liquid nitrogen, liquid hydrogen and liquid helium. Not all gases can be liquified at atmospheric pressure, for example carbon dioxide can only be liquified at pressures above 5.1 atm.
Some materials cannot be classified within the classical three states of matter; they possess solid-like and liquid-like properties. Examples include liquid crystals, used in LCD displays, and biological membranes.
[edit] Applications
Liquids have a variety of uses, as lubricants, solvents, and coolants. In hydraulic systems, liquid is used to transmit power.
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.[2]
Many liquids are used as solvents, to dissolve other liquids or solids. Solutions 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. Body fluids are water based solutions.
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.[3]
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.[4] Water or glycol coolants are used to keep engines from overheating.[5] The coolants used in nuclear reactors include water or liquid metals, such as sodium or bismuth.[6] Liquid propellant films are used to cool the thrust chambers of rockets.[7] 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.[8]
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.[9]
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.[10]
[edit] 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. Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception.
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, requires a pressure of the order of 200 bar to increase its density by 1/1000. 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.
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.
[edit] Phase transitions
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 coexist. For the opposite transition from solid to liquid, see melting.
[edit] Structure
[edit] 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.[11]
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:
- 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—analogous known to the diffraction pattern of the X-rays reflected from crystalline planes. [12]
[edit] Dynamics
[edit] Elastic waves
In a static sense, the fundamental difference between a liquid and a solid is that the solid has elastic resistance against a shearing stress while a liquid does not. Thus, thermal motion in liquids can be decomposed into elementary longitudinal vibrations (or acoustic phonons) while transverse vibrations (or shear waves) were originally described only in elastic solids exhibiting the highly ordered crystalline state of matter. In other words, simple liquids cannot support an applied force in the form of a shearing stress, and will yield mechanically via macroscopic plastic deformation (or viscous flow). Furthermore, the fact that a solid deforms locally while retaining its rigidity -- while a liquid yields to macroscopic viscous flow in response to the application of an applied shearing force -- is accepted by many as the mechanical distinction between the two. [13]
It can be concluded from these observations on continuity that transverse vibrations can be propagated not only in crystalline bodies, but also in liquids. The fact that this conclusion is not verified experimentally in the case of ordinary liquids is due to the short time scale of relaxation compared to the period of vibrations which can be obtained with the help of modern opto-acoustic methods: lasers and ultrasonics. Under such conditions, the transverse vibrations must be strongly damped.
Experimental verification of these conclusions have been obtained using molecular dynamics computer simulation studies in monatomic liquids and glasses where it has been shown that, at short wavelengths, monatomic liquids can support a propagating shear wave, i.e. a collective excitation which is analogous to the transverse phonons found in solids. The onset of this viscoelastic behavior is linked to the fact that as the wave number increases the rigidity of the liquid becomes an important factor.[14][15][16][17][18][19][20]
Mechanisms of attenuation of high-frequency shear modes and longitudinal waves were considered with viscous liquids, polymers and glasses.[21] The subsequent work led to a fresh interpretation of the glass transition in viscous liquids in terms of a spectrum of structural relaxation phenomena occurring over a wide range of spatial and temporal scales. Experimentally, the use of dynamic light scattering experiments (or photon correlation spectroscopy) allows to study molecular processes down to time intervals of 10−11 sec. This is equivalent to extending the available frequency range to 109 Hz or greater.[22][23]
Thus we see the intimate correlation between transverse acoustic phonons (or shear waves) and the onset of rigidity or vitrification. The frequency dependence of this phenomenon becomes apparent when one considers the increasing wavelength over which such rigidity can be observed. The relationship between these transverse waves and the mechanism of vitrification is described further in the glass transition.
The representation of thermal motion in liquids as a superposition of elastic sound waves was first introduced by Brillouin. Atomic motion in condensed matter can therefore be represented by a Fourier series of standing waves whose physical interpretation consists of a superposition of supersonic longitudinal and transverse atomic displacement waves (or density fluctuations) with varying directions and wavelengths. With respect to sound wave propagation, the speed of longitudinal or compression waves will be limited by the bulk modulus of compressibility. The square root of the ratio of the bulk modulus K to the density p will be equal to the velocity of propagation of longitudinal phonons. In the case of transverse vibrations or shear waves, for which the density remains constant, the speed of such waves will be limited by the shear modulus or rigidity.[24]
The square root of the ratio of the shear modulus G to the density will be equal to the velocity of transverse phonons. Thus, the wave velocities will be given by:
where the constant of proportionality ρ in both cases is the particle density or reciprocal specific volume.
[edit] Molecular 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.[25][26]
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. [27]
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. [28]
[edit] 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[29] 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.[30]
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.
[edit] 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. [31] [32] [33]
[edit] See also
[edit] References
- ^ Theodore Gray, The Elements: A Visual Exploration of Every Known Atom in the Universe New York: Workman Publishing, 2009 p.127 ISBN 1579128149
- ^ Theo Mang, Wilfried Dressel ’’Lubricants and lubrication’’, Wiley-VCH 2007 ISBN 3527314970
- ^ George Wypych ’’Handbook of solvents’’ William Andrew Publishing 2001 pp. 847-881 ISBN 1895198240
- ^ N. B. Vargaftik ’’Handbook of thermal conductivity of liquids and gases’’ CRC Press 1994 ISBN 0849393450
- ^ Jack Erjavec ’’Automotive technology: a systems approach’’ Delmar Learning 2000 p. 309 ISBN 1401848311
- ^ Gerald Wendt ’’The prospects of nuclear power and technology’’ D. Van Nostrand Company 1957 p. 266
- ^ ’’Modern engineering for design of liquid-propellant rocket engines’’ by Dieter K. Huzel, David H. Huang – American Institute of Aeronautics and Astronautics 1992 p. 99 ISBN 1563470136
- ^ Thomas E Mull ’’HVAC principles and applications manual’’ McGraw-Hill 1997 ISBN 007044451X
- ^ R. Keith Mobley Fluid power dynamics Butterworth-Heinemann 2000 p. vii ISBN 0750671742
- ^ Bela G. Liptak ’’Instrument engineers’ handbook: process control’’ CRC Press 1999 p. 807 ISBN 0849310814
- ^ McQuarrie, D.A., Statistical Mechanics (Harper Collins, 1976)
- ^ Berry, R.S. and Rice, S.A., Physical Chemistry, App.23A: X-Ray Scattering in Liquids: Determination of the Structure of a Liquid (Oxford University Press, 2000)
- ^ Born, M., The Stability of Crystal Lattices, Proc. Camb. Phil. Soc., Vol. 36, p.160, (1940) doi=10.1017/S0305004100017138; Thermodynamics of Crystals and Melting, J. Chem. Phys., Vol. 7, p. 591 (1939) doi=10.1063/1.1750497; A General Kinetic Theory of Liquids, University Press (1949)
- ^ C.A. Angell, J.H.R. Clarke, I.V. Woodcock (1981). "Interaction Potentials and Glass Formation: A Survey of Computer Experiments". Adv. Chem. Phys. 48: 397. doi:10.1002/9780470142684.ch5.
- ^ C.A. Angell (1981). "The Glass Transition: Comparison of Computer Simulation and Laboratory Studies". Trans. N.Y. Acad. Sci. 371: 136. doi:10.1111/j.1749-6632.1981.tb55657.x.
- ^ D. Frenkel, J.P. McTague (1980). "Computer Simulations of Freezing and Supercooled Liquids". Ann. Rev. Phys. Chem. 31: 491. doi:10.1146/annurev.pc.31.100180.002423.
- ^ Levesque, D. et al., Computer "Experiments" on Classical Fluids, Phys. Rev. A, Vol. 2, p. 2514 (1970); Phys. Rev. A, Vol. 7, p. 1690 (1973); Phys. Rev. B, Vol. 20, p. 1077 (1979)
- ^ G. Jacucci, I.R McDonald (1980). "Shear waves in liquid metals". Molec. Phys. 39: 515. doi:10.1080/00268978000100411.
- ^ M.H. Cohen and G.S. Grest (1980). "Liquid-glass transition: Dependence of the glass transition on heating and cooling rates". Phys. Rev. B 21: 4113. doi:10.1103/PhysRevB.21.4113.
- ^ G.S. Grest, S.R. Nagel, A. Rahman (1980). "Longitudinal and Transverse Excitations in a Glass". Phys. Rev. Lett. 49: 1271. doi:10.1103/PhysRevLett.49.1271.
- ^ Mason, W.P., et al., Mechanical Properties of Long Chain Molecule Liquids at Ultrasonic Frequencies, Phys. Rev., Vol. 73, p. 1074 (1948); Measurement of Shear Elasticity and Viscosity of Liquids by Means of Ultrasonic Shear Waves, J. Acoust. Soc. Amer., Vol. 21, p. 58 (1949)
- ^ Litovitz, T.A., et al., Ultrasonic Spectroscopy in Liquids, J. Acoust. Soc. Amer., Vol. 431, p. 681 (1959); Ultrasonic Relaxation and Its Relation to Structure in Viscous Liquids, Vol. 26, p. 566 (1954); Mean Free Path and Ultrasonic Vibrational Relaxation in Liquids, J. Acoust. Soc. Amer., Vol. 32, p. 928 (1960); On the Relation of the Intensity of Scattered Light to the Viscoelastic Properties of Liquids and Glasses, Vol. 41, p. 1601 (1967); Montrose, C.J., et al., Brillouin Scattering and Relaxation in Liquids, Vol. 43, p. 117 (1968); Lamacchia, B.T., Brillouin Scattering in Viscoelastic Liquids, Dissertation Abstracts International, Vol. 27-09, p. 3218 (1967)
- ^ I.L. Fabelinskii (1957). "Molecular Scattering of Light in Liquids". Uspekhi Fizicheskikh Nauk 63: 355.
- ^ L. Brillouin (1922). "Diffusion de la lumière et des rayons X par un corps transparent homogène; influence de l'agitation thermique". Annales de Physique 17: 88.
- ^ E.N. Andrade (1934). "Theory of viscosity of liquids". Phil. Mag. 17: 497, 698.
- ^ C. Lindemann (1911). "Kinetic theory of melting". Phys. Zeitschr. 11: 609.
- ^ Frenkel, J., Kinetic Theory of Liquids, Translated from Russian (Oxford University Press, 1946)
- ^ Fleury, P.A., Central-Peak Dynamics at the Ferroelectric Transition in Lead Germanate, Phys. Rev. Lett., Vol. 37, p. 1088 (1976); in Anharmonic Lattices, Structural Transitions and Melting, Ed. T. Riste (Noordhoff, 1974); in Light Scattering Near Phase Transitions, Eds. H.Z. Cummins, A. P. Levanyuk (North-Holland, 1983)
- ^ D.B. Macleod (1923). "On a relation between the viscosity of a liquid and its coefficient of expansion". Trans. Farad. Soc. 19: 6. doi:10.1039/tf9231900006.
- ^ G.W Stewart (1930). "The Cybotactic (Molecular Group) Condition in Liquids; the Association of Molecules". Phys. Rev. 35: 726. doi:10.1103/PhysRev.35.726.
- ^ Scherer, G.W., Relaxation in Glass and Composites, Krieger, 1992 ISBN 0471819913
- ^ Mason, W.P., et al. (1948). "Mechanical Properties of Long Chain Molecule Liquids at Ultrasonic Frequencies". Phys. Rev. 73: 1074. doi:10.1103/PhysRev.73.1074.
- ^ Montrose, C.J., et al. (1968). "Brillouin Scattering and Relaxation in Liquids". J. Acoust. Soc. Am. 43: 117. doi:10.1121/1.1910741.Litovits, T.A. (1959). "Ultrasonic Spectroscopy in Liquids". J. Acoust. Soc. Am. 31: 681. "Ultrasonic Relaxation and Its Relation to Structure in Viscous Liquids". J. Acoust. Soc. Am. 26: 566. 1954.Candau, S., et al. (1967). "Brillouin Scattering in Viscoelastic Liquids". J. Acoust. Soc. Am. 41: 1601. doi:10.1121/1.2143675.Pinnow, D. et al. (1967). "On the Relation of the Intensity of Scattered Light to the Viscoelastic Properties of Liquids and Glasses". J. Acoust. Soc. Am. 41: 1601. doi:10.1121/1.2143676.
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