Atomic Structure Of Liquids

Type of physical science: Condensed matter physics

Field of study: Liquids

The atomic structure of a liquid is characterized by short-range order but long-range disorder, thus having partly solidlike and partly gaslike properties.

Overview

Liquids exist in a narrow range of temperature and pressure between solids and gases.

When cooled to a sufficiently low temperature, any liquid becomes a solid. When heated to a sufficiently high temperature, any liquid becomes a gas. Ice cubes will melt to liquid water when removed from a freezer. The liquid water boils and turns to steam when heated. Dry ice, or solid carbon dioxide, is sometimes used in camp freezers or ice-cream freezers. If dry ice is removed from the freezer, it warms directly from a solid to a gas. The force of the air pressure is great enough for ice to warm to a liquid, but it is not great enough for dry ice to do so. The 1-atmosphere pressure of the air at sea level must be raised to 5 atmospheres or above in order for solid carbon dioxide to become liquid.

Gases are easily compressed and expanded. They have no shape but expand to fill whatever contains them, they mix rapidly, and they are light (low in density). The kinetic molecular theory explains these properties by suggesting that gases consist of very tiny particles that are called molecules if a substance is a compound, or atoms if a substance is an element.

Compounds can be decomposed into more than one substance but elements cannot. Some elements such as oxygen form molecules by combining two identical atoms. The molecules are so tiny that there is enormous space between them. Gases expand and compress easily, mix rapidly, and have a low density because the molecules have diffusive motion; they move rapidly in straight lines until they collide. There is no pattern in their positions. Each small portion would appear differently than any other small portion. Thus, gases have no order.

Solids are difficult to expand or compress. They resist deformation, have a definite shape bounded by their surfaces, and mix very slowly over a long period of time. Solids are heavy compared to gases. Ice has a density of 0.9 gram per cubic centimeter. Iron has a density of 8 grams per cubic centimeter. Air, a mixture of gases, has a density of 0.013 gram per cubic centimeter. The molecules of solids are closely packed. They vibrate rapidly about fixed positions but in general do not change their positions. A two-dimensional model of a solid can be visualized by imagining a set of marbles packed as close as possible. The spheres touch each other but have small spaces between them. They form a pattern that is repeated throughout the rows and columns. Thus, the atomic structures of solids are ordered over a long range in three dimensions in the same way.

Liquids resemble solids in that they are difficult to expand or compress, but, like gases, they do not resist forces that change their shape. The shape of liquids is not definite but takes on the bottom part of whatever contains them. The top of their shape is bounded by a surface that has some strength, or a surface tension. Surface tension is manifested when water bugs skim over the surface of a gently flowing stream. Some liquids mix if they are dissolved in each other, but they do not mix as rapidly as gases. The molecules of liquids both diffuse, as do those of gases, and vibrate about their positions, as do the molecules of solids. The molecules of liquids show a rocking motion relative to each other, which is an additional type of vibration that is not manifested by the molecules of solids. Liquids have densities that are close to solids. The molecules of liquids are closely packed, but not as closely packed as solids. A two-dimensional model of a liquid can be visualized by imagining a set of marbles packed tightly, but not as tightly as possible. Thermal motion from collisions with the surrounding environment, as a result of thermal equilibrium, gives all the motions attributed directly above to the molecules of a liquid. The ordered pattern of the solid is not present throughout all the marbles. Yet, a cluster of eight or ten marbles often shows a pattern that forms and deforms over time. The number of marbles in the area of a thin, circular shell about any one increases and then decreases several times, as the shell varies from one to five marble lengths from the center one, before becoming uniform. The atomic structure of liquids shows short-range order but long-range disorder in this sense.

Pictures of short-range order and long-range disorder in liquids are provided by scattering experiments with X rays and with neutrons. X rays are high-energy, invisible radiations with wavelengths of the same order of magnitude as spacings between molecules in closely packed structures. Neutrons are uncharged subatomic particles. The solar system model of the atom suggests that most of the mass of the atom is in a tiny nucleus that consists of neutrons and positively charged protons. Negatively charged electrons orbit around the nucleus.

The atom is held together by electrostatic forces. Neutrons are emitted from nuclei in nuclear fission and fusion reactions, and can be formed into a ray and directed against a substance for scattering. X rays are scattered by the outer electrons in a substance. Neutrons are scattered by the inner nuclei in a substance.

The scattering patterns of both X rays and neutrons show rays of light and dark diffraction patterns, which are characteristic of the spacings between molecules near one another in liquids. The nearest molecules next to a particular one in a liquid, and in a solid, are about the same distance away. The second and third nearest molecules in a liquid are a little farther away and less common on the average than they are in a solid. Thereafter, molecules in a liquid beyond the third nearest molecule show no pattern, while those in a solid repeat the former pattern. In aluminum metal, for example, any neighbor pair of molecules is 0.25 nanometer (one-billionth of a meter) apart in both liquid and solid. Every added 0.25 nanometer away in the solid locates another molecule; in the liquid, it is more probable, but not as certain as in the solid, to find a second and third molecule 0.30 and 0.60 nanometer away from the original pair.

Beyond that distance away from the original pair in a liquid, there is no greater probability of locating a molecule in any one position than in any other. This picture of short-range order and long-range disorder in the liquid state is demonstrated by scattering experiments. Scattering experiments also show that there is no difference between a liquid and a gas under high pressure.

Neutron diffraction is especially useful for mixtures of different substances with different nuclei, because the nuclei can be distinguished from one another in the diffraction pattern. Regardless of the type of scattering, the picture of short-range order but long-range disorder for pure liquids and for liquid mixtures is established experimentally. Determining exact ranges of order for liquids is an exciting object of research.

The short-range order of liquids suggests that liquids resemble solids. Supercooling liquids, however, indicates otherwise. For example, water freezes normally at 0 degrees Celsius.

A salt-ice mixture can be used to obtain temperatures below freezing. If a clean, small container of pure water is placed in a salt-ice mixture and kept without movement for about thirty minutes, its temperature may drop to -5 degrees Celsius or more without freezing. The water is said to be supercooled. If a small piece of ice is dropped into some supercooled water, the water is transformed into beautiful crystals quickly. The fact that liquids can be supercooled indicates that their structure does not consist of tiny solidlike crystals.

When a liquid is inserted in a closed container, it has a certain gas-phase pressure--known as its vapor pressure--because some of the liquid evaporates. If the closed container is made to withstand high pressures, is filled with liquid at a special critical density, and is heated, the liquid will pass through the critical state. When the contents of such a container are observed through a window, the surface of the liquid appears as a line that is called a meniscus. As the temperature is raised, the density of the liquid phase decreases and the density of the gas phase increases. Just below the critical temperature, the densities of the gas and liquid become so close that the meniscus appears broadened and vertical streaming of the substance can be seen. At the critical temperature, the density of the liquid and the gas are identical, and the meniscus vanishes. A scattering of light appears as mixed colors, then the substance becomes transparent once again at a temperature above the critical temperature; at this point, there is no meniscus and no distinction between liquid and gas. The substance is called a fluid. Below the critical temperature, there is a certain heat energy required to change the liquid to a gas. This energy of vaporization vanishes at the critical state. The critical state of carbon dioxide has a temperature of 31 degrees Celsius, a pressure of 73 atmospheres, and a density of 0.5 gram per cubic centimeter. The critical state of water has a temperature of 374 degrees Celsius, a pressure of 218 atmospheres, and a density of 0.3 gram per cubic centimeter. Liquid and gas water have densities of 1.0 and 0.008 gram per cubic centimeter, respectively, at room temperature and 1 atmosphere of pressure. The difference between a gas and a liquid is, in part, one of density; in other words, it is the degree to which the molecules or atoms of a substance are closely packed that determines whether it is a liquid or a gas.

There is an attraction between molecules that pulls them together and forms them into clusters. If those forces are large, then a substance will liquefy at a low temperature and low pressure. These intermolecular forces are electrostatic in nature and are much stronger than gravity. They are responsible for the energies of vaporization. The size of the molecules and their intermolecular forces determine how closely packed the molecules of a liquid will be and the range of temperature and pressure within which a particular substance exists as a liquid.

Applications

Knowledge of the atomic structure of liquids has encouraged a search for explanations and models that emphasize the continuity between liquids and gases through density, rather than through microcrystalline structures. Scientists are also trying to determine the intermolecular forces of substances. Equations of state can then be developed to predict liquid and gas properties for substances that have not yet been measured. Mixtures are involved in the recovery of fuels and minerals from underground, and in the extraction of oils and nutrients from foods. Equations of state can predict energies and changes in density that accompany mixing.

Liquids are useful for heat transfer and heat storage. Liquid water under pressure is used as a coolant for nuclear reactors because of its large specific heat, or the amount of energy that is absorbed to raise the temperature of 1 gram 1 degree Celsius. Liquids have higher specific heats than solids or gases because their unique combination of order and disorder allows their molecules not only to vibrate but also to rock relative to one another. Water has been the liquid of choice to store solar energy. Certain compounds that are related to common table salt absorb energy when they dissolve in water. In an attempt to enhance the heat-storage ability of water, such salts have been used to dissolve in sunlight, absorbing solar energy; they release it when the temperature lowers and the salt resolidifies. This process is reversible.

Sound is transmitted in alternate pulses of high and low density or pressure. When a guitar twangs, alternate high and low pressures in the air are transmitted from the guitar to the eardrum to create the sensation of hearing. Pulses can be heard from 20 to 20,000 per second.

Whales communicate with each other by "singing" in the sea. Sonar, which detects underwater objects by reflection of sound, is used to detect submarines, underwater mines, fish, and navigation obstacles. An understanding of the order-disorder structure of water mixtures was necessary for the development of sonar. The limit of the low density or pressure of a sound pulse is a cavity in which the liquid breaks down. Cavitation is important in the transmission of projectiles underwater and in the study of underwater and underground explosions. Such work cannot be undertaken without understanding the atomic structure of liquids.

Liquid crystals are compounds that are crystalline along one direction or two but liquid along another; as a result, they show long- or short-range order dependent upon the direction.

When such a substance is at a sufficiently low temperature, it is a normal solid crystal, having long-range disorder in all directions. As it is warmed, it melts through several mesophases that appear cloudy in appearance and show short-range order only in one direction or two; however, the liquid crystal retains long-range disorder in position or orientation in another direction. As warming continues, melting occurs in all directions into a normal liquid. Liquid crystals reflect or transmit light of different colors depending on their structure. Since their structure may change with temperature, their color serves as a sensitive thermometer. Their structure may change with pressure as well, such as sound. Liquid-crystal displays are commonplace in calculators, digital watches, blood pressure monitors, and the like. The liquid crystals are aligned on the inner surface of the display. A small electric current near a portion of the display causes the liquid crystal to change alignment and reflect or transmit light differently. Gentle pressure shows colors or striations as the surface is stressed and then relaxed. Some liquid crystals form only when mixed with another liquid. A medicine or beneficial drug might be trapped within such a liquid crystal, transported to the site of an infection, then released by the higher temperature in the environment of the infection or by a sonar pulse.

Context

The kinetic theory of gases evolved in the eighteenth and nineteenth centuries. It was necessary for scientists to think in terms of molecules and atoms before an atomic structure of liquids could be conceived. In 1850, Marcelin Berthelot discovered that whereas liquids experienced a change in shape under stress, they did not change volume or density even when 10 atmospheres of pressure were applied. In 1888, Johannes Diderik van der Waals published his classic ON THE CONTINUITY OF THE GASEOUS AND LIQUID STATES, and reported an equation of state that he used to relate the properties of both liquids and gases. Liquids were considered highly compressed gases. His equation accounted both for an attraction between molecules and for their size.

In the second decade of the twentieth century, Sir William Henry Bragg and his son, Sir Lawrence, discovered that X rays could be reflected and scattered from solids. The results showed a long-range order and led to the concept of a crystal lattice. The X-ray experiments were then applied to liquids during the next decade by Peter Debye and others. These scientists concluded that liquids showed a crystallike order over several molecular diameters, after which the order diminished and disappeared. This conclusion was the first indication that liquids might have a short-range order. Several structure theories were developed, including ones by Joseph Mayer and Henry Eyring.

In 1935, John Kirkwood related the short-range order functionally to intermolecular forces. He showed that repulsive hard spheres could lead to a solid-to-liquid transition. Robert Scott and Joel Hildebrand physically packed spheres, and John Desmond Bernal packed polyhedra, in separate experiments, which established that close packing was less dense than in ordered solids and could not be equivalent to the close packing observed in solid crystals.

Hildebrand championed the view that liquids were not crystallike. In 1969, a two-dimensional model of steel ball bearings was agitated and demonstrated that, at a certain intensity of agitation, a solid-to-liquid transition could occur.

In the late twentieth century, high-speed computers became increasingly important.

Computer experiments were undertaken for numerous models of hard spheres and ellipsoids with and without different kinds of intermolecular forces. These experiments established that short-range order can be the result of closely packed spheres on the basis of repulsive forces only and that intermolecular attractions control the energy to expand the liquid to a gaslike disorder.

Research continues to establish more accurate intermolecular forces, which were applied to mixtures in order to discover new ranges of pressure and temperature within which liquid crystals of different substances can be found.

Principal terms

ATOM: the smallest particle of an element that can exist

COMPOUND: a pure material composed of more than one element; water and carbon dioxide are examples of compounds

CRITICAL STATE: the pressure and density at the critical temperature above which a liquid cannot exist

DENSITY: the weight of a substance or mixture divided by its volume; it is used to compare the weights of different substances when they occupy the same volume

ELECTROSTATIC: referring to a force that pulls electrons toward or away from one another

ELEMENT: a primary substance discovered in nature, such as carbon or neon, that can be decomposed only by extraordinary means such as a nuclear explosion

MOLECULE: the smallest part of a compound that can exist

PRESSURE: a force per unit area caused by the impact of all the molecules of a substance against the molecules of another substance; the pressure of the air at sea level is 1 atmosphere

STRUCTURE: the repeating pattern in space that the atoms or molecules of a substance may or may not have, often expressed as order or disorder

VOLUME: the amount of space that a substance occupies (visualized as a point moving a distance to generate a line, the line moving to generate a square, then the square moving to generate a cube)

Bibliography

Barker, J. A., and Douglas Henderson. "What Is `Liquid'? Understanding the States of Matter." REVIEWS OF MODERN PHYSICS 48 (October, 1976): 5. The first four paragraphs of the introduction present the authors' view of the status of liquids.

Contreras, Martin, and Jorge Valenzuela. "A Two-Dimensional Model of a Liquid: The Pair-Correlation Function." JOURNAL OF CHEMICAL EDUCATION 63 (January, 1986): 7-9. Shows how the short-range order and long-range disorder of liquids can be observed from a picture of collections of spheres, triangles, or objects of any shape.

Demus, Dietrich, and Lothar Richter. TEXTURES OF LIQUID CRYSTALS. New York: Springer-Verlag, 1978. Contains beautiful pictures of liquid crystals, with excellent details.

Dreisbach, Dale. LIQUIDS AND SOLUTIONS. Boston: Houghton Mifflin, 1966. Dreisbach's unique approach intersperses quotes from original works of the masters throughout the text. Especially interesting are the descriptions of the packing experiments in chapters 5 and 6, and Joel Hildebrand's critique of crystallike models of liquids in chapter 7.

Hildebrand, Joel. SCIENCE IN THE MAKING. New York: Columbia University Press, 1957. A fascinating series of lectures given in 1956 for the layperson by one of the major contributors to liquid and mixture theory. Provides valuable personal accounts and insight into the way that scientists work and think.

Murrell, J. N., and E. A. Boucher. PROPERTIES OF LIQUIDS AND SOLUTIONS. New York: Wiley, 1982. The introduction, chapter 5, and sections 4 and 5 of chapter 13 are interesting and well written.

Temperly, H. N. V., and D. H. Trevena. LIQUIDS AND THEIR PROPERTIES. New York: Wiley, 1978. Although the majority of the book is too complex for the general reader, the historical introduction in chapter 1 is worth reading.

Trevena, D. H. THE LIQUID PHASE. New York: Springer-Verlag, 1975. A small book that is replete with equations and pictures. The pictures are accompanied by ample discussion, much of which is geared to the general reader. Sections 5 and 6 of chapter 5 have a brief but well-written account of the two- and three-dimensional ball-bearing models of a liquid.

Diffusion in Gases and Liquids

Equation of State

The Behavior of Gases

Liquefaction of Gases