liquid crystal, liquid whose component particles, atoms or molecules, tend to arrange themselves with a degree of order far exceeding that found in ordinary liquids and approaching that of solid crystals. As a result, liquid crystals have many of the optical properties of solid crystals. Moreover, since the order is not as firmly fixed as that of a solid crystal, it can be easily modified with corresponding changes in the optical properties; typically, a small electrical impulse darkens the crystal so that it is clearly visible against the lighter background of neutral crystals. An array of seven lozenges, each of which can be darkened by a separate impulse, can yield any digit. Thus, liquid crystal displays (LCDs) have become the most common means of producing visual readouts on such devices as digital clocks and electronic calculators. Some liquid crystals vary the color of the light that they reflect as their temperature changes. Since the colors reflected at any given temperature are quite specific, temperature can be measured by this means to an accuracy of 0.1°C;.

liquid air, ordinary air that has been liquefied by compression and cooling to extremely low temperatures (see liquefaction). Its commercial preparation involves purification by washing to remove soluble impurities and by passage over calcium oxide (lime) to remove the carbon dioxide; compression, under a pressure of 200 atmospheres, or about 3,000 lb per sq in.; cooling, by passage through pipes immersed in cold water; treatment with sodium hydroxide to remove excess water; and rapid expansion, the expanding air passing back over the pipe from which it has just escaped absorbing so much heat that the air remaining in the pipe becomes liquid. Freshly liquefied air consists of 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and very small amounts of rare gases and hydrogen in solution. Its boiling point is approximately -195°C;. Because of fractional evaporation, its oxygen concentration and its boiling point increase with time. It must be kept in a specially designed container, the Dewar flask, because at ordinary temperatures it absorbs heat rapidly and reverts to the gaseous state. Liquid air is used commercially for freezing other substances and especially as an intermediate step in the production of nitrogen, oxygen, and argon and the other inert gases. As the temperature of liquid air rises, the nitrogen evaporates first at -195.8°C;, the argon next at -185.7°C;, and the oxygen last at -183°C;. See low-temperature physics.

liquid, one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from solid and gas, in which a substance has a definite volume but no definite shape.

Properties of Liquids

In general, liquids show expansion on heating, contraction on cooling; water, however, does not follow the rule exactly. A liquid changes at its boiling point to a gas and at its freezing point, or melting point, to a solid. The boiling point is especially important because, since liquids change their states at different temperatures, those in a mixture can be separated from one another by raising the temperature of the mixture gradually so that each component in turn undergoes vaporization at its boiling point. This process is known as fractional distillation.

Liquids, like gases, exhibit the property of diffusion. When two miscible liquids (i.e., they mix without separation) are poured carefully into a container so that the denser one forms a separate layer on the bottom, each will diffuse slowly into the other until they are thoroughly mixed. Liquids, like gases, differ from solids in that they are fluids, that is, they flow into the shape of a containing vessel. Liquids exert pressure on the sides of a containing vessel and on any body immersed in them, and pressure is transmitted through a liquid undiminished and in all directions. Liquids exert a buoyant force on an immersed body equal to the weight of the liquid displaced by the body (see Archimedes' principle and specific gravity). Unlike gases, liquids are very nearly incompressible, and for that reason are useful in such devices as the hydraulic press. Liquids are useful as solvents. No one liquid can dissolve all substances; each takes into solution only certain specific substances.

Molecular Structure of Liquids

The molecules (or atoms or ions) of a liquid, like those of a solid (and unlike those of a gas), are quite close together; however, while molecules in a solid are held in fixed positions by intermolecular forces, molecules in a liquid have too much thermal energy to be bound by these forces and move about freely within the liquid, although they cannot escape the liquid easily. Although the molecules of a liquid have greater cohesion than those of a gas, it is not sufficient to prevent some of those at the free surface of the liquid from bounding off (see evaporation). On the other hand, the cohesive forces between the molecules at the surface of a mass of liquid and those within cause the free surface to act somewhat like a stretched elastic membrane; it tends to draw inward toward the center of the liquid mass, to draw the liquid into the shape of a sphere, thus exhibiting the phenomenon known as surface tension.

A liquid is said to "wet" a solid substance when the attractive force between the molecules of the liquid and those of the solid is great enough to hold the liquid's molecules at the solid surface. For example, water "wets" glass since its molecules cling to glass surfaces, whereas mercury does not since the adhesive force between its molecules and those of glass is not strong enough to hold them together. Capillarity is an example of surface tension and adhesion acting at the same time.

Optoelectronic device used in displays for watches, calculators, notebook computers, and other electronic devices. Current passed through specific portions of the liquid crystal solution causes the crystals to align, blocking the passage of light. Doing so in a controlled and organized manner produces visual images on the display screen. The advantage of LCDs is that they are much lighter and consume less power than other display technologies (e.g., cathode-ray tubes). These characteristics make them an ideal choice for flat-panel displays, as in portable laptop and notebook computers.

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Substance that flows like a liquid but maintains some of the ordered structure characteristic of a crystal. Some organic substances do not melt directly when heated but instead turn from a crystalline solid to a liquid crystalline state. When heated further, a true liquid is formed. Liquid crystals have unique properties. The structures are easily affected by changes in mechanical stress, electromagnetic fields, temperature, and chemical environment. Seealso liquid crystal display.

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One of the three principal states of matter, intermediate between a gas and a solid. A liquid has neither the orderliness of a solid nor the randomness of a gas. Liquids have the ability to flow under the action of very small shear stresses. Liquids in contact with their own vapour or air have a surface tension that causes the interface to assume the configuration of minimum area (i.e., spherical). Surfaces between liquids and solids have interfacial tensions that determine whether the liquid will wet the other material. With the exception of liquid metals, molten salts, and solutions of salts, the electrical conductivities of liquids are small.

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