Physical Properties Of Molecular Crystals Liquids And Glasses Pdf
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- Crystals, Liquids and Glasses
- 12.1: Crystalline and Amorphous Solids
- Physical Properties Molecular Crystals Liquids Glasses
Crystals, Liquids and Glasses
Liquid , in physics , one of the three principal states of matter, intermediate between gas and crystalline solid. The most obvious physical properties of a liquid are its retention of volume and its conformation to the shape of its container. When a liquid substance is poured into a vessel, it takes the shape of the vessel, and, as long as the substance stays in the liquid state, it will remain inside the vessel.
Furthermore, when a liquid is poured from one vessel to another, it retains its volume as long as there is no vaporization or change in temperature but not its shape. These properties serve as convenient criteria for distinguishing the liquid state from the solid and gaseous states. Gases, for example, expand to fill their container so that the volume they occupy is the same as that of the container. Solids retain both their shape and volume when moved from one container to another.
Liquids may be divided into two general categories: pure liquids and liquid mixtures. On Earth, water is the most abundant liquid, although much of the water with which organisms come into contact is not in pure form but is a mixture in which various substances are dissolved. Such mixtures include those fluids essential to life—blood, for example—beverages, and seawater. Seawater is a liquid mixture in which a variety of salts have been dissolved in water.
Even though in pure form these salts are solids, in oceans they are part of the liquid phase. Thus, liquid mixtures contain substances that in their pure form may themselves be liquids, solids, or even gases.
The liquid state sometimes is described simply as the state that occurs between the solid and gaseous states, and for simple molecules this distinction is unambiguous. However, clear distinction between the liquid, gaseous, and solid states holds only for those substances whose molecules are composed of a small number of atoms.
When the number exceeds about 20, the liquid may often be cooled below the true melting point to form a glass , which has many of the mechanical properties of a solid but lacks crystalline order. If the number of atoms in the molecule exceeds about —, the classification into solid, liquid, and gas ceases to be useful. At low temperatures such substances are usually glasses or amorphous solids, and their rigidity falls with increasing temperature—i. With these large molecules, the gaseous state is not attainable, because they decompose chemically before the temperature is high enough for the liquid to evaporate.
Synthetic and natural high polymers e. If the molecules are large, rigid, and either roughly planar or linear, as in cholesteryl acetate or p -azoxyanisole, the solid may melt to an anisotropic liquid i. Such a state is called a liquid crystal , and the anisotropy produces changes of the refractive index a measure of the change in direction of light when it passes from one medium into another with the direction of the incident light and hence leads to unusual optical effects.
Liquid crystals have found widespread applications in temperature-sensing devices and in displays for watches and calculators.
However, no inorganic compounds and only about 5 percent of the known organic compounds form liquid crystals. The theory of normal liquids is, therefore, predominantly the theory of the behaviour of substances consisting of simple molecules. A liquid lacks both the strong spatial order of a solid, though it has the high density of solids, and the absence of order of a gas that results from the low density of gases—i.
The combination of high density and of partial order in liquids has led to difficulties in developing quantitatively acceptable theories of liquids. Understanding of the liquid state, as of all states of matter, came with the kinetic molecular theory, which stated that matter consisted of particles in constant motion and that this motion was the manifestation of thermal energy.
The greater the thermal energy of the particle, the faster it moved. In very general terms, the particles that constitute matter include molecules, atoms, ions, and electrons. The motion of particles is in a straight line, and the collisions that result occur with no loss of energy, although an exchange of energies may result between colliding particles.
Each particle, when it joins others in the liquid state, gives up a measure of heat called the latent heat of liquefaction, but each continues to move at the same speed within the liquid as long as the temperature remains at the condensation point.
The distances that the particles can travel in a liquid without colliding are on the order of molecular diameters. As the liquid is cooled, the particles move more slowly still, until at the freezing temperature the attractive energy produces so high a density that the liquid freezes into the solid state. They continue to vibrate, however, at the same speed as long as the temperature remains at the freezing point , and their latent heat of fusion is released in the freezing process.
Further heating provides the liquid particles with their heat of evaporation , which enables them to escape one another completely and enter the vapour, or gaseous, state. This starkly simplified view of the states of matter ignores many complicating factors, the most important being the fact that no two particles need be moving at the same speed in a gas, liquid, or solid and the related fact that even in a solid some particles may have acquired the energy necessary to exist as gas particles, while even in a gas some particles may be practically motionless for a brief time.
It is the average kinetic energy of the particles that must be considered, together with the fact that the motion is random. At the interface between liquid and gas and between liquid and solid, an exchange of particles is always taking place: slow gas molecules condensing at the liquid surface and fast liquid molecules escaping into the gas. An equilibrium state is reached in any closed system, so that the number of exchanges in either direction is the same. Because the kinetic energy of particles in the liquid state can be defined only in statistical terms i.
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Articles from Britannica Encyclopedias for elementary and high school students. John M. See Article History. Physical properties of liquids The most obvious physical properties of a liquid are its retention of volume and its conformation to the shape of its container. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now. Load Next Page.
12.1: Crystalline and Amorphous Solids
Crystalline solids have regular ordered arrays of components held together by uniform intermolecular forces, whereas the components of amorphous solids are not arranged in regular arrays. The learning objective of this module is to know the characteristic properties of crystalline and amorphous solids. With few exceptions, the particles that compose a solid material, whether ionic, molecular, covalent, or metallic, are held in place by strong attractive forces between them. When we discuss solids, therefore, we consider the positions of the atoms, molecules, or ions, which are essentially fixed in space, rather than their motions which are more important in liquids and gases. The faces of crystals can intersect at right angles, as in galena PbS and pyrite FeS 2 , or at other angles, as in quartz. Right Cleavage surfaces of an amorphous solid. Obsidian, a volcanic glass with the same chemical composition as granite typically KAlSi 3 O 8 , tends to have curved, irregular surfaces when cleaved.
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Physical Properties Molecular Crystals Liquids Glasses
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