An amorphous (noncrystalline) form of matter intermediate between gases and solids in which the molecules are much more highly concentrated than in gases but much less concentrated than in solids. The molecules of liquids are free to move within the limits set by intermolecular attractive forces. At the air-liquid interface the vibration of the molecules causes some to be ejected from the liquid at a rate depending on the surface tension. The tendency of molecules to escape from a liquid surface is called fugacity and is largely responsible for evaporation, which occurs when the air space above the liquid is unrestricted. In a closed system, where the air space is restricted, the escaping molecules eventually saturate the air, and thus the number of molecules leaving the liquid will be equal to those returning to it as a result of molecular attraction. In these circumstances the liquid-air system is said to be in equilibrium.Liquids vary greatly in viscosity, melting point, vapor pressure, and surface tension. Mercury has a density of 13.6 and the highest surface tension of all liquids. Glass has the highest viscosity. Polar liquids are those whose molecules have opposite electrical charges on their terminal atoms or groups, that impart a force called dipole moment. Water is a polar liquid with high dielectric constant. Pure hydrocarbon liquids are generally nonpolar.See Liquid, Newtonian; Glass; Amorphous; Solid; Liquid Crystal; Kinetic Theory.
