STATES OF MATTER PHYSICAL CHEMISTRY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site gas The state of matter in which the constituent atoms or molecules always occupy the whole of the space in which they are contained. A gas consists of particles that would move freely in an ideal gas, but which in a real gas are subject to small intermolecular foces known as van der Waal's forces. A gas is distinguished from a vapor in that a gas is above the critical temperature of the substance. Gases have low densities, are highly compressible over wide ranges of volumes, and have no rigidity and low viscosities. The molecules are usually a large distance apart compared with their diameter and there is no regularity in their arrangement in space. Given the positions of two or three molecules, it is not possible to predict where a further one will be with any precision – the molecules are distributed at random throughout the whole volume. Gas molecules move randomly with a mean velocity comparable with that of sound, of the order of 10 m/s. Occasionally two or three of them may be found very close to one another so that their electron clouds overlap and they bind together. Such clusters are common at high pressures but they are usually short-lived. The low density can be readily understood in terms of the comparatively small number of molecules per unit volume, and the high compressibility follows from the fact that the average distance between molecules can be altered over wide limits. The lack of rigidity can be explained by the molecules being able to take up any configuration with equal ease. Further, the molecules can move long distances without encountering one another, so that there is little resistance to motion of any kind, which is the basis of the explanation of the low viscosity. See also kinetic theory of gases. Related categories    • STATES OF MATTER    • PHYSICAL CHEMISTRY Source: Properties of Matter, B. H. Flowers and E. Mendoza, Wiley & Sons, 1970. Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP