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gas molecules in a container
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 forces known as van der Waal's forces. Gas particles move randomly at a wide range of speeds of the order of 100 m/s. A gas is distinguished from a vapor in that a gas is above the critical temperature of the substance. See also liquid; solid.

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.

For a given temperature and pressure, equal volumes of gas contain the same number of molecules (2.7 × 1025 m-3 at room temperature and atmospheric pressure). The impacts of the molecules on the walls of the container are responsible for the pressure exerted by gases, which is much larger than is often appreciated: the atmosphere exerts on everything a pressure many times larger than a person's weight and without it we would quite simply boil and burst.

For a given mass of ideal gas, the product of the pressure (P) and the volume (V) is proportional to the absolute temperature (T):
PV = RT (the general gas law).
The constant of proportionality (R) is known as the universal gas constant and has the value 8.314 J/K-mole. The general gas law and most of the other properties of gases can be explained in terms of the kinetic theory without reference to the internal structure of the molecules.

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Source: Properties of Matter, B. H. Flowers and E. Mendoza, Wiley & Sons, 1970.