Kinetic Theory of Gases
The atomistic principle rendered a very great service in the development of a picture of gases which could account for such properties as pressure. Newton’s mechanics, which contributed so much toward our knowledge of the stellar universe, finds a beautiful application to the molecules of a gas. Although the conception of the idea of the kinetic theory of gases can be traced further back, Clausius ( 1857) really founded it after Joule (185 I) had attempted to calculate the velocity of a hydrogen molecule. The calculations were inexact, but they mark the beginning of an actual atomic view of matter as distinguished from the mere symbolic view of the chemist up to that time.
The molecules of a solid are considered to be bound to certain positions which result in equilibrium, but this is not the case in gases. The molecules of a gas are continually in motion, excepting at absolute zero, about 491 degrees Fahrenheit below the melting-point of ice. At first they were likened to elastic spheres shooting in all directions and colliding with each other. At ordinary temperatures and pressures a gas-molecule will suffer several billion collisions each second. Although the modern ideas of the atom are not fully developed in relation to the kinetic theory of gases, it is likely that the molecules do not actually collide in the ordinary sense. The modern conception is that atoms are tiny planetary systems of electrons revolving around a positive nucleus. Doubtless they are surrounded by electric forces so that instead of a collision we have a repulsion of the electric forces. At any rate the true picture is of little consequence here because it will not alter the actual facts.
The pressure on the sides of a vessel containing a gas is due to the billions of collisions of the molecules with the sides. The force of an impact is determined by the mass and the velocity. The pressure is the result of the number of impacts per second. If the density of the gas is doubled there will be twice as many impacts in a second on a given area of the vessel and, therefore, the pressure will be doubled. This relation of density and number of molecules was suggested by Boyle (1627-1691) which gave rise to Boyle’s law. This law states that, for a given temperature, the product of the pressure and the volume is constant.
The temperature of a gas is determined by the mean energy of motion of a molecule. Here again Newton’s mechanics enter, for the energy of motion of a molecule is proportional to its mass and to the square of its velocity. As the temperature decreases, the energy of motion (kinetic energy) of a molecule decreases until at absolute zero there is no motion. Hence, at absolute zero the pressure of any gas is zero. It may be of interest to know that absolute zero has not quite been reached in the laboratory, but it has been approached to within less than a degree Centigrade.