Chemical Sciences

The Structure of Matter

The Nature of Gases

Introduction to the Kinetic-Molecular Theory of Gases

Any scientific law is an empirical generalization which describes the results of several experiments. A law, however, only describes results; it does not explain why they have been obtained. A theory is a description which explains the results of experiments. The kinetic-molecular theory of gases is a theory of great explanatory power. We shall see how it explains the ideal gas law, which includes the laws of Boyle and of Charles; Dalton's law of partial pressures; and the law of combining volumes.

The kinetic-molecular theory of gases can be stated as four postulates:

  1. A gas consists of molecules in constant random motion.
  2. Gas molecules influence each other only by collision; they exert no other forces on each other.
  3. All collisions between gas molecules are perfectly elastic; all kinetic energy is conserved.
  4. The volume actually occupied by the molecules of a gas is negligibly small; the vast majority of the volume of the gas is empty space through which the gas molecules are moving.
These postulates, which correspond to a physical model of a gas much like a group of billiard balls moving around on a billiard table, describe the behavior of an ideal gas. At room temperatures and pressures at or below normal atmospheric pressure, real gases seem to be accurately described by these postulates, and the consequences of this model correspond to the empirical gas laws in a quantitative way.
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Copyright 1995 James A. Plambeck (Jim.Plambeck@ualberta.ca). Updated October 26, 1996 jp.