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:
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.
- A gas consists of molecules in constant random motion.
- Gas molecules influence each other only by collision; they exert no other forces on
- All collisions between gas molecules are perfectly elastic; all kinetic energy is
- 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
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Copyright 1995 James A. Plambeck (Jim.Plambeck@ualberta.ca).
Updated October 26, 1996 jp.