What is the Kinetic Gas Law

   The gas laws developed by Boyle, Charles, and Gay-Lussac are based upon empirical observations and describe the behavior of a gas in macroscopic terms, that is, in terms of properties that a person can directly observe and experience. An alternative approach to understanding the behavior of a gas is to begin with the atomic theory, which states that all substances are composed of a large number of very small particles (molecules or atoms). In principle, the observable properties of gas (pressure, volume, temperature) are the consequence of the actions of the molecules making up the gas.

   The Kinetic Molecular Theory of Gases begins with five postulates that describe the behavior of molecules in a gas. These postulates are based upon some simple, basic scientific notions, but they also involve some simplying assumptions. In reading a postulate, do two things. First, try to understand and appreciate the basic physical idea embodied in the postulate; this idea will ultimately be important in understanding the macroscopic properties of the gas in terms of the behavior the microscopic molecules making up the gas. Second, identify possible weakness or flaws in the postulates. Inaccurate predictions by a theory derive from flawed postulates used in the derivation of the theory.

Definition Provided Davidson College 


  1. A gas consists of a collection of small particles traveling in straight-line motion and obeying Newton's Laws.
  2. The molecules in a gas occupy no volume (that is, they are points).
  3. Collisions between molecules are perfectly elastic (that is, no energy is gained or lost during the collision).
  4. There are no attractive or repulsive forces between the molecules.
  5. The average kinetic energy of a molecule for diatomic gases is 3/2 kT. (T is the absolute temperature and k is the Boltzmann constant.)