The ideal-gas equation, which is empirical, describes the behavior of gases by establishing relationships between their macroscopic properties. For example, Charles’ law states that volume and temperature are directly related. Gases, therefore, expand when heated at constant pressure. Although gas laws explain how the macroscopic properties change relative to one another, it does not explain the rationale behind it.
The kinetic molecular theory is a microscopic model that helps understand what happens to gas particles at the molecular or atomic level when conditions such as pressure or temperature change. In 1857 Rudolf Clausius published a complete and satisfactory form of the theory, which effectively explains the different gas laws through the postulates that were developed based on hundreds of experimental observations of the behavior of gases.
The salient features of this theory are:
This text is adapted from Openstax, Chemistry 2e, Chapter 9.5 The Kinetic-Molecular Theory.