Why is it called kinetic theory of gases and not kinetic theory of solids or liquids?
The particles (atoms or molecules) composing a gas are in much more simple motion. Gas molecules in an idealized gas function as ideal billiard balls, which drift around, and do not exert forces on each other until they collide, and fully rebound, conserving both momentum and kinetic energy. Also, gas particles are spaced out a lot more, such that their particle size is negligible compared to the mean free path of the particles. This enables us to construct a simple theory of the motion of molecules in the easiest to understand situation, and develop an equation of state for any gas which can be treated as an ideal gas. Parts of the kinetic theory of gases is applicable to solids and liquids. However, fundamentally, liquids and solids exist because they complicate their kinetic situation. Intermolecular forces for liquids and solids cause the particles to be much closer and the substances to be much denser. Trying to model the intermolecular forces involved is an incredible mathematica
