Molecular nature of matterWe have learnt that every substance (solid, liquid or gas) consists of extremely tiny particles called molecules. In a gas, the molecules move rapidly because the forces between them are weak. Various phenomena, such as diffusion, evaporation, elasticity and Brownian movement, provide conclusive evidence of molecular motion.
Based on these observations and the fact that heat can be identified with molecular motion, a theory has been developed which satisfactorily explains physical concepts like temperature, pressure, elasticity, etc. This theory is called the kinetic theory of matter.
The kinetic theory of solids and liquid is too complicated to be discussed. We shall consider only the kinetic theory of gases because of its simplicity.