Statistical thermodynamics connects the microscopic world of atoms and molecules to the macroscopic world we can measure in the lab.
At the microscopic level, every system is made up of a huge number of tiny particles, each moving and interacting in many possible ways. Each of these possible arrangements is called a microstate.
Since we can’t track every particle individually, statistical thermodynamics uses ideas from probability to describe how likely each microstate is. It then averages over all these possibilities to predict the overall, or macroscopic, behavior of the system.
Through this process, we can explain quantities like temperature, pressure, and entropy as results of the collective motion and energy of many particles.
In short, statistical thermodynamics provides the link between the random motions of countless particles and the smooth, predictable laws of thermodynamics that describe gases, liquids, and solids at the large scale.