CHAPTER 4. PHASE EQUILIBRIUM
When two phases are in equilibrium, they have the same chemical potential. Let’s examine a simple one component, two phase system, steam and liquid water at equilibrium. The equality of chemical potential means that the vapor has just as much tendency to become liquid as the liquid has tendency to become vapor. This means that while individual molecules of water are changing from liquid to vapor and from vapor to liquid, both processes are happening at the same rate. Thus, the system as a whole is in equilibrium.
Seader, JD, Henley, Ernest, and Roper, Keith. Separation Process Principles With Applications Using Process Simulators. Wiley Publishing. USA. 2016.