Change of Phase Equilibrium Phase Equilibrium . Phase is defined as a substance's several states (solid, liquid, and gas). Equilibrium: When a system is in equilibrium, neither its internal energy level nor its state of motion change over time. The study of equilibrium that exists within the various phases of matter (solid, liquid, and gas) is what is meant by the phrase phase equilibrium. Figure 1:- Phase diagram Understanding the connections between the various stages is made easier with the help of the phase diagram above. It often illustrates how temperature and pressure affect a system's phase as a function of each other. The triple point is a point on the graph where all three states coincide, and it differs for each component. The liquid and gaseous phases unite to form one phase at critical temperatures and pressures, which is where the term "critical point"...