Topic overview

Start with the big picture

Phase transitions occur at boundaries where Gibbs free-energy change is zero; at equilibrium, each component has the same chemical potential across coexisting phases. The Gibbs phase rule relates the number of components and phases to the variables that can be set. The lesson then considers pharmaceutical examples: polymorphs and hydrate or solvate transitions can affect material properties, while glass transition and melting behavior inform stability and compatibility. Eutectics, sublimation, and supercritical fluids illustrate how phase behavior can shape processing. Relationships involving vapor pressure, ideal solutions, partitioning, and interfacial tension extend the discussion to drying, distribution between phases, and formulation behavior. Together, these ideas link physical chemistry to practical pharmaceutical considerations.