In a temperature-composition diagram, what do the boundaries indicate?

The boundaries in a temperature-composition diagram indicate the composition of phases in equilibrium at various temperatures. This diagram displays how the equilibrium between different phases, such as liquid and vapor or solid and liquid, changes with temperature and composition. Each boundary line represents the specific conditions (temperature and composition) at which two phases coexist in equilibrium.

For instance, in a liquid-vapor equilibrium, the boundary marks the point where the liquid and vapor phases can coexist. By examining the diagram, one can determine the exact compositions of the phases present at a given temperature, allowing for deeper insights into phase behavior and transition between different states of matter. This is essential in thermodynamics and physical chemistry, as it aids in understanding the properties of mixtures and solutions, as well as predicting phase changes under varying conditions.