What is an isopleth in the context of phase diagrams?

An isopleth, in the context of phase diagrams, refers to a line of constant composition. This is crucial for understanding how different phases coexist within a mixture given changes in temperature and pressure. In phase diagrams, isopleths illustrate how the composition of the phases varies and helps in visualizing how mixtures behave when subjected to different conditions.

When examining a phase diagram, isopleths enable you to identify regions where specific compositions will exhibit certain phase behaviors, such as liquid, solid, or gas states. By following a specific isopleth, one can predict how the equilibrium phases will respond to temperature or pressure changes while maintaining a fixed composition. This is distinct from the other options, which represent different conditions or characteristics that do not specifically denote constant composition. For instance, lines of constant temperature or pressure are referred to differently and do not convey the same information about the composition of the phases present in the system.