What does a tie line represent in a phase diagram?

A tie line in a phase diagram represents a line that connects points of two coexisting phases at equilibrium. In a typical phase diagram, such as those for a binary mixture, the tie line indicates the compositions of the phases in equilibrium with each other at a specific temperature and pressure. Essentially, it shows how much of each component is present in each phase, allowing one to determine the ratios of the phases when considering phase separation.

At a given temperature and composition along the tie line, you can observe two phases, and the ends of the tie line correspond to the compositions of these phases. This is particularly relevant in understanding phase separation and the behavior of mixtures at different temperatures, pressures, and compositions.

In the context of the other options, while a line of constant temperature does exist in phase diagrams, it does not specifically define the relationship between phases. Similarly, a line of varying pressure is also present, but it does not indicate equilibrium between phases. Finally, a line indicating a phase change is often referred to as a phase boundary, rather than a tie line, highlighting the distinction in terminology used in phase diagrams.