What characterizes a tie line between two phases in equilibrium?

A tie line in a phase diagram represents the relative concentrations of two coexisting phases at equilibrium. When two phases are in equilibrium, such as liquid and vapor or solid and liquid, the tie line connects points that denote the compositions of these phases at a specific temperature and pressure. The ends of the tie line indicate the compositions of the phases, allowing for the determination of how much of each phase is present in a system.

For example, in a binary liquid-liquid phase diagram, the tie line can help to illustrate how the concentrations of two components in a solution can shift between the two phases, depending on their relative amounts. Understanding this aspect is crucial in thermodynamics and physical chemistry, as it enables the prediction of phase behavior in mixtures.

The other responses do not correctly describe the nature of a tie line. While temperature can influence the position of tie lines in a phase diagram, it does not characterize a tie line itself. Moreover, tie lines do not specifically pertain to constant pressure conditions, nor do they indicate components undergoing a reaction. Instead, they focus solely on the equilibrium compositions of phases.