In a phase diagram, what do the lines represent?

In a phase diagram, the lines represent the boundaries between different phases of a substance, indicating the conditions (such as temperature and pressure) under which distinct phases coexist in dynamic equilibrium. These equilibrium lines show the points at which two phases can exist together; for example, the line between the solid and liquid phases indicates the melting point at various pressures. When a system is at a point along one of these lines, it is in dynamic equilibrium, meaning that the two phases are continuously transforming into each other at equal rates.

The significance of this is that it provides critical insight into how substances behave under varying temperature and pressure conditions, which is essential in many applications, from manufacturing processes to understanding environmental changes. Each line’s slope and position give valuable information about the thermodynamic behavior of the material involved.