What does a phase diagram represent?

A phase diagram is a graphical representation that illustrates the physical states of a substance as they vary with changes in temperature and pressure. It typically shows the boundaries between different phases — solid, liquid, and gas — indicating the conditions under which a substance exists as each phase. The axes of a phase diagram typically correspond to temperature (on one axis) and pressure (on the other), with lines that denote the phase transitions, such as melting, boiling, and sublimation.

By understanding a phase diagram, one can predict the state of a substance at any given temperature and pressure. For example, within a diagram, specific regions indicate where the substance is in solid form, liquid form, or gaseous form. The critical point and triple point marked on these diagrams are vital for understanding phase behavior, particularly in applications involving changes in temperature and pressure.

The other options do not accurately encapsulate the comprehensive role of a phase diagram. While some may touch upon elements of phase behavior or properties, they do not fully represent the interrelation of temperature and pressure conditions with the physical states of a substance, which is central to what a phase diagram conveys.