What point on a phase diagram indicates all three phases are in mutual equilibrium?

The point on a phase diagram where all three phases—solid, liquid, and gas—coexist in mutual equilibrium is known as the triple point. At this unique set of conditions, the temperature and pressure are precisely suited such that the three phases can exist simultaneously without any tendency for one phase to convert completely into another. This equilibrium condition is significant because it illustrates the fundamental aspects of phase behavior and thermodynamics.

In addition, the triple point is defined for a specific substance; for example, the triple point of water occurs at approximately 0.01 degrees Celsius and 611.657 pascals. This is a critical reference point for defining the Kelvin temperature scale.

While the critical point represents the end of the liquid-gas phase boundary where distinct liquid and gas phases cease to exist, and the melting and boiling points correspond to first-order phase transitions, they do not denote the simultaneous coexistence of all three phases in equilibrium. Hence, the triple point uniquely encapsulates this concept.