The triple point of a substance is a specific temperature and pressure at which the three phases (solid, liquid, and gas) of that substance coexist in thermodynamic equilibrium. This means that at the triple point, a substance can exist as a solid, a liquid, and a gas simultaneously, and there is no net change in the amounts of each phase over time, as the rates of phase transitions (like melting, freezing, boiling, condensation, sublimation, and deposition) are equal.

Triple Point Definition

The triple point is a unique set of temperature and pressure conditions for a pure substance. It is a specific point on its phase diagram where the solid, liquid, and gas phase boundaries meet. For water, the triple point is defined as 273.16 K (0.01 °C) and 611.657 Pa (approximately 0.006 atm). This exact point is crucial because it is used to define the Kelvin temperature scale, which is the SI unit for temperature.

Equilibrium at the Triple Point

At the triple point, the substance reaches a perfect balance where all phase changes occur simultaneously and at equal rates. Consider water at its triple point:

• Ice is melting into water at the same rate that water is freezing into ice.
• Water is boiling into water vapor (gas) at the same rate that water vapor is condensing into water.
• Ice is sublimating directly into water vapor at the same rate that water vapor is depositing directly onto ice.

Because all these transitions are balanced, the macroscopic amounts of solid, liquid, and gas remain constant, creating a state of dynamic equilibrium. This means that if you were to observe a substance at its triple point, you would see all three states of matter existing together in a stable condition.

Understanding Phase Transitions

The options provided describe various phase transitions. Let's look at why they are not the complete answer for the triple point:

• Matter solidifies: Solidification (freezing) is a transition from liquid to solid. This occurs at the freezing point, which is a line on the phase diagram, not a single point where all three phases coexist.
• Matter liquifies: Liquefaction (melting) is a transition from solid to liquid. This occurs at the melting point. Again, it's a phase transition, not a state of three-phase equilibrium.
• Matter sublimates: Sublimation is a direct transition from solid to gas. This occurs below the triple point pressure on the phase diagram. While sublimation does occur at the triple point, it is only one of the simultaneous transitions, not the sole process.
• Matter exists in equilibrium in all the three states (solid, liquid and gas): This statement accurately describes the unique condition at the triple point, where solid, liquid, and gas phases coexist in stable equilibrium. This is the defining characteristic of the triple point.

Summary of Triple Point Conditions

In summary, the triple point represents a very specific and unique set of temperature and pressure conditions for a substance. It is the only point on a phase diagram where solid, liquid, and gas phases can exist together in a stable, dynamic equilibrium. This concept is fundamental in thermodynamics and phase transitions, providing a critical reference point for material properties.