Triple Point Definition: Unique Phase Coexistence

The triple point of a pure substance is a specific temperature and pressure at which three thermodynamic phases—solid, liquid, and gas—coexist in equilibrium.

Analyzing the Options:

• Option 1: Incorrect. The triple point relates to the phases of a single pure substance, not a mixture of gases.
• Option 2: Correct. This accurately describes the triple point as the unique condition where solid, liquid, and gas phases exist together.
• Option 3: Incorrect. Sublimation is a phase transition (solid to gas) that can occur at various conditions, not exclusively at the triple point. The triple point is a singular point of three-phase equilibrium.
• Option 4: Incorrect. Since Option 2 is correct, this option is invalid.

Key Characteristics of the Triple Point:

• It is an invariant point, meaning it occurs at only one specific temperature and pressure for each substance.
• At the triple point, all three phases (solid, liquid, gas) are stable and can coexist indefinitely.
• For water, the triple point occurs at approximately 273.16 K (0.01°C) and 611.657 Pa (about 0.006 atm).

Therefore, the triple point is defined as the point where three phases exist together.