What happens at the triple point of water?

At the triple point of water, the three physical states: ice, liquid water, and water vapor—coexist in perfect equilibrium. This means that under these precise conditions, the rate of melting equals the rate of freezing, and the rate of evaporation equals the rate of condensation. 

There is no net change among the phases even though molecular activity continues constantly. If temperature or pressure slightly changes, this delicate balance breaks, causing one or two phases to disappear.

 For instance, lowering the pressure turns liquid water directly into vapor (sublimation), while increasing temperature shifts the system entirely into vapor. This point reveals how phase transitions depend on both variables. In laboratories, scientists reproduce the triple point using sealed cells with pure water and precisely controlled environments to calibrate thermometers and pressure sensors. 

It’s a physical demonstration of equilibrium, illustrating how matter transitions smoothly between phases. The triple point also serves as a fundamental concept in thermodynamics because it’s one of the few natural states where multiple phases of a substance can coexist stably without external interference or continuous change.