What is the triple point of water?

The triple point of water is the specific temperature and pressure at which all three phases of water—solid (ice), liquid (water), and gas (vapor)—can exist simultaneously in equilibrium. It represents a unique thermodynamic state where phase boundaries intersect on a phase diagram. For pure water, this occurs at a temperature of 0.01°C (273.16 K) and a pressure of approximately 611.657 pascals. 

At this precise condition, none of the phases dominates; ice melts, water evaporates, and vapor condenses at equal rates. The triple point serves as a reference for calibrating thermometers and defining temperature scales because it is constant and reproducible under controlled laboratory conditions. It’s used in physics and engineering to illustrate phase relationships and study substance behavior under different conditions. 

In scientific standards, the Kelvin temperature scale was once defined based on the triple point of water. This point highlights how temperature and pressure together determine the state of matter. In short, the triple point of water is fundamental in thermodynamics because it marks the exact conditions where all three states of water coexist perfectly in balance.