Question

why is 0° known as triple point ?

Answer 1

Water exists in three distinct phases at something called the triple point. Zero degrees celsius is defined by the triple point of water which is 273.16K at 611.2 Pa. At a certain energy molecules will have enough energy to evaporate, even if the water temperature is 0 degrees C.

Answer 2

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Triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium .