triple point In a single component system (e.g. pure water), if all the three phases, ice water and water vapor, coexist in equilibrium, application of phase rule * gives the number of degrees of freedom, F

F = C - P + 2

= 0

because the number of components (C) is 1, and the number of phases (P), 3. This indicates that it is a point in the phase (pT) diagram shown in fig. t16. The pressure and temperature are fixed for such equilibrium to exist. This point is called the triple point of water. It is used as temperature standard in the International Practical Scale of Temperature. The temperature and pressure at the triple point of water are 273.16 K and 4.579 mm of Hg..