Triple Point of Water

 

Triple Point of Water/substance :- The temperature and pressure at which all the three phases of a substance coexist is called the triple point of a substance.

We have studied that during change of state(phase change) the temperature of a substance remains constant. A graph between the temperature T and the Pressure P of the substance is called a phase diagram or P – T diagram. The following figure shows the phase diagram of water :-

The above phase diagram divides the P – T plane into three regions :

1. The solid-region
2. The vapour-region and
3. The liquid-region.

These regions are separated by following curves :

1. Fusion curve or ice line (AB) : Points on the fusion curve AB represent states in which solid and liquid phase coexist. This curve shows that the melting point of ice decreases with increase in pressure.
2. Vaporization curve or steam line (BC) : Points on the vaporization curve BC represent states in which the liquid and vapour phases coexist. This curve shows that the boiling point of water increases with increase in pressure.
3. Sublimation curve or hoar frost line (OB) : Points on the sublimation curve OB represent states in which the solid and vapour phases coexist.

All the three curves meet at point B which is called triple point of water/substance. Hence we may define :

Triple Point of Water

The temperature and pressure at which the fusion curve, the vaporization curve and the sublimation curve meet and all the three phases of a water(or any substance) coexist is called the triple point of water.

At triple point of water the values of temperature and pressure are :

T_triple = 0.0098ºC ≅ 0.01ºC = 273.16 K

P_triple = 4.6 mm. of Hg column = 605.26 N/m² = 0.00625 atm