Triple Point of a Pure Substance

Definition:- Triple point is define as the state of the pure substance existing as an equilibrium mixture of solid, liquid, vapour phase of that substance so that the state is define by fixed
temperature and pressure.

In the fig. shown                                                                         

1.       OA is the vapourisation curve along which liquid and vapour phase of the substance coexist in equilibrium condition.                                                                                      

2.       OC is the fusion curve along which liquid and solid phase of the substance coexist in equilibrium condition.

3.       OB is the sublimation curve along which the solid and vapour phase are coexist in equilibrium condition.

  The intersection the curves must meet at a point O, where the substance can exist as a equilibrium mixture of solid, liquid and vapour. Thus the point O define by a fixed temperature and pressure is known a Triple Point of that substance.

The Triple Point is the only fixed point of a substance which can be measure accurately in any time at any place of our earth.

Foe water the triple point temp. = 273.16 K

                   the triple point pressure = 4.58 mm Hg 

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