Triple point with reference to one component water system:

• In one component system, the equilibrium conditions may be represented with the help of diagrams taking pressure and temperature at two axes. This diagram is called as pressure-temperature (p-t) diagram.
• In this diagram any line or curve represents an univariant system, because the equilibrium conditions at any point on line can define by fixing pressure or temperature.
• All areas represents bivariant, to define the system completely at any point in the area both the pressure and temperature should be fixed.
• In the diagram all the three phases that is solid, liquid and vapour are in contact with each other at a specific point called astriple point represents zero variant system because system is completely defined by itself.
• The triple point occurs at 0.0075 degree C and 4.58 mm Hg pressure, since there are three phases and one component, hence F=C - P + 2, hence F = 1 - 3 + 2 = 0.
• The system at triple point is zero variant or non-variant. Thus neither temperature nor pressure can be altered. Even slight changed three phases would not exist if one of the phase disappears.
• Water system under normal condition is of three phases and one component system. The system involved three phases such as solid-ice, liquid-water and gas-vapour. All these phases can be represented by one chemical entity H2O, hence it is one component system.