Explanation of the triple point of water.

It can be defined as the vapor pressure - temperature point at which the three states (gaseous, liquid, and solid) of pure water exist in equilibrium. At this point water molecules in gaseous, liquid, and solid states have the same chemical potential. According to the Phase Rule, the number of components, C, is equal to 1, the number of phases at equilibrium, P, is equal to 3, and the degrees of freedom, F, is equal to 0.  This means that it is sufficient to mention water at the triple point to fully describe the state (energy state of the molecules) of the system without mentioning any variable. In fact, the triple point of pure water can only exist at a particular pressure - temperature point. That is at pressure equal to 4.58 mm Hg and temperature of 0.0098 ˚C.  Below pressure value of 4.58 mm Hg water can never exist in liquid state. It can either be solid or gaseous (vapor).