Figure 1 shows the p-T projection of the p- -T surface of a typical pure fluid. The point at which the lines representing the boundaries between solid + gas, liquid + gas and solid + liquid phases intersect is the triple point. It is a fixed or invariant point since it is a result of the phase rule that a single component system with three phases has no degrees of freedom. The consequence is that the solid, liquid and vapor phases can be in equilibrium only at one thermodynamic state of pressure and temperature. The triple point is relatively easily realized for water and it is therefore frequently used as a standard temperature for the calibration of thermometers.
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