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The phase boundaries are defined by the fact that they represent the precise conditions of temperature and pressure under which the chemical potentials of the two phases on either side of the boundary are equal.
We derived, on the previous page, the Clapeyron Equation. It now remains to show how it may be applied to the most commonly encountered phase boundaries.
It is a consequence of the Second Law that at equilibrium, the chemical potential of a substance must be the same throughout, regardless of the number of phases present.
This diagram is a highly simplified representation of the phase diagram of water. At high pressures (greater than 2000 atmospheres) various allotropes of solid ice have been observed – these are omitted for clarity.
When a liquid is heated in an open vessel, its temperature and vapour pressure will both increase. At the temperature at which the liquid’s vapour pressure (the pressure at any given temperature at which both liquid and vapour are in equilibrium) would be equal to the external pressure, vaporisation can occur throughout the bulk of the liquid, and free expansion of the gas into its surroundings may take place.
A phase of a substance is a form of matter that is uniform throughout in chemical composition and physical state.