Uniform composition, line states

We are now in a position to study the series of states of uniform composition which are obtained when T,p and a 2 all vary simultaneously. These three variables are always related by the two equatoms (28.17) it follows therefore that only one of them can be varied independently, so that the states of uniform composition fall on a line called the azeotropic line. The differential relations which this line must satisfy are examined below. ... 

It is assumed here that solidification is being carried out as a series of equilibrium states in which the solid continually adjusts through solid-state diffusion to the value corresponding to the intersection of the tie-line with the solidus. Eventually as the temperature falls to T3, the final solid will have a uniform composition equal to the starting value. 

Let us first discuss the phase diagram of water. The phase diagrams show the thermodynamically stable physical states of matter at different temperatures and pressures. Within each phase, the material is uniform with respect to its chemical composition and physical state. At typical temperatures and pressures on Earth (T = 298 K (25°C) andV = 1 atm) water is a liquid, but it becomes solid (that is, ice) if its temperature is lowered below 273 K (0°C) and gaseous (that is, water vapor) if its temperature is raised above 373 K (100°C), at the same pressure. Each line (phase coexistence line) on a phase diagram represents a phase boundary and gives the conditions when two phases may stably coexist (in any relative proportions) [1]. 

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