Phase Rule as applied to Binary Diagrams

Binary diagrams are drawn for atmospheric pressures, thus P is no longer a variable and the Phase Rule becomes:

With reference to the hypothetical binary diagram:

1. At X
   Point X lies within the liquid field,
   
   • P = 1 - Liquid
   • C = 2 - two components A and B
   • F = 2 - two degrees of freedom, giving a divariant area.
   • To maintain equilibrium, i.e. to keep the single phase Liquid stable, T and X may be varied independently.
2. At Y
   Point Y lies on the boundary curve between the fields of Liquid and A + Liq.
   • P = 2 - Solid A and Liquid
   • C = 2 - two components A and B,
   • F = 1 - one degree of freedom yielding a univariant curve.
   • To maintain equilibrium, i.e. to stay on the boundary curve, Fixing T automatically fixes X.
3. At E
   Point E is the eutectic, the point where the liquidus and solidus intersect.
   • P = 3 - Solid A, Solid B and Liquid,
   • C = 2 - two components A and B,
   • F = 0 - no degrees of freedom, an invariant point.
   • No degrees of freedom, all 3 phases coexist in equilibrium, changing either temperature or compostion from the point represented by E will cause one or more of the phases to disappear, thus changing the value for P in the phase rule equation.