The maximum number of phases in a two component system will be four. Maximum number of phases exists when degree of freedom is zero
$P = C – F + 2 \\ = 2 – 0 + 2 \\ = 4$
Maximum degree of freedom in a 2 component system will be 3 when system exists as one phase
$F = C – P + 2 \\ = 2 – 1 + 2 \\ = 3$
System will have three variables namely temperature, pressure and concentration. For constructing a phase diagram of 2-component system, a three dimensional space model is required using these 3 variables as its coordinates.
It is possible to split such a 3D diagram into 2D by keeping the third variable in each case as constant.
It is convenient to prepare temperature – composition (T-C) diagrams keeping pressure constant, such diagrams are called Isobaric.
Similarly, P-C diagrams at constant temperature are called isothermal.
P-T diagrams at constant composition are called isoplethal.
Any such restriction in phase rule equation regarding constancy of one of the variables reduces the phase rule equation to following form –
F = C – P + 1
F + P = C + 1
This is known as reduced phase rule equation