Question

What is reduced phase role?

Answer 1

In some systems, an equilibrium exists between solid -liquid phases and gaseous phase is practically absent. Hence the effect of pressure on such system can be neglected.
Then it is
Necessary to take into account only two variables viz. temperature and concentration. Such system showing solid -liquid equilibrium is called condensed system and phase rule applied to such system is as follows:-
F = C - P + 1 ... known as condensed phase rule.