Liquid water is stable over arangeof temperatures and pressures - that is, within certain well-defined limits, we can arbitrarily choose a pressure and temperature, and the system will still be in the liquid water field on thephase diagramfor water.On the other hand, the equilibrium (i.e., mechanically, thermally and chemically balanced) coexistence of pure liquidwaterAND pureiceis rather restricted: at an arbitrarily chosen pressure, there is onlyonetemperature at which liquid and solid H2O can coexist in equilibrium. When only onephaseis stable, there is agreater degree of freedomin choosing P and T; when two phases are stable, the system has *less freedom*. We say that thenumber of degrees of freedomhas been reduced by one when there is an additional phase in an assemblage. Examining this relationship further, we find that ice/water coexistence at any given pressure is expanded from a unique single temperature at the given pressure to a RANGE of temperatures by adding a second component (such as NaCl) to the system. That is, the degrees of freedom can beincreasedby increasing the number of components.
In general, the number of degrees of freedom in a system of phases is equal to the number ofsystem components+ 2minusthe number of phases:
F = C + 2 - p
the greater the number of components C, thegreaterthe number of degrees of freedom; the greater the number of coexisting phases, p, thefewerthe number of degrees of freedom


Thank You
Ruchi
Askiitians Faculty