Engineering Chemistry S Datta

PHASE RULE 271
From the diagram it is also found that under ordinary pressure, cooling of liquid sulfur
leads to solid monoclinic variety along CE but under high pressure, rhombic sulfur separates
on cooling liquid sulfur along EG.
Thus actual observation can be explained perfectly in terms of phase rule.
Phase Rule for Two-Component Alloy Systems
For a two component system, the highest degree of freedom F = C – P + 2 = 2 – 1 + 2 = 3
i.e., the behaviour of a system may be represented by a three-dimensional diagram of pressure,
temperature and composition which cannot be drawn on a paper.
In the case of a two component alloy system there is a solid-liquid equilibrium of the
alloy and the vapor phase is practically absent and hence the effect of pressure can be considered
to be negligible. Thus now the system is present at atmospheric pressure and it remains constant,
which will reduce the degrees of freedom by one i.e., F = C – P + 1 and the system is called
condensed system. The variables reduce to temperature and composition and hence the
diagrams are represented on temperature-composition equilibrium.
Eutectic Systems
The cooling curve of a pure substance can be represented as Fig. 11.3 (a). When the pure
liquid is cooled it reaches its freezing point b when the pure solid makes its appearance and
the temperature remains constant until the total liquid is solidified (bc) and then again
temperature falls along cd.
The cooling curve of a mixture of solid is represented by Fig. 11.3 (b). When a solid
solution is cooled at freezing point, a solid phase appears, the rate of cooling is altered and
there is a break in the cooling curve. The temperature decreases continuously but at different
rate than before freezing point. If the mixture becomes eutectic the temperature falls until the
eutectic point is reached. The system becomes invariant, temperature remains constant until
total solidification occurs (along cd) and again temperature falls uniformly along de.
a
a
Freezing
point
Temperature
b
Freezing
point
c
d
Temperature
b
c
Eutectic
temperature
d
e
Time
Time
(a)
(b)
Fig. 11.3 Cooling curves.
From the cooling curve of any mixture of definite composition freezing point and eutectic
temperature both can be obtained.
The freezing point of a system varies with the composition but the eutectic point remains
constant.