Engineering Chemistry S Datta

PHASE RULE 275
Highlights:
Condensed phase rule. For two-component alloy systems it is considered that
there is practically no gas phase and the effect of pressure is negligible. So the
system is considered to be under atmospheric pressure which reduces the degrees
of freedom by one and hence,
F = C – P + 1
Triple point. It is the point of a system at which the gaseous, liquid and solid
phases of a substance can co-exist in equilibrium. At triple point the system is nonvariant.
Eutectic point. A solid solution of two or more substances having lowest freezing
point is the eutectic mixture and the minimum freezing point attained is the eutectic
point.
Congruent melting point. A compound possesses congruent melting point if it
melts to give a liquid having the composition same as the solid.
SOLVED PROBLEMS
Problem 1. An alloy of Cd and Bi contains 25% Cd. Find the mass of eutectic in 1 kg of
the alloy, if the eutectic system contains 40% Cd.
Sol. In the eutectic system, 40 % is Cd and hence 60 % is Bi.
In 1 kg alloy content of Cd is 25%, i.e., 250 gm.
250 × 60
∴ Corresponding to 250 gm Cd, the content of Bi in eutectic is gm = 375 gms.
40
∴ The mass of eutectic in 1 kg alloy is (250 + 375) gm = 625 gm. Ans.
Problem 2. An alloy AB contains 40% A in 1 kg. AB on cooling forms eutectic containing
60% A and 40% B and the rest of B. What is the amount of B formed during eutectic formation?
Sol. In 1 kg of AB, A = 40 % = 400 gm and B = 60% = 600 gm.
The eutectic mixture contains 60 % A and 40 % B.
400 × 40
∴ Corresponding to 400 gm A, the content of B = = 266. 7 gm
60
∴ Amount of B separated out = (600 – 266.7) gm = 333.3 gm. Ans.
SHORT QUESTIONS AND ANSWERS
Q. 1. State Phase rule.
Ans. See page 267.
Q. 2. What is condensed phase rule?
Ans. See page 272.
Q. 3. Define the following terms:
(a) Phase (b) Component (c) Degrees of freedom.