Silver and Lead in the molten state mix with each other in all proportions, giving a homogeneous solution.
Also lead and silver do not react chemically and there are four possible phases in equilibrium. They are (i) Solid Lead (ii) Solid Silver (iii) Molten Solution of Ag + Pb (iv) Vapour of Ag + Pb.
The boiling points of lead and silver (1893 K and 2228K) are very high and hence the gaseous system is practically absent. The pressure will have no effect on the equilibrium and hence the phase diagram will be temperature vs. composition.
The equilibrium diagram consists of two curves, AO showing the effect of addition of silver on the melting point of pure lead, and OB the effect of addition of lead on the melting point of Pure Silver. The melting point of Pb (600K) gradually falls along the curve AO during the addition of Ag. Similarly, the melting point of Ag (1233K) gradually falls along BO on addition of Pb. The degree of freedom on any curve will be
F = C - P +1 = 2 - 2 + 1 = 1
Hence the system is univarient.
The two curves meet at point O, called the Eutectic Point 576 K. This point O, represents the condition at which solid Pb, solid Ag and the solution coexist. The degree of freedom,
F = C - P + 1 = 2 - 3 + 1 = 0
At this point, the system is non-variant.
Eutectic point is the lowest temperature at which the liquid can exist and if the liquid is cooled below this temperature, both the components separate simultaneously in the solid form, without change in temperature of composition. The corresponding temperature and composition are known as Eutectic temperature and eutectic con1position respectively. For this system the eutectic composition is 2.4% Ag and 97.6% Pb.
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