PHASE RULETWO COMPONENT SYSTEMTHEORY 1 TwoComponentSystem(LeadSilver System ) Thissystemhastwocomponent&fourphasesthesearei) Solid Silver, ii) Solid lead, iii) Solution of molten silver & lead & iv) Vapour. ButtheB.P.ofAg&Pbiscompletelyhighthevapourphaseis completely absent. Since the press has nearly no effect on equilibrium. so the system can be convinently represented by temp conc diagram. Suchsolidliquidsystemwiththegasphaseisabsentiscalled condensed system Theexperimentalmeasurementsoftemp&concincondensed system are usually carried at under atm press. Sincethedegreeoffreedomissuchsystemisreducedbyone, therefore,itcanbealsotermedasreducedphaserule& represented by the equation. PHASE RULETWO COMPONENT SYSTEMTHEORY 2 F = C P + 1 Itismoreconvenienttoapplytosolidliquidtwocomponents condensed system. e.g. Pb Sb, Ag Pb, Zn Cd. The complete phase diagram of systemSilver Lead. Diagram F= 2P+ 1= 33= 0100%Pb0%Ag303SolidPb+Sol (Pb+ Ag)n961 ALiquidFreezing curve of AgSolidelectricmixture+ SolidAgSol getsaturatedWr+ Pbn0Freezing curveofPbB 327Solid electricmixture+ SolidPb100%Pb0%Ag97.4%Pb2.6%AgSolidAg+ LiquidF= 2P+ 1= 32= 1UnivaliantTempC%CompositionF= 2P+ 1= 32= 1Univariant % Composition PHASE RULETWO COMPONENT SYSTEMTHEORY 3 From fig following silent features are observed: i)The Curve OA (Freezing Curve of Ag) ii)The Curve BO ( Freezing Curve of Pb)iii)The eutectic Point O (i)Curve OA Freezing Curve of Ag a)It shows the effect on freezing point of Ag on addition of Pb in small quantities. b)The curve starts from A (961oC) the M.P. of Ag, where pureAgcoexistassolid&liquid(Vapourbeing neglected). c)ThiscurveindicatesthatthemeltingpointofAgfall gradually on adding Pb, along AO, till the lowest point O (303oC)isreachedwheresolutiongetssaturatedwith respecttolead.AtOmorePbcangoinsolution& consequently, the M.P. does not fall any further & if any Pb is added it separates as the solid phase. Alongthiscurve,solidAg&Solution(Vapourbeingnegligible)Coexist & hence, according to reduced phase rule equation PHASE RULETWO COMPONENT SYSTEMTHEORY 4 E= 2 P+ 1 i.e.F =3 P= 3 2 = 1 i.e.systemisunivariant.ThepointO(303oC)Correspondstoa fixed composition of 2.6%Ag & 91.4% Pb & is known as eutectic composition. On cooling the whole mass crystallizes out as such. ii)Curve BO Freeing curve of Pb i)ItrepresentstheeffectonfreezingpointofPbon gradualadditionofsmallamount ofAgtoit.Point Bis the M.P. of pure Pb-32. ii)Along BO, the M.P. Gradually falls on the addition of Ag till lowest point O is reached. iii)At this point solution gets saturated with respect to Ag & M.P. of Pb does not fall any more. iv)On cooling whole mass crystallizes out. The system is univariant like AO. PHASE RULETWO COMPONENT SYSTEMTHEORY 5 iii)Point O Eutectic Point ThetwocurveAO+BOmeetatO.Wherethreephases solidAg,Solid-Pb&theirsolutionCoexist&accordingto condensedphaserule.TheptofOrepresentsafixed compositionofAg2.6%,Pb-97.4%iscalledeutectic composition. Ateutecticcomp.temp.remainsconstantuntilthewholeofthe meltsolidifiesinblocktobecomesolidofeutecticcomposition. However, further cooling results in the simultaneous crystallization ofamixtureofAg&Pbinrelativeamountscorrespondingto eutectic point O. Below temp line there are two regions eutectic solid + solid Ag in crystalline- stable eutectic + solid Pb in crystalline stable. ItrepresentsolutionofPbAgsampleofPbcontaininglessthan 2.6%Ag is taken, at anarbitrary pt on curve. On following mass cool temp gradually falls without any charge in composition till this point reached to curve BO (point may be P) PHASE RULETWO COMPONENT SYSTEMTHEORY 6 Onloweringthetempleadbeginstoseparatesout&the composition varies along pb till point O is reached. On cooling whole mass solidifies in block. Applicationsi)Used in pattinsons process of desilverization of Pb. ii)If Pb is less than 2.6Pb will separate out from Solution. iii)If Pb is more than 2.6 Ag will separate out from solution.