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Unformatted text preview: Dr. P. Lucas U of A MSE 110 Phase Diagrams • Plotting the free energy curve G=H-TS for each phase: solid, liquid, gas gives the relative stability domain of the three phases. • The phase with the lowest G is the stable phase. • The intersection of G sol and G liq gives the melting point and the intersection of G liq and G gas gives the vaporization temperature. • Because S gas >S liq >S sol the slope of G gas is steeper than G liq which is steeper than G sol . SUMMARY FROM LAST CLASS • One component phase diagrams: • The one-component P-T phase diagram is built by combining the free energy diagrams generated for different pressure P. • The P-T phase diagram defines the relative stability of each phase at any P and T. • The gas, liquid and solid phase can coexist only at the triple point. • Several phases can only exist in equilibrium according to the phase rule: F=C-P+2 . • For most materials, the solid phase is denser than the liquid phase, however for H 2 O ice has lower density than water and the ice-water slope is negative on the P-T diagram. Therefore, ice liquefies upon compression. Dr. P. Lucas U of A MSE 110 Binary phase diagrams PHASE DIAGRAMS • At ambient pressure, pure water melts at 0ºC. • This means the salt solution is more stable than ice even below 0ºC, therefore ice melt below 0ºC. This is why we put salt on roads to melt snow and ice. • A solution of salt water has a higher entropy than pure water because it is a mixture of NaCl in water which increase molecular disorder and the statistical weigh Ω . • The larger the amount of salt, the lower the free energy curve will shift and the lower the melting point. • The dependence of the melting point on the composition of a binary (two component) system is depicted on the binary phase diagram which gives the relative phase stability as a function of composition and temperature....
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This note was uploaded on 11/19/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at Arizona.
- Spring '08