Solid-liquid equilibria The simple eutectic diagram

3 SOLID-LIQUID EQUILIBRIA THE SIMPLE EUTECTIC DIAGRAM 

If a liquid solution of two substances A and B is cooled to a sufficiently low temperature, a solid will appear. This temperature is the freezing point of the solution, which depends on the composition. In the discussion of freezing-point depression. Section 13.6, we obtained the equation 

A point such as a represents solution of composition b in equilibrium with solid of composition c, that is, pure A. By the lever rule, the ratio of the number of moles of solution to the number of moles of solid A present is equal to the ratio of segments of the tie line ac/ab. The lower the temperature, the greater the relative amount of solid for a specified overall composition. 

This curve cannot represent the situation over the entire range of composition. As Xg 1, we would expect solid B to freeze out far above the temperatures indicated by the curve in this region. If the solution is ideal, the same law holds for substance B  

The lead-antimony system has the simple eutectic type of phase diagram (Fig. 15.7). The regions are labeled L signifies liquid, Sb or Pb signifies pure solid antimony or pure solid 

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