Equilibrium displacements in open systems

Here we consider two single phase open systems, the independent variables now being 

We have already obtained an equation (c/. 17.24) for (dAldni) in an ideal system, so that for this particular case, (18.7) becomes  

If we suppose that the only difference between the two systems lies in p and then 

In the same way one can write a series of other equations by supposing that only two of the c + 2 increments ST, Sn. .. Sn are different from zero. 

First of all we shall examine (18.9) in more detail, and to fix our ideas we suppose that the system is an ideal solution and that 3, 0. The 

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