Two States Can Be Stable at the Same Time

At the midpoint of a phase transition, two phases are in equilibrium with each other. In this chapter our first example is liquid-liquid immiscibility. Put oil and water together at equilibrium, you get two phases having different compositions—one is mostly oil and the other is mostly water. We also consider boiling w ater at equilibrium, you get two phases having different densities— one is steam and the other is liquid water. 

To see how two different phases can be stable at the same time, recall that if a system has a degree of freedom x, the stable state is identified as the value 

JN Murrell and AD Jenkins, Properties of Liquids and Solutions, John Wiley Sons, New York, 1982. Data are fromJH Hildebrand, BB Fisher, and HA Benesi, J Am ChcmSoc 72, 4348(1950). 

What is p at the boiling temperature, T = 100 °C At the boiling point, there are two equal minima m the free energy function. Both the liquid and the vapor states of w ater are stable at the same time. There is more than one valley on the energy landscape, and balls roll with equal tendency into either of them. Changing the temperature changes the relative depths of the two minima. 

Let s look first at liquid-liquid immiscibility before returning to boiling. 

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