Factors governing Physical and Chemical Equilibrium

When a new phase is in process of formation, it may be dispersed in droplets, or minute particles, so small that the free energy per unit mass is no longer independent of the state of mechanical division, and the phenomenon of delayed transformation—which is connected with this—may appear. 

The transition from gas to liquid, in particular, follows a different course according as there is a continuous vapour-liquid interface present initially or not. If there is, then the liquid phase simply increases and the vapour phase decreases. If there is not, then droplets must form and grow safely past the limit of the region where redispersion is their likely fate. 

In Fig. 9 the pressure-volume relations of a gas-liquid system are represented. A corresponds to a dilute unsaturated vapour. On compression at constant temperature the pressure and volume change more or less in accordance with Boyle s law and the curve AB is followed. Imagine the vapour to be tested at various points by being placed in contact with a continuous surface of its liquid. Up to B, the saturation point, it would take up liquid which would evaporate into it. At B there would be equilibrium, and if in presence of the liquid the pressure were infinitesimally raised, complete condensation would occur at constant pressure the line BC would be followed to the point C. If pressure were raised further, the compression curve of the liquid, CD, would be traversed. The only variable 

Now suppose the compression from A to occur in complete absence of liquid. This time the point B possesses no special significance. From JB to X the average aggregates of molecules in the vapour (formed in conformity with the Boltzmann principle) are too small to grow. Although beyond B the free energy per unit mass of vapour 

Van der Waals equation and other equations of state. Critical phenomena 

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