Point for Pure Substances

Let us consider the family of isotherms of a pure substance. At sufficiently high temperatures each isotherm is a continuous curve, but at low temperatures the isotherm consists of three portions. The section of the curve at high pressures corresponds to the liquid state, while that at low pressures refers to the gaseous state. The two curves are joined by a horizontal line corresponding to the simultaneous presence of two phases, liquid and gas. 

At some temperature we have the transition between isotherms corresponding to the gas phase only and those including a horizontal portion corresponding to a liquid-gas equilibrium. At this temperature the horizontal segment of the isotherm has contracted to a simple point of inflexion. This is the criticalpoi nioi the system It is characterized by the conditions for the existence of a point of inflexion with a horizontal tangent 

The various parts of the continuous curve A V N M L D have distinct physical significance. First of aU we see that between N and M 

Stable, metastable and unstable states in the vapourisation of a pure substance 

It can be diown that such states are unstable. This means that thermal fluctuations will destroy spontaneously this phase (cf. e.g. Prigogine and Defay [1950], Ch. XV and XVI). The portion V N corresponds to a supersaturated vapour which is metastahle and disappears spontaneously if condensation nuclei are introduced into the system. Similarly M L corresponds to an overexpanded liquid, which again is a metastable state. If we take the loci of the points M and N of a series of isotherms we obtain the curves aC and bC (cf. Fig. 12.2.1) characterized by the equation 

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While vaguely equivalent to melting point for pure substances, the pour point for oil, unlike the melting point of a pure chemical, will increase as the oil weathers. The most commonly used formula to describe this change is an algorithm proposed by Mackay et al. [6]... 

Introduction 233. 2. Critical Point for Pure Substances 234. 3. Critical Point for... 

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