Some physico-chemical features peculiar to closed systems

Some physico-chemical features peculiar to closed systems 

General introduction. The physical chemistry of closed systems, such as an evacuated line isolated from the pumps, is of course implicit in the general physical chemistry which should be familiar to the readers of 

The phase relationships for a normal substance are represented in the phase diagram shown in Fig. 1.1, which is interpreted in terms of the Phase 

The normal freezing point of the liquid under pressure is given by Tp, and OS is the melting curve of the substance, i.e. the locus of the points defining the co-existence of solid and liquid. If we measure the freezing point of a liquid in a closed system, the Phase Rule tells us that since at that temperature all three phases will be in equilibrium, F=0, and we obtain the 

The phase diagram also illustrates why some substances which melt at normal pressure, will sublime at a lower pressure the line p = Pa intersects at Tg the locus OR of the points defining the solid-vapour equilibrium, i.e. at the pressure pj, the substance will sublime at the temperature T. Sometimes the opposite behaviour is observed, namely that a substance which sublimes at normal pressure will melt in a vacuum system under its own vapour pressure This is a non-equilibrium phenomenon and occurs if the substance is heated so rapidly that its vapour pressure rises above that of the triple point this happens quite frequently with aluminium bromide and with iodine. 

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