VOLUMETRIC PROPERTIES OF PURE FLUIDS

The vaporization curve 2-C temrinates at point C, the critical point. The coordinates of this point are the critical pressure and the critical temperature Tc, the highest pressure and highest temperature at wlriclr a pure chemical species can exist in vapor/liquid eqnilibrinm. 

The area existing at temperatures and pressures greater than Tc and Pc is marked off by dashed lines in Fig. 3.1, which do not represent phase boundaries, but rather are limits fixed by the meanings accorded the words liquid and gas. A phase is generally considered a liquid if vaporization results from pressure reduction at constant temperature. A phase is considered a gas if condensation results from temperature reduction at constant pressure. Since neither process occurs in the area beyond the dashed lines, it is called ihe, fluid region. 

The lines labeled T and T2 are for subcritical temperatures, and consist of three segments. The horizontal segment of each isothemi represents all possiblemixturesofliqnid and vapor in equilibrium, rangingfrom 100% liquid at the left end to 100% vapor at the right end. The locns of these end points is the dome-shaped curve labeled BCD, the left half of which (from B to C) 

The horizontal segments of the isothemis in the two-phase region become progressively shorter at liigher temperatures, being ultimately reduced to a point at C. Thus, the critical isothemi, labeled Tc, exhibits a horizontal inflection at the critical point C at the top of the dome. Here the liquid and vapor phases camiot be distinguishedfrom each other, because their properties are the same. 

If tlie tube is only partially filled with liquid (the remainder being vapor in equilibrium with the liquid), heating at first causes changes described by the vapor-pressure curve (solid line) of Fig. 3.3. For the process indicated by line J Q on Fig. 3.2(b), the meniscus is initially near tire top of the hibe (point J), and the liquid expands upon heating until it completely fills tire hibe (point Q). On Fig. 3.3 the process traces a path from (J, K) to Q, and with further heating departs from the vapor-pressure curve along the line of constant molar volume Vl- 

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