The critical volume, and the corresponding critical pressure p c, so that the conditionsĭefine the critical point. Critical temperature may also be defined as the highest temperature at which the liquid phase of a substance can exist.Īs can be seen from Figure 1, along the critical isotherm T c, the relationship between fluid pressure and its molar volume shows a point of inflection at the molar volume At this temperature, the critical temperature, T c, the meniscus disappears and the molar volumes of the two phases are equal to If this process is continued, the liquid phase and the vapor phase eventually become indistinguishable at some temperature, which is the situation indicated by point c in Figures 1 and 2. At the same time, the molar volume of the liquid phase is increased and that of the vapor decreased, as shown in Figure 1. If tube temperature is increased to T 2 in Figure 1, vapor pressure is increased along 1c of Figure 2.
In Figure 2, both points 1 and 2 lie on the vapor-pressure curve which separates liquid and vapor phases. Such a situation corresponds to the isotherm T 1 in Figure 1, where point 2 represents the system at the vapor-pressure of the liquid and corresponds to the liquid molar volume, whereas point 1 corresponds to the larger molar volume of the vapor phase at the same pressure and temperature. If a pure liquid is contained in a sealed tube in equilibrium with its vapor at a temperature T 1 then, in a gravitational field, the top of the liquid is indicated by an interface (meniscus).
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