The thermodynamic requirements of mechanical, thermal, and material equilibrium together with the equation specify two points on the curve, ( T s , v f ) , and ( T s , v g ) , shown as green circles that designate the coexisting boiling liquid and condensing gas respectively. Heating the fluid in this state increases the fraction of gas in the mixture; its v , an average of v f and v g weighted by this fraction, increases while T s remains the same. This is shown as the dotted gray line, because it does not represent a solution of the equation; however, it does describe the observed behavior. The points above T s , superheated liquid, and those below it, subcooled vapor, are metastable; a sufficiently strong disturbance causes them to transform to the stable alternative. Consequently they are shown dashed. Finally the points in the region of negative slope are unstable. All this describes a fluid as a stable gas for T > T s , a stable liquid for T < T s , and a mixture of liquid and gas at T = T s , that also supports metastable states of subcooled gas and superheated liquid. It is characteristic of all isobars 0 < p < p c , where T s is a function of p . The orange isobar is the critical one on which the minimum and maximum are equal. The black isobar is the limit of positive pressures, although drawn solid none of its points represent stable solutions, they are either metastable (positive or zero slope) or unstable (negative slope. All this is a good explanation of the observed behavior of fluids.