In reference to the graph at the bottom, my textbook has a statement '''..A process of vaporization or condensation at a constant temperature is illustrated by the lines G-H-I or I-H-G, respectively, in figure 16.3. Water would vaporize or condense at a constant temperature as the pressure-reaches point H on the vapor-pressure curve. The change that occurs at H is the increase or decrease in the fraction of vapor or liquid, respectively, at a fixed temperature. The pressure does not change until all of the vapor, or liquid, has completed the phase transition.'' This part of the statement is not clear to me. How is it that there is no pressure change until all of the vapor, or liquid, has completed the phase transition? It seems like as vapor is condensing the pressure would decrease.