Thus as evaporation of the mixture takes place, the composition of the remaining liquid increases in B as excess A is drawn off in the vapour, and the boiling point rises. If the vapour is removed and condensed elsewhere, then the composition of the liquid changes to one less rich in A (to take account of the excess of A in the removed vapour), for example Φ2.Īs this mixture boils, the vapour that is produced (of composition Φ2′, again richer in A than the liquid from which is produced) may again be removed, causing the composition of the liquid to shift to one still less rich in A. From the diagram, it may be seen that this vapour is richer in A than the liquid is. If a liquid of initial composition I is heated to boiling (the blue line), then vapour of composition Φ1′ is produced. Though many mixtures have temperature composition diagrams resembling the idealised version given above, there can be marked deviations:Ī maximum in the phase diagram may be observed if favourable interactions between the A and B molecules can stabilise the liquid phase with respect to the vapour phase: Again the relative abundance of each phase is given by the lever rule. At points between the two lines, the two phases coexist, and the composition of each phase is given by the x coordinate of the phase boundary at that temperature (just as in pressure composition diagrams). For an ideal mixture, in which A is more volatile than B, the vapour-composition diagram has the following form: The interpretation of these diagrams is in many respects entirely analogous to the interpretation of pressure composition diagrams, the only difference being that the liquid phase lies below the vapour phase, not above it: Below the lower curved line, only liquid exists, and above the upper curved line, only vapour exists. These are phase diagrams which show the composition of two phases in equilibrium at a given pressure, and how these compositions change with temperature (as opposed to the pressure composition diagrams which showed the pressure dependence of the composition at a fixed temperature).