The curve AMF (Fig. 32) is the vapour-pressure curve of the saturated solutions of the salt, _i.e._ it represents, as we have seen, the maximum vapour pressure at which salt can exist in contact with solution and vapour. The dotted line _aa_ represents atmospheric pressure. If, now, an unsaturated solution, the composition of which is represented by the point _x_, is heated in an open vessel, the temperature will rise, and the vapour pressure of the solution will increase. The system will, therefore, pass along a line represented diagrammatically by _xx'_. At the point _x'_ the vapour pressure of the system becomes equal to 1 atm.; and as the vessel is open to the air, the pressure cannot further rise; the solution boils. If the heating is continued, water passes off, the concentration increases, and the boiling point rises. The system will therefore pass along the line _x'm_, until at the point _m_ solid salt separates out (provided supersaturation is excluded). The system is now univariant, and continued heating will no longer cause an alteration of the concentration; as water passes off, solid salt will be deposited, and the solution will evaporate to dryness. If, however, the atmospheric pressure is represented not by _aa_ but by _bb_, then, as Fig. 32 shows, the maximum vapour {131} pressure of the system salt--solution--vapour never reaches the pressure of 1 atm. Further, since the curve _bb_ lies in the area of the bivariant system solution--vapour there can at no point be a separation of the solid form; for the system solid--solution--vapour can exist only along the curve AMF. On evaporating the solution of a salt in an open vessel, therefore, salt can be deposited only if at some temperature the pressure of the saturated solution is equal to the atmospheric pressure. This is found to be the case with most salts. In the case of aqueous solutions of sodium and potassium hydroxide, however, the vapour pressure of the saturated solution never reaches the value of 1 atm., and on evaporating these solutions, therefore, in an open vessel, there is no separation of the solid. Only a homogeneous fused mass is obtained. If, however, the evaporation be carried out under a pressure which is lower than the maximum pressure of the saturated solution, separation of the solid substance will be possible.