The complete investigation of the equilibria between sulphur dioxide and potassium iodide, therefore, shows that these two components form the compounds KI,14SO_{2} and KI,4SO_{2}; and that when solutions having a concentration between those represented by the points G and K are heated, separation into two layers occurs. The temperatures and concentrations of the different characteristic points are as follows:-- ------------------------------------------------------------- | | Composition of Point. | Temperature. | the solution | | per cent. KI. ------------------------------------------------------------- A (m.p. of SO_{2}) | -72.7° | -- B (eutectic point) | -- | 0.86 C (m.p. of KI,14SO_{2}) | -23.4° | 17.63 E (m.p. of KI,4SO_{2}) | +0.26° | 39.33 G (KI + two liquid phases) | (about) 88° | 24.0 H (critical solution point) | 77.3° | 12 K (KI + two liquid phases) | (about) 88° | 2.7 ------------------------------------------------------------- * * * * * {161} CHAPTER IX EQUILIBRIA BETWEEN TWO VOLATILE COMPONENTS General.--In the two preceding chapters certain restrictions were imposed on the discussion of the equilibria between two components; but in the present chapter the restriction that only one of the components is volatile will be allowed to fall, and the general behaviour of two volatile[239] components, each of which is capable of forming a liquid solution with the other, will be studied. As we shall see, however, the removal of the previous restriction produces no alteration in the general aspect of the equilibrium curves for concentration and temperature, but changes to some extent the appearance of the pressure-temperature diagram. The latter would become still more complicated if account were taken not only of the total pressure but also of the partial pressures of the two components in the vapour phase; this complication, however, will not be introduced in the present discussion.[240] In this chapter we shall consider the systems formed by the two components iodine and chlorine, and sulphur dioxide and water. Iodine and Chlorine.--The different systems furnished by iodine and chlorine, rendered classical by the studies of Stortenbeker,[241] form a very complete example of equilibria in a two-component system. We shall first of all consider the {162} relations between concentration and temperature, with the help of the accompanying diagram, Fig. 42. [Illustration: FIG. 42.]