II. Melting points.
A. Iodine,[[246]] 114.15° (pressure 89.8 mm.).
C. α-Iodine monochloride, 27.2° (pressure 37 mm.).
E. Iodine trichloride, 101° (pressure 16 atm.).
G. β-Iodine monochloride, 13.9°.
Since the vapour pressure at the melting point of iodine trichloride amounts to 16 atm., the experiments must of course be carried out in closed vessels. At 63.7° the vapour pressure of the system trichloride—solution—vapour is equal to 1 atm.
Pressure-Temperature Diagram.—In this diagram there are represented the values of the vapour pressure of the saturated solutions of chlorine and iodine. To give a complete picture of the relations between pressure, temperature, and concentration, a solid model would be required, with three axes at right angles to one another along which could be measured the values of pressure, temperature, and concentration of the components in the solution. Instead of this, however, there may be employed the accompanying projection figure[[247]] (Fig. 43), the lower portion of which shows the projection of the equilibrium curve on the surface containing the concentration and temperature axes, while the upper portion is the projection on the plane containing the pressure and temperature axes. The lower portion is therefore a concentration-temperature diagram;
the upper portion, a pressure-temperature diagram. The corresponding points of the two diagrams are joined by dotted lines.
Corresponding to the point C, the melting point of pure iodine, there is the point C1, which represents the vapour pressure of iodine at its melting point. At this point three curves cut: 1, the sublimation curve of iodine; 2, the vaporization curve of fused iodine; 3, C1B1, the vapour-pressure curve of the saturated solutions in equilibrium with solid iodine. Starting, therefore, with the system solid iodine—liquid iodine, addition of chlorine will cause the temperature of equilibrium to fall continuously, while the vapour pressure will first increase, pass through a maximum and then fall continuously