(c) Hydrate, vapour.
(d) Hydrate, ice.
(e) Solution I., solution II.
(f) Solution I., vapour.
(g) Solution I., ice.
(h) Solution II., vapour.
(i) Solution II., ice.
(j) Ice, vapour.
Pressure-Temperature Diagram.[[251]]—If sulphur dioxide is passed into water at 0°, a solution will be formed and the temperature at which ice can exist in equilibrium with this solution will fall more and more as the concentration of the sulphur dioxide increases. At -2.6°, however, a cryohydric point is reached at which solid hydrate separates out, and the system becomes invariant. The curve AB (Fig. 45) therefore represents the pressure of the system ice—solution II.—vapour, and B represents the temperature and pressure at which the invariant system ice—hydrate—solution II.—vapour can exist. At this point the temperature is -2.6°, and the pressure 21.1 cm. If heat is withdrawn from this system, the solution will ultimately