The Phase Rule and Its Applications

s; but in the present chapter the restriction that only one of the components is volatile will be allowed to fall, and the general

e 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 pha

f Stortenbeker,[241] form a very complete example of equilibria in a two-component system. We shall first of all

g.

ilibrium with the liquid solution will be all the lower the greater the concentration of the chlorine. We therefore obtain the curve ABF, which represents the composition of the solution with which solid iodine is in equilibrium at different temperatures. This curve can be followed down to 0°, but at temperatures below 7.9° (B) it represents metastable equilibria. At B iodine monochloride can be formed, and if present the system

ibrium with iodine, the other containing a higher proportion of chlorine in equilibrium with iodine monochloride. The composition of the latter solution is represented by the curve BCD. As the concentration of chlorine is increased, the temperature at which there is equilibrium between iodine monochloride and solution rises until a point is reached at which the composition of the solu

At this temperature iodine trichloride can separate out, and a second quadruple point (D

ature is raised, until at the point E, where the solution has the same composition as the solid, the maximum temperature is reached; the iodine trichloride melts. On increasing still further the concentration of chlorine in the solution, the temperature of equilibrium falls, and a continuous curve, sim

which melts at 27.2°, is called the α-form, while the less stable variety, melting at 13.9°, is known as the β-form. If, now, the presence of α-ICl is excluded, it is possible to obtain the β-form, and to study the conditions of equilibrium between it and solu

of the numerical data from whic

and C

riant s

Pressure. P

Liquid.

2,α-ICl ICl0.

2,β-ICl

-ICl,ICl3 ICl1

m. ICl3,Cl2

lting

] 114.15° (pre

chloride, 27.2°

loride, 101° (p

e monochlo

6 atm., the experiments must of course be carried out in closed vessels. At 63.7°

r 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 c

g.

rough a maximum and then fall continuously until the eutectic point, B (B1), is reached.[248] At this point the system is invariant, and the pressure will therefore remain constant until all the iodine has disappeared. As the concentration of the chlorine increases in the manner represented by the curve BfH, the pressure of the vapour also increases as represented by the curve B1f1H1. At H1, the eutectic point for iodine monochloride and iodine trichloride, the pressure again remains constant until all the monochloride has disappeared. As the concentration of t

e iodine to pure chlorine have not been made, the experimental data are nevertheless

s, two phases will form a bivariant system. The fields in which these systems can exist are sho

dine-

lution

e trichlor

monochlor

g.

entration of chlorine in the vapour is greater than in the solution, condensation of vapour would increase the concentration of chlorine in the solution; a certain amount of iodine must therefore pass into solution in order that the composition of the latter shall remain unchanged.[249] If, therefore, the volume of vapour be sufficiently great, continued diminution of volume will ultimately lead to the disappearance of all the iodine, and there will remain only solution and vapour (field II.). As the diminution of volume is continued, the vapour pressure and the concentration of the chlorine in the solu

lead successively to the univariant system (c), iodine monochloride-solution-vapour; the bivariant system solution-vapour (field II.); the univariant system (d), iodine trichloride-solution-vapour; and

, or of ferric chloride and water. In the case of sulphur dioxide and water, however, the melting point of the compound formed cannot be realized, because transition to another system occurs; retroflex concentration-temperature curves are therefore not found here,

water, and represented by the symbols SO2 xH2O (solution I.), and H2O ySO2 (solution II.). Vapour: a mixture of sulphur dioxide and water vapour in varying propo

stems: Four co-

drate, solu

lution I., solu

ystems: Three co

, solution

, solution

I., solutio

solution I.

ate, ice

olution II

ydrate, so

Systems: Two co

ate, sol

ate, sol

drate,

ydrat

ion I., s

tion I.,

ution I

tion II.

ution I

ce, v

g.

.6°, and the pressure 21.1 cm. If heat is withdrawn from this system, the solution will ultimately solidify to a mixture of ice and hydrate, and there will be obtained the univariant system ice-hydrate-vapour. The vapour pressure of this system has been determined down to a temperature of -9.5°, at which temperature the pressure amounts to 15 cm. The pressures for this system are represented by the curve BC. If at the point B the volume is diminished, the pressure must remain constant, but the relative amounts of the different phases will undergo change. If suitable quantities of these are present, diminution of volume will ult

hloride-solution-vapour, this curve cannot be followed to the melting point of the hydrate. Before this point is reached, a second liquid phase appears, and an invariant system consisting of hydrate-solution I.-solution II.-vapour is formed. We have here, therefore, the phenomenon of melting under the solution as in the case of succini

riant system will then be obtained. Thus, if the vapour phase is made to disappear, the univariant system solution I.-solution II.-hydrate, will be left, and the temperature at which this system is in equilibrium will var

r. In the case of this system the vapour pressure increases as the temperature rises, as represented by the curve FG. Such a system is analogous to the case of ether and water, or

ve has been followed to a temperature of 0°, the pressure at this point being 113 cm. The metastable prolongation of GF has also been determined. Altho

am, and the fields of the different bivariant systems are indicated by letters, corresponding to the letters on p. 170. Just as in the case of one-component systems (p. 29), we found that the field lying between any two curves gave the conditions of existence of that phase

te (which is relatively poor in sulphur dioxide) into the solution. If, therefore, the amount of hydrate present is relatively very small, the final result of the compression will be the production of the system f, solution I.-vapour. On the other hand, if the vapour is present in relatively small amount, it will be the first phase to disappear, and the bivariant system a, hydrate-solution I., will be obtained. Finally, if we start with the invariant system at F, compression will cause the condensation of vapour, while the composition of the two solutions will remain unchang

an be taken of the formation o

is formed which possesses a definite melting point, as in the case of iodine trichloride. In these cases, therefore, a retroflex curve is obtained. Further, just as in the case of the chlorides of iodine the upper branch of the retroflex curve ended in a eutectic point, so also in th