Finally, in the case of the system FeCl3—HCl—H2O, we find closed isothermal curves. Since, as already stated, the salt 2FeCl3,2HCl,8H2O has a definite melting point, the temperature of which is therefore higher than that at which this compound is in equilibrium with solutions of other composition, it follows that the line of intersection of an isothermal plane corresponding with a temperature immediately below the melting point of the salt with the cone-shaped surface of its region of existence, will form a closed curve. This is shown by the isotherm for -4.5°, which surrounds the point Q, the melting point of the ternary salt.

The following table gives some of the numerical data from which the curves and the model have been constructed:—

Point. Solid Phases. Temperature. Composition of the
solution in gm.-mols. salt
to 100 gm.-mols. water.
HCl FeCl3
A 2FeCl3,12H2O -20° — 6.56
C 2FeCl3,12H2O; 2FeCl3,7H2O 27.4° — 24.30
E 2FeCl3,7H2O; 2FeCl3,5H2O 30° — 30.24
G 2FeCl3,5H2O; 2FeCl3,4H2O 55° — 40.64
J 2FeCl3,4H2O; FeCl3 66° — 58.40
L

2FeCl3,12H2O; 2FeCl3,7H2O;
2FeCl3,2HCl,8H2O

-7.5° 19.22 23.72
M

2FeCl3,7H2O; 2FeCl3,5H2O;
2FeCl3,2HCl,8H2O

-7.3° 23.08 28.55
N

2FeCl3,5H2O; 2FeCl3,4H2O;
2FeCl3,2HCl,8H2O

-16° 28.40 31.89
S

2FeCl3,4H2O;
2FeCl3,2HCl,8H2O;
2FeCl3,2HCl,4H2O

-27.5° 32.33 34.21
O

2FeCl3,4H2O; FeCl3;
2FeCl3,2HCl,4H2O

29° 33.71 49.84
U

2FeCl3,7H2O;
2FeCl3,2HCl,8H2O

-4.5° 20.66 25.74
V

2FeCl3,12H2O;
2FeCl3,2HCl,12H2O;
2FeCl3,2HCl,8H2O

-13° 22.40 18.00
X

2FeCl3,12H2O;
2FeCl3,2HCl,12H2O

-12.5° 22.14 16.69
Q 2FeCl3,2HCl,8H2O -3° (melting point)

Basic Salts.—Another class of systems in the study of

which the Phase Rule has performed exceptional service, is that of the basic salts. In many cases it is impossible, by the ordinary methods of analysis, to decide whether one is dealing with a definite chemical individual or with a mixture. The question whether a solid phase is a chemical individual can, however, be answered, in most cases, with the help of the principles which we have already learnt. Let us consider, for example, the formation of basic salts from bismuth nitrate, and water. In this case we can choose as components Bi2O3, N2O5, and H2O; since all the systems consist of these in varying amounts. If we are dealing with a condition of equilibrium at constant temperature between liquid and solid phases, three cases can be distinguished,[[371]] viz.—

1. The solutions in different experiments have the same composition, but the composition of the precipitate alters. In this case there must be two solid phases.

2. The solutions in different experiments can have varying composition, while the composition of the precipitate remains unchanged. In this case only one solid phase exists, a definite compound.

3. The composition both of the solution and of the precipitate varies. In this case the solid phase is a solid solution or a mixed crystal.

In order, therefore, to decide what is the nature of a precipitate produced by the hydrolysis of a normal salt, it is only necessary to ascertain whether and how the composition of the precipitate alters with alteration in the composition of the solution. If the composition of the solution is represented by abscissæ, and the composition of the precipitate by ordinates, the form of the curves obtained would enable us to answer our question; for vertical lines would indicate the presence of two solid phases (1st case), horizontal lines the presence of only one solid phase (2nd case), and slanting lines the presence of mixed crystals (3rd case). This method of representation cannot, however, be carried out in most cases. It is, however,

generally possible to find one pair or several pairs of components, the relative amounts of which in the solution or in the precipitate undergo change when, and only when, the composition of the solution or of the precipitate changes. Thus, in the case of bismuth, nitrate, and water, we can represent the ratio of Bi2O3 : N2O5 in the precipitate as ordinates, and N2O5 : H2O in the solution as abscissæ. A horizontal line then indicates a single solid phase, and a vertical line two solid phases. An example of this is given in Fig. 118.[[372]]