MELTING-POINT MAXIMUM. 191
salt can be answered only in a few special cases. Undoubtedly in certain
metal-ammonia compounds it belongs to the complex, positive component of
the salt, (cf. especially No. 134). Probably the water is also attached to the
metal atom as one of the constituents of the complex in such of the hydrated
salts as are closely related to the metal-ammonia salts in their stoichiomet-
rical composition. For example, cupric sulphate hydrate is analogous to
tetramminecupric sulphate (No. 126), whereas the violet modification of
chromic chloride corresponds to the hexammine salts (No. 140):
[Cr(NHa) 6 ]Cl 3
[Cr(NH 3 ) 5 H 2 O]CI 3
[Cr(H 2 O) 6 ]Cl 3.
The fact that the number of molecules of water in hydrated salts is fre-
quently four or six is quite in accord with the views just stated, inasmuch as
four and six are also the most usual coordination numbers. It cannot be
claimed, however, that all compounds containing water of crystallization
must possess complex cations; since, among other reasons, it is quite certain
that the negative components of compounds have the power of attaching
water molecules.
How much, and in what manner, water is united with the ions in solution
is not positively known. A number of facts make it seem extremely prob-
able, however, that the ions are hydrated.^1
- The Melting-point Maximum for Magnesium Nitrate
Hexahydrate; Eutectic Mixture of Barium Chloride
Dihydrate and Water. - The freezing-point diagram of a series of mixtures of anhydrous mag-
nesium nitrate and water shows a pronounced maximum for the composition
corresponding to that of the hexahydrate. Measurements from which such
a diagram can be constructed can be made with the very simplest apparatus.
Starting with the crystallized hexahydrate, one branch of the curve is
obtained through successive additions of water, which by dissolving in the
hexahydrate depresses its freeziug-point; on the other hand, another branch
of the curve is obtained by adding successive portions of a less hydrated
magnesium nitrate which likewise dissolves in the hexahydrate and depresses
its freezing-point. The two branches of the curve unite in a very pronounced
maximum at the composition Mg(NO 3 ) 2 + 6H 2 O. As a rule the maxima
obtained in the investigation of hydrates are less pronounced, and often they
are entirely concealed. Regarding the so-called "concealed maxima," see
Tammann. A critical compilation of all the known equilibria between
water and inorganic substances is given in Table 133, in Landolt-Bornstein-
Roth's physikalisch-chemischen Tabellen, 4th edition, 1912.
(^1) E. W. Washburn. The Hydration of Ions. J. Am. Chem. Soc. 31, 322
(1909); Hydrates in Solution; Review of Recent Experimental and Theoretical
Contributions, Technology Quarterly, 1908, 360, or Jahrbuch der Radio-
aktivitat u. Elektronik, 1908, December.