190 HYDRATES.This method of analysis does not apply solely to the mixtures of a salt
and water, but it can be used very generally to prove the existence of com-
pounds. It has acquired a high importance in the study of alloys (Tammann
and his students: see articles in Z. anorg. Chem. mostly later than 1903).- VAPOR TENSION ANALYSIS. When a hydrated crystallized salt is in
equilibrium with water vapor, its water of hydration can be progressively with-
drawn in the same manner that ammonia is withdrawn from the metal ammo-
nia compounds (cf. p. 163). Two methods are available by which salts can be
investigated from this point of view. According to the first the vapor pressure
of the salt is determined for varying water content, and a curve is constructed
with the aqueous tensions as ordinates and the corresponding percentages of
water as abscissas; if the salt forms a number of different hydrates, the vapor
pressure above any given mixture of the hydrates remains constant, when water
is slowly withdrawn, as long as any of the hydrate richest in water is still pres-
ent. When this hydrate is entirely exhausted, the pressure sinks abruptly,
and a second, and lower, horizontal line on the plot corresponds to the tension
of the next lower hydrate, etc. (cf. Fig. 25). The tension of aqueous vapor in
the case of hydrated cupric sulphate is given in the following table:
CuSO 4 + 4.5 H 2 O 46.3 mm. CuSO 4 + 1.5 H 2 O 29.7 mm.
CuSO 4 + 3.5 H 2 0 47.1 " CuSO 4 + 0.5 H 2 0 4.4 "
CuSO 4 + 2.5 H 2 O 29.9 "
From this we may conclude that the following hydrates exist: CuSO 4 • 5H 2 O;
CuSO 4 • 3 H 2 O; CuSO 4 • 1 H 2 O. The vapor pressure of the pentahydrate is
about 46 mm. and remains constant, irrespective of the amount present,
until the pentahydrate has entirely disappeared. Then the pressure drops to
that of the trihydrate (about 30 mm.), and again remains constant until
this is completely changed into monohydrate (vapor pressure about 4.5 mm.).
The other method of vapor tension analysis consists in determining the
decomposition temperature under a constant pressure of aqueous vapor
(cf. p. 163). Approximate values for the decomposition temperature can
be obtained, however, by finding the point at which
water is given up when the salt is heated in any
indifferent atmosphere. In this way crystallized
copper sulphate loses the last molecule of water at
220° to 240°, and from this and the fact that the
other molecules of water escape at much lower tem-
peratures, the existence of a definite monohydrate
has been recognized for a long time.
Preset) re
40 mm-
80 " •
!0 " -
10 ".
0 "—
(^1) •jBo In the cases where the water is not chemically
pIG 25 bound in a compound, but is merely adsorbed (cf.
pp. 41 and 44), either of these methods of investi-
gation shows merely a continuous loss of water with rising temperature, or
diminishing pressure.
The question as to how the water is united with the components of the