PHYSICO-CHEMICAL DETECTION 69
fact that the velocity at which compact substances enter into reaction is often
very slight. (Cf. TiS 2 , No. 58.)
If the phosphides, nitrides, and certain carbides are regarded as the metal
salts of phosphine, ammonia, and the hydrocarbons, that is to say, as the salts
of hydrogen compounds the acid nature of which is almost infinitesimal (the
acidic constants, K 2 , of which are therefore extremely small), it then becomes
easy to understand why these substances can be prepared only in the absence
of water (best by direct union of the elements themselves). The readiness
with which the preparation of phosphine from calcium or magnesium phos-
phide (No. 63), of acetylene from calcium carbide (No. 64), and of ammonia
from magnesium or boron nitride (Nos. 60, 61), is carried out, depends on the
ease with which the salts of such extremely weak acids hydrolyze.(a) Acids and Bases.- Physico-chemical Detection of Electrolytic Dissociation.
To demonstrate the dissociation of a substance in aqueous solu-
tion, determine the molecular weight of nitric acid by the freezing-
point method, first with nitrobenzene and then with water as the
solvent. Dehydrate some concentrated nitric acid by distilling
it with an equal volume of concentrated sulphuric acid, and
free the distillate from oxides of nitrogen by passing dry air
through it.
Determine the molecular weight by measuring the lowering of
the freezing-point of nitrobenzene, using about 25 g. of the sol-
vent, and 0.2, 0.4, 1.0, and 1.5 g. of nitric acid, then repeat, using
about 22 g. of water instead of nitrobenzene. The details for
carrying out the determinations may be found in H. Biltz; Prac-
tical Methods for Determining Molecular Weights, Translated by
H. C. Jones, 1899. In nitrobenzene the molecular weight corre-
sponds-approximately to the formula HN0 3 , whereas in water the
molecular weight is about half as large.
In order to show that this dissociation causes the aqueous solu-
tion to become a conductor of electricity, insert two platinum
electrodes in a beaker of water, and connect them through an
electric incandescent lamp with the terminals of a lighting circuit.
The lamp does not glow, because water is a very poor conductor
of electricity, and the circuit is therefore practically open. As
soon, however, as a few drops of nitric acid are added to the
water, the lamp begins to glow, and at the same time an evolution
of gas takes place at the electrodes.