CRYSTALLIZATION 13
The finer particles of the solid dissolve first; as the solution
becomes more concentrated the rate of solution grows slower, and
it takes a very long time to dissolve the remaining coarser par-
ticles. Hence when a limited amount of solvent or reagent
is used, as for example when copper is to be dissolved in a minimum
amount of nitric acid, it is best to hold in reserve perhaps one-tenth
of the reagent; when the nine-tenths are almost exhausted and the
reaction with the coarser particles has almost stopped, pour off the
solution already obtained, and treat the small residue with the
fresh acid held in reserve.
- CRYSTALLIZATION
(a) A great number of pure substances are capable of assuming
the crystalline condition when in the solid form. Crystals are
bounded by plane surfaces, which make definite and characteristic
angles with each other and with the so-called axes of the crystals.
The external form of a crystal reflects in some manner the shape
or structure of the individual molecules of the substance, for the
crystal must be regarded as being built up by the deposition of
layer on layer of molecules, all of which are placed in the same
definite spatial relation to the neighboring molecules.
When a substance takes on the solid form very rapidly (as when
melted glass or wax cools) its molecules do not have an opportunity
to arrange themselves in a regular order, and consequently the
solid body is amorphous. The axes of the individual molecules
point in every direction without regularity, and consequently the
solid body possesses no crystalline axes or planes.
It is evident from the above that the essential condition favor-
ing the formation of perfect crystals is that the solid shall be built
up very slowly. This is the only general rule which can be given
in regard to the formation of perfect crystals.
The excellence of a chemical preparation is judged largely
from its appearance. The more uniform and perfect the crystals,
the better appearance the preparation presents.
In the following preparations sometimes a pure melted sub-
stance is allowed to crystallize by simply cooling; the cooling
should then take place slowly. More often crystals are formed
by the separation of a dissolved substance from a saturated solu-
tion. Perfect crystals can best be obtained in this case by keep-
ing the solution at a constant temperature and allowing it to
