12 NOTES ON LABORATORY MANIPULATIONand then when in any one place the liquid does penetrate, the
sudden cooling causes the porcelain to crack. Usually when a
solid begins to separate from a boiling liquid the evaporation should
be stopped and the liquid left to crystallize. After that the
mother liquor may be evaporated further in a smaller dish.
(6) Evaporating to Dryness. The only circumstances under
which a direct flame may be used to evaporate to dryness are
that the dish shall be held in the hand all the time and the contents
rotated to keep the sides of the dish wet.
Steam Bath. Laboratories are sometimes
equipped with general steam baths, which
are copper or soapstone chests kept filled
with steam, and provided with round open-
ings in the top into which evaporating
dishes may be set.
Each student, however, may set up a
steam bath, as shown in Fig. 5, at his own
desk. After the water in the beaker
reaches the boiling point a very small
flame is all that should be used, because
the steam that escapes around the sides of
the dish is wasted; only as much as will
condense on the bottom of the dish is
effective. With such a steam bath there
is no danger of spattering or of decom-
Steam Bath for posing the solid product while evaporating
a solution to dryness.
Hot Plate. A large, thick iron plate
kept hot with a burner or with steam coils is useful for drying
certain damp preparations.
- DISSOLVING SOLID SUBSTANCES
The process of dissolving solid substances is hastened, first
by powdering the substance as finely as possible, and second by
raising the temperature. The solid and solvent should be heated
together in a porcelain dish (not in a beaker), and care should be
taken to keep the mixture well stirred, for if the solid should settle
in a layer on the bottom, that part of the dish would become
superheated and would be likely to break (see last paragraph in
Note 6 (a)).
FIG. 5.
Evaporating to Dry-
ness. Note 6 (6)