NOTES ON LABORATORY MANIPULATIONThese notes are intended to help the student in foreseeing
and in overcoming some of the difficulties that arise in experi-
mental work. They by no means make it unnecessary for him
to exercise ingenuity and originality in planning and carrying out
the details of laboratory work. At the outset these notes should
be read through carefully; then, when in the later work references
to specific notes are made, their general bearing will be better
appreciated.
- PRECIPITATION; CRYSTALLIZATION
In the majority of chemical processes which are carried out
in the wet way, separations are accomplished by taking advantage
of differences in solubility. If a certain product is extremely
insoluble and is formed almost instantaneously when solutions
containing the requisite components are mixed, the process is called
precipitation and the insoluble substance is called the precipitate.
If the product to be formed is less insoluble, so that it separates
more slowly, or only after evaporating away a part of the solvent,
the process is called crystallization.
In some cases the precipitate, or the crystals, constitute the
desired product; in others, a product which it is necessary to
remove from the solution before the desired product can be ob-
tained pure. In either case it is necessary to make as complete
a separation as possible of the solid from the liquid. This in-
volves the manipulations described under Notes 2, 3 and 4. - POURING
In pouring a liquid from a vessel, either into a filter or into
another vessel, care must be taken not to slop the liquid or to
allow it to run down the outside of the vessel from which it is
poured. To this end touch a stirring rod to the lip of the dish
or beaker (Fig. 1) and allow the liquid to run down the rod.
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