14 NOTES ON LABORATORY MANIPULATIONevaporate very slowly. This is easily accomplished in industrial
works where large vats of solution can be kept at a uniform tem-
perature with steam coils and allowed to evaporate day and night.
On the laboratory scale it is almost impossible, first on account
of variations in temperature, and next on account of dust which
inevitably falls into an uncovered dish.
The majority of substances are more soluble at higher temper-
atures than at lower. If a solution just saturated at a high
temperature is allowed to cool very slowly, it is possible for the
solid to separate so slowly as to build up perfect crystals. This is
an expedient that can be adopted to advantage in several of the
preparations. In many cases, however, when a saturated solution
cools it becomes supersaturated, sometimes to a high degree. Then
when crystallization is once induced it occurs with such rapidity
that a mass of minute crystals, instead of a few large, perfect ones,
is produced. To avoid this supersaturation a few seed crystals
(i.e., very small crystals of the kind desired) may be placed in the
solution before it has cooled quite to the saturation point. These
form nuclei on which large crystals can be built up, and when they
are present it is impossible for the solution to remain super-
saturated.
In carrying out the following preparations the principles just
stated should be kept carefully in mind; but in many instances
specific suggestions will be given as to the easiest method for ob-
taining good crystals of any particular substance.
Large crystals, it is true, present a pleasing appearance, but
oftentimes they contain a considerable quantity of the mother
liquor inclosed between their crystal layers. Hence if purity of
product is the sole requisite, it is often more desirable to obtain
a meal of very fine crystals. Such a meal is obtained by crystal-
lizing rapidly and stirring while crystallizing. Some substances
are so difficult to obtain in large crystals that it is more satisfactory
to try only to obtain a uniform crystal meal.
(b) Purification by Recrystallization. When a given substance
crystallizes from a solution, it generally separates in a pure con-
dition irrespective of any other dissolved substances the solution
may contain. Thus a substance can be obtained in an approxi-
mate state of purity by a single crystallization. Portions of the
mother liquor (containing dissolved impurities) are, however,
usually entrapped between the layers of the single crystals, not to