Page 12 - A HANDBOOK OF ORGANIC ANALYSIS QUALITATIVE AND QUANTITATIVE

PRELIMINARY INVESTIGATION 7
repeated until the melting-point is sharp and shows no change
on further recrystallisation ; it may then be regarded as pure.
If, however, the melting-point after one recrystallisation is
identical with that of the original substance, no further
purification is necessary.
Should the substance appear to contain tarry or coloured
impurities, it is advisable to add some decolorising carbon
to the hot solution before filtering.
It is well to ascertain whether a solid can be distilled or
sublimed. Not only is the boiling-point of a substance solid
at ordinary temperatures a valuable additional characteristic
to be taken into consideration, but solids can often be obtained
in a higher state of purity by distillation or sublimation than
by crystallisation. The precaution should of course be taken
of heating a small quantity, in order to make sure that it
can be distilled without decomposition. When this is pos-
sible, a distilling flask with a wide side-tube should be em-
ployed, without a condenser, and any distillate solidifying at
the outset melted by gently warming the tube. This distil-
lation of solids may often advantageously be carried out
under reduced pressure ; in such a case a distilling flask with
a wide (ro--r5 mm.) side-arm should be employed, in order
to avoid stoppage by solidified distillate.

General and Physical Characteristics
It is of great importance to investigate the solubility
of the substance in various solvents, for information as
to the chemical nature of the compound can frequently
be obtained from consideration of this characteristic property.
Thus, in general, salts are more or less soluble in water,
but insoluble in ether; acids are often soluble in hot water,
and sparingly so in cold water, while being as a rule soluble
in alcohol or ether ; hydrocarbons are all insoluble in water,
but soluble in ether, and so on. As a general rule, compounds
are dissolved by liquids containing similar groups of atoms.
The boiling- and melting-points should always be kept in