g. can be
calculated. Some of these calculations have a practical interest as well
as an educational value. Students should practise them so as to become
familiar with the relations between weight and volume.
_When substances are mixed by volume_, the sp. g. of the mixture is the
mean of those of its constituents, and may be calculated in the usual
way for obtaining averages. 1 c.c. of a substance having a sp. g. of 1.4
mixed with 1 c.c. of another having a sp. g. of 1.0 will yield 2 c.c. of
a substance having a sp. g. of 1.2. If, however, we write gram instead
of c.c. in the above statement, the resulting sp. g. will be 1.16. The
simplest plan is to remember that the sp. g. is the weight divided by
the volume (sp. g. = w/v) and the sp. g. of a mixture is the sum of the
weights divided by the sum of the volumes (sp. g. = (w + w' + w",
&c.)/(v + v' + v", &c.)). In the above example the sum of the volumes is
2 c.c.; the weights (got by multiplying each volume by its
corresponding sp. g.) are 1.4 gram and 1 gram. The sum of the weights
divided by the sum of the volumes is 2.4/2 or 1.2.
The sp. g. of a mixture of 10 c.c. of a substance having a sp. g. of
1.2, with 15 c.c. of another having a sp. g. of 1.5 may be thus found:--
sp. g. = (12+22.5)/(10+15) = 1.38
multiply each volume by its sp.
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