PHYSICAL PROCESSES INVOLVING HEAT
a. Evaporation. In pharmacy, evaporation signifies the process of applying
heat to drive off a portion of the liquid as a vapor. Such a liquid may consist of a
solution of a nonvolatile substance in water, alcohol, or other solvent. The purpose of
evaporation is to concentrate the solution or to obtain the solute in dry form.
Evaporation may be hastened by (1) using a shallow evaporating dish which exposes
as large a surface of the solution as possible, (2) stirring the solution constantly which
allows the free escape of vapors and ensures the even heating of the entire quantity of
liquid, (3) using sufficient heat, and (4) reducing atmospheric pressure.
b. Distillation. Distillation is the separation of the constituents of a liquid
mixture by vaporization and subsequent condensation of the vapors. Such a separation
is made possible by the different boiling points of the various volatile constituents.
c. Sublimation. This is the process of distilling volatile solids. The product
obtained from the process is called a sublimate. Sublimation is similar to distillation, but
there is no intermediate liquid state. The volatile solid evaporates and is resolidified.
d. Desiccation. Desiccation is a dehydration process for removing moisture
from solid substances. The moisture thus driven off is called hygroscopic moisture as
distinguished from moisture that is chemically combined, as in water of crystallization.
The process should be conducted at the lowest effective temperature; this may be
accomplished by exposing the substance to a dry atmosphere at ordinary temperature
or by placing it in a drying oven at a moderate temperature.
Section IV. COMMINUTION
Comminution is a mechanical process in which the particle size of a solid is
reduced or the degree of subdivision of a solid is increased. Substances may be
comminuted for the following three reasons:
a. To increase the rate of solution of solids.
b. To increase the ease and thoroughness of extraction of animal and vegetable
c. To obtain a uniform powder or mixture of powders that can be used to
prepare medicinal solids in their most desirable form.