EVAPORATION 11original volume of the mixture (Fig. 4), any soluble substances will
still remain uniformly distributed throughout the whole volume.
If now the upper four-fifths, consisting of the clear solution, is
drawn away, it follows that practically one-fifth of the solution,
containing one-fifth of the soluble impurities, remains with the
precipitate. By stirring up the solid again with pure water, the
soluble impurities become uniformly distributed through the larger
volume, and on letting the precipitate settle and drawing off
four-fifths of the liquid, as before, there will remain with the wet
precipitate only £ X i = •£% of the original soluble matter. After
the third decantation the remaining suspension will contain
i X ^ = j^s of the original impurities, and so on.
- EVAPORATION
(a) When it is necessary to remove a part of the solvent from a
solution, as when a dissolved substance is to be crystallized from it,
the solution is evaporated. In some cases, where the dissolved
substance is volatile or is decomposed by heat, the evaporation
must take place at room temperature, but ordinarily the liquid
may be boiled. The concentration of a solution should always be
carried out in a porcelain dish of such size that at the outset it is
well filled with the liquid. The flame should be applied directly
under the middle of the dish where the liquid is deepest; the part
of the dish against which the flame plays directly should be pro-
tected with wire gauze. Under no circumstances should the flame
be allowed to play up over the sides of the dish: first, because, by
heating the dish where it is only partly cooled by liquid, there is
great danger of breakage; second, because, by heating the sides, the
film of liquid which creeps up is evaporated and the solid deposited
becomes baked hard and in some cases is decomposed. To prevent
the formation of a solid crust around the edges, which even at best
will take place to some extent, the dish should occasionally be tilted
back and forth a little, so that the crust may be dissolved, or
loosened, and washed back into the middle of the dish.
While evaporating a solution over a flame it should be carefully
watched, for if it should be allowed to evaporate to dryness the
dish would probably break and the product be spoiled. If a pre-
cipitate or crystals separate from the liquid and collect in a layer
at the bottom, the dish may break, because where the solid pre-
vents a free circulation of the liquid the dish becomes superheated,