Organic Chemistry Laboratory Techniques, 2016a

1.3.B CONTROLLED BOILING
Boiling solutions always have the potential to “bump,” where bubbles vigorously erupt from superheated
areas of the solution: areas where the temperature is above the boiling point of the solvent, but gas bubbles
have not yet formed due to lack of a nucleation site. Bumping can splash hot material out of a flask: onto
your hand or onto a hotplate surface where it might start a fire. Bumping is hazardous, not to mention
frightening when a bubble unexpectedly erupts. Several methods can be used to prevent bumping and
ensure smooth boiling.
1.3.B.1 BOILING STONES (BOILING CHIPS)
Figure 1.39: a) Boiling stones in water, b) Vigorous boiling, c) Boiling stones used in crystallization.
Boiling stones (or boiling chips) are small pieces of black porous rock (often silicon carbide) that are added
to a solvent or solution. They contain trapped air that bubbles out as a liquid is heated, and have high
surface area that can act as nucleation sites for formation of solvent bubbles. They should be added to a
cool liquid, not one that is near its boiling point, or a vigorous eruption of bubbles may ensue. When a
liquid is brought to a boil using boiling stones, the bubbles tend to originate primarily from the stones
(Figure 1.39b). Boiling stones cannot be reused, as after one use, their crevices fill with solvent and they
can no longer create bubbles.
Boiling stones should not be used when heating concentrated solutions of sulfuric or phosphoric acid, as
they may degrade and contaminate the solution. For example, Figure 1.40 shows a Fischer esterification
reaction that uses concentrated sulfuric acid. When a stir bar is used for bump prevention, the solution
remains colorless (Figure 1.40a). When the same reaction is conducted using a boiling stone, the solution
darkens during heating (Figure 1.40b) and eventually turns the entire solution a deep purple-brown color
(Figure 1.40c). Besides contaminating the solution, the dark color makes manipulation of the material with
a separatory funnel difficult: two layers are present in Figure 1.40d, although it is very difficult to see.
Figure 1.40: a) Fischer esterification reaction using a stir bar (solution is colorless), b) Same reaction using boiling stones, c)
Same reaction after a few minutes of heating, d) Two dark layers in the separatory funnel as a result of the darkened solution.
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