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.
Boiling Stones (Boiling Chips)
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.
Boiling Sticks (Wood Splints)
"Boiling sticks" (wood splints) are also used to encourage smooth boiling. They are plunged directly into a solvent or solution, and act much the same as boiling stones: they too are highly porous and contain nucleation sites. When a liquid is brought to a boil using a boiling stick, the bubbles tend to originate primarily from the surface of the stick (Figure 1.41b).
When choosing between boiling stones and boiling sticks, the main advantage of boiling stones is that they are small and so will fit in any flask. They also absorb very little compound, unlike boiling sticks. The main advantage of boiling sticks is that they can be easily removed from a solution. This is useful in crystallization, as the stick can be easily removed before crystals form (Figure 1.41 c+d).
Stir Bars and Spin Vanes
Stir bars (stirring bars, or spin vanes in microscale work, Figure 1.42a) are Teflon-coated magnets that can be made to spin with a magnetic stir plate (Figure 1.42b). Stirring is often used with heating as stirring encourages homogeneity, allowing for liquids to more quickly heat or cool, and disrupts superheated areas. In the context of chemical reactions, stirring also increases the rate of mixing (especially for heterogeneous mixtures) and thus increases reaction rates.
A stir bar can be removed from a flask with a magnet, called a "stir bar retriever" (Figure 1.42d). If a solution is to be subsequently poured through a funnel and into a separatory funnel, they are also easily removed by the funnel.