What is the purpose of a boiling stick when heating a reaction mixture?
I read some stuff online and still don't understand how it works. Does it just ensure that the mixture will boil evenly? If so, then what's wrong when a reaction doesn't boil evenly?
This is the LibreTexts page I was reading information from. I don't understand the terms "bump" and "superheated". If the liquid is above the boiling point, why hasn't it turned into a gas then?
Boiling chips and boiling sticks are practical solutions to problems chemists face in the lab.
When boiling liquids they often observe the the boiling does not happen evenly. Instead of a gentle warming giving gentle bubbling we find that nothing happens for a while until, suddenly, a great deal of bubbling happens and this causes the liquid to splash overwhelming the capacity of the vessel and throwing the liquid into places we don't want it to be (during distillations, for example, the splash may throw liquid to the top of the distillation column, contaminating the product with raw liquid; during reactions the product can be expelled from the vessel entirely which is rarely what you want).
Boiling chips or sticks ensure the boiling happens evenly, keeping things under control.
The problem they solve is that, in a smooth vessel, there are often not many rough patches to initiate bubbles. Bubbles are much easier to initiate if there is a small rough patch on the vessel wall. Chips or sticks provide such a rough patch. In the absence of a rough patch some liquids will superheat and may spontaneously vaporise but suddenly and violently. Roughness ensures bubbles start to form evenly and in many places avoiding superheating and making the vaporisation much more even.
The idea that things won't happen this way as "If the liquid is above the boiling point, why hasn't it turned into a gas then?" is a mistake from applying the ideas of thermodynamic equilibrium to a situation that is clearly not in equilibrium. If you are heating something it is clearly not in equilibrium and you have to worry about the practical effects not the ideal thermodynamic view. In practice, vaporisation is encouraged by nucleation sites (rough patches on the glass, for example). If there are not enough of those, it may not happen evenly.
The boiling point of a substance is defined as the temperature at which the vapor pressure of a liquid or solution is equal to atmospheric pressure; it does not mean that gas has started precipitating out of solution. The key here is to focus on the fact that boiling is gas precipitating out of solution in much the same way that a solid precipitates out of a supersaturated solution. Have you seen the precipitation demonstrations where a supersaturated solution has a crystal of the solute introduced and the precipitate immediately starts growing in solution? The boiling chip is serving a similar purpose to the solute being introduced to a supersaturated solution.
In order for the precipitation to begin happening, it requires enough of the precipitating material to overcome the viscosity of the solvent and join together into its own unit. Boiling chips provide an uneven surface upon which gases can form bubbles and escape the solution; lab glassware typically is too smooth.
As for bumping, you can think of the superheated liquid as being a supersaturated solution where any disturbance can provide the nucleation site. Once the liquid is disturbed, you get precipitation just like in the sodium acetate demos, except this precipitation is a gas which occupies a much greater volume than a solid or liquid, and it splashes all over the place.
