Definition for Boiling chips

Bog

Bolus

In distillation, boiling chips are small, insoluble, porous stones made of calcium carbonate or silicon carbide. These stones have pores inside which provide cavities both to trap air and to provide spaces where bubbles of solvent vapor can form. When a boiling chip is heated in a solvent, it releases tiny bubbles. These bubbles ensure even boiling and prevent bumping and boiling over and loss of the solution.

Liquids often boil in an uneven fashion, or "bump" as it's called in chemistry. Bumping occurs frequently when you are distilling solvents or mixtures and there aren't any scratches or irregularities on the walls of the boiling pot that would help boiling bubbles to form. If this happens, a thing called superheating occurs- because the walls of your boiling pot are smooth and bubbles of vaporized liquid cannot easily form, the temperature of the liquid can actually rise above it's boiling point without boiling- the solution is now a superheated liquid. This can be dangerous because bubbles eventually will form, and when finally do, they usually erupt violently because so much of the liquid is just itching to boil but hasn't been able to, that it all goes bloooop and releases all of it's superheated boiling energy at once.

The result is that the entire solution turns into a mass of gas and bubbles, and with a resounding "thump" it can send a shockwave of undistilled boiling solution shooting up and through the entire distillation system- at the very least contaminating everything you've just distilled, and at worst it can actually cause the glassware to come apart or even break due to the sudden pressure wave.

Bumping is easily prevented by adding boiling chips to the liquid.