Why use boiling stones when boiling liquids?

Boiling stones, more commonly referred to as boiling chips, are used when heating liquids in organic chemistry experiments to ensure even boiling and prevent boiling over of the solution. These chips are made from small flakes of calcium carbonate or silicon carbide.

Boiling chips are generally porous, trapping heating bubbles and allowing solvent vapors to form in the air cavities. They should always be added to a solvent prior to the beginning of a heating cycle; adding to a hot liquid may cause it to violently boil over. Boiling chips and stones are only used once, as the absorptive material becomes impregnated with the first solvent used in.