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A boiling chip, also known as boiling stone, or anti-bumping granule is an unevenly shaped rough granular material added to liquids to make them boil more calmly.
Boiling chips are frequently employed in distillation and heating. When a liquid becomes superheated, a particle of dust or a stirring rod can cause flash boiling. This very rapid boiling can be extremely violent and cause reagents to splatter, possibly causing severe burns, ruining experiments, and damaging equipment. Boiling chips work by providing nucleation sites so the liquid boils smoothly without becoming superheated or bumping.
Boiling chips are made of a chemically stable material, like glass, quartz, or calcinated porcelain. They appear like flakes or beads, with a rough surface, necessary for nucleation.
Boiling chips can be purchased from lab suppliers.
DIY boiling chips
The easiest and cheapest way to make boiling chips is to smash porcelain or similar ceramic materials into small bits. Avoid using glazed ceramic, as it doesn't have rough surface necessary for nucleation.
Glass shards or quartz pieces can be used, but because the surface of these materials is smooth they are not suitable untreated. To make their surface rough, there are two ways: sandpaper the glass/quartz surface using a very abrasive material (SiC, WC, diamond, etc.) to make it rough, or chemical treatment using hydrofluoric acid or aq. sodium hydroxide.
Silica gel can also be used as boiling stones.
Chemically cleaned sand can also be used when distilling liquids that tend to bump violently.
Boiling stones help reducing bumping during boiling, but do not eliminate it.