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3 Limitations of Using Boiling Chips in Superheated Fluids

When microwaves first became popular, I was warned that using one to heat a mug of water beyond its boiling point could lead to an explosion. There have been reports such occurrences, with people supposedly being scalded. In the lab, superheating is

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Liquids will not boil smoothly unless a source of nuclei is present on which the bubbles of vapor may readily form. Boiling liquids that suddenly vaporize can lurch out of vessels with almost explosive force and scatter hot liquid over innocent bystanders. This problem is especially troublesome in vacuum distillations and in systems containing precipitates.

For work at atmospheric pressure, you can overcome most problems by very good stirring. A magnetic stirrer will provide enough nuclei if the liquid is devoid of precipitates. If dense precipitates are present, use a paddle stirrer.

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Boiling chips are either porous, solid lumps that contain trapped air that is released on heating, thus providing a stream of nuclei for even boiling, or they have sharp edges such as carborundum chips. It is important to realize that boiling chips have several limitations:

  • They will not prevent "bumping" or lurching (see above) in systems which contain precipitates.
     
  • They will not work if they are used in a hot system that is later cooled and then reheated. A new chip must be added, since the pores of the used boiling chips will be clogged by liquid.
     
  • They are useless in vacuum; after about five seconds all the air is sucked out.

In a vacuum distillation, the easiest way for providing nuclei is to stir the solution in a magnetic stirrer.

CAUTION: Never add a boiling chip or initiate stirring when the liquid is hot! An eruption will result.