Bumping is a phenomenon in chemistry where homogeneous liquids
boiled
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. T ...
in a
test tube
A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom.
Test tubes are usually placed in ...
or other container will
superheat and, upon
nucleation, rapid boiling will expel the liquid from the container. In extreme cases, the container may be broken.
Cause
Bumping occurs when a liquid is heated or has its pressure reduced very rapidly, typically in smooth, clean glassware. The hardest part of bubble formation is the initial formation of the bubble; once a bubble has formed, it can grow quickly. Because the liquid is typically above its boiling point, when the liquid finally starts to boil, a large vapor
bubble is formed that pushes the liquid out of the test tube, typically at high speed. This rapid expulsion of boiling liquid poses a serious hazard to others and oneself in the
lab. Furthermore, if a liquid is boiled and cooled back down, the chance of bumping increases on each subsequent boil, because each heating cycle progressively de-gasses the liquid, reducing the number of remaining nucleation sites.
Prevention
The most common way of preventing bumping is by adding one or two
boiling chips
A boiling chip, boiling stone, porous bit anti-bumping granule is a tiny, unevenly shaped piece of substance added to liquids to make them boil more calmly. Boiling chips are frequently employed in distillation and heating. When a liquid becomes ...
to the reaction vessel. However, these alone may not prevent bumping and for this reason it is advisable to boil liquids in a
boiling tube
A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom.
Test tubes are usually placed in s ...
, a
boiling flask, or an
Erlenmeyer flask
An Erlenmeyer flask, also known as a conical flask (British English) or a titration flask, is a type of laboratory flask which features a flat bottom, a conical body, and a cylindrical neck. It is named after the German chemist Emil Erlenmeyer ...
. In addition, heating test tubes should never be pointed towards any person, just in case bumping does occur. Whenever a liquid is cooled below its boiling point and re-heated to a boil, a new boiling chip will be needed, as the pores in the old boiling chip tend to fill with solvent, rendering it ineffective.
A sealed capillary tube can also be placed in a boiling solution to provide a nucleation site, reducing the bumping risk and allowing its easy removal from a system.
Stirring a liquid also lessens the chances of bumping, as the resulting vortex breaks up any large bubbles that might form, and the stirring itself creates bubbles.
References
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Phase transitions
Chemical safety