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Bumping is a phenomenon in chemistry, where homogenous liquids boiled in a test tube or some other container will superheat and, upon nucleation, rapid boiling will expel the liquid from the container. If the bumping is violent enough, the container may be broken.
Bumping occurs when liquids are heated, or has its pressure reduced very rapidly, typically in smooth, clean glass flasks. As the liquid's temperature rises above its boiling point, it becomes superheated. Once a bubble has formed, it will grow quickly, turning into a large vapor bubble that rises, pushing 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.
Viscous liquids with a high boiling point, like conc. sulfuric acid will bump violently during distillation.
The most common way of preventing bumping is by adding boiling chips to the reaction vessel. This however, does not completely eliminate the bumping, but it can prevent violent the liquid from splashing all over the flask or even rupturing the flask.
Stirring the liquid using a stir bar can also work, though not always.
Another technique is to use a glass capillary tube to bubble air (or nitrogen) below the liquid level. This is particularly effective, but is less convenient to set up than boiling chips.
Bumping is a problem when distilling sulfuric acid.