A stopper is a truncated conical piece of rubber, cork, glass, or plastic used to close off a glass tube, piece of laboratory glassware, a wine bottle or barrel and other containers with orifices.
A rubber stopper is sometimes called a rubber bung, and a cork stopper is called a cork.
Ground glass stoppers and rubber stoppers are commonly used with laboratory glassware, mainly because of their nonreactivity. Some stoppers used in labs have holes in them to allow the insertion of glass or rubber tubing. This is often used when a reaction is taking place in the flask or test tube and the byproduct or result of the reaction is desired to be collected. For instance, if one were to boil water in a test tube in an attempt to collect the water vapor, one can seal the test tube with a stopper with holes in it. Upon inserting tubing into the hole(s) and exposing the tube to heat, the water vapor will rise through the hole, make its way through the tubing, and into the collection chamber of choice. The water vapor would not be able to escape into the air, because the stopper and the tubing, if set up correctly, should be airtight.
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