A boiling tube is a small cylindrical vessel used to strongly heat substances in the flame of a Bunsen burner

Bunsen burner

A Bunsen burner, named after Robert Bunsen, is a common piece of laboratory equipment that produces a single open gas flame, which is used for heating, sterilization, and combustion.- Operation:...

. A boiling tube is essentially a scaled-up test tube

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, usually with a rounded U-shaped bottom....

, being about 50% larger in every aspect.

They are designed to be wide enough to allow substances to boil violently as opposed to a test tube which is too narrow for this task; boiling liquid can explode out of the end of test tubes when they are heated as there is no room for the bubbles of gas to escape independently of the surrounding liquid.

Boiling tubes are made from borosilicate glass that can withstand high temperatures.