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Jessica, certain reactions like Diels-Alder reactions and catalytic hydrogenations involve working with gases at a pressure greater than 1 atm. In such a scenario it becomes necessity to seal a tube.

You are right Radhey Shyam. Jessica, under the circumstance which is mentioned by name1, the reaction must be performed in a sealed reaction vessel. Although the ordinary glass reaction setup is able to hold a perfect vacuum without imploding, this equipment cannot withstand pressure from within and may explode if the internal pressure is allowed to rise and is sealed off. The content is cooled in a dry Ice or may be in a liquid nitrogen bath. This is done to prevent ignition during the sealing process. Then a glass rod is sealed to the open end of the tube and the tube is heated while it is rotated. As the glass rod melts, it is allowed to thicken and to collapse to one-third of the original diameter of the tube tube and drawn to give a thick-walled capillary. After the cooling process, the capillary is sealed in the flame.

Thank you for the answer sir.

Jessica, if an inert atmosphere is desired within the tube, the capillary must be broken before sealing, a rubber tubing that is connected to a three-way stopcock is then slipped over the open capillary end, the tube is evacuated and is then filled with an inert gas.