Certain experiments in organic chemistry involve reactions that must be carried out in the absence of water. Unfortunately, air contains moisture. The solution to this problem is a drying tube.
Anhydrous calcium chloride is a very hygroscopic compound, meaning that it readily absorbs water. It can even dissolve, in fact, in the water it absorbs from the surrounding atmosphere. Calcium chloride pellets in a drying tube act as desiccants or drying agents, removing moisture from the air that flows through them.
A calcium chloride drying tube contains calcium chloride pellets at the top and bottom, held in place by plugs made of glass wool. As air flows through the wool and the calcium chloride, it is de-humidified so that the air entering the reaction chamber contains little or no moisture.
A reaction involving gases could potentially shatter a sealed vessel; it may not be desirable, however, to allow air moisture to enter the chamber. A drying tube relieves pressure inside the reaction vessel by allowing gases to escape while preventing moisture from contaminating the reactants.
Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.