Which one of the following materials is present in a guard tube (drying tube) that is used for preparation of HCl gas?
Calcium chloride is commonly used as a drying agent in guard tubes (drying tubes) during the preparation of dry HCl gas.
Hydrogen chloride (HCl) gas is acidic and reacts with basic drying agents like quicklime (CaO) or NaOH. Therefore, an acidic or neutral drying agent is required. Anhydrous calcium chloride (CaCl2) is a neutral salt and a highly efficient drying agent that does not react with HCl gas.
Concentrated sulfuric acid is another common drying agent for HCl, but calcium chloride is often used in solid form in drying tubes. Other calcium halides like calcium bromide, iodide, and fluoride are less commonly used or less effective as drying agents for general laboratory purposes compared to calcium chloride.