Which one of the following materials is present in a guard tube (drying tube) that is used for preparation of HCl gas ?

During the laboratory preparation of hydrogen chloride (HCl) gas, the gas is typically produced by reacting solid sodium chloride with concentrated sulphuric acid [1]. To ensure the gas is dry, especially in humid conditions, it is passed through a guard tube (drying tube) containing anhydrous calcium chloride [1]. Calcium chloride is a highly effective desiccant and drying agent due to its high hygroscopicity, allowing it to absorb moisture from gases like nitrogen, oxygen, and hydrogen chloride without reacting with them. It is commonly used in granular form within drying tubes to extract water vapor and prevent atmospheric moisture from entering the reaction vessel [1]. While other calcium halides like calcium bromide, iodide, or fluoride exist, they are not standard laboratory desiccants for this specific application [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.9 > p. 23