Which one of the following materials is present in a guard tube (drying tube) that is used for preparation of HCl gas ?
- A.Calcium chloride
- B.Calcium bromide
- C.Calcium iodide
- D.Calcium fluoride
Correct answer
Explanation
The correct answer is A, Calcium chloride. A guard tube is filled with anhydrous calcium chloride, which absorbs moisture and keeps the hydrogen chloride gas dry.
In the school experiment, hydrogen chloride gas is made by heating sodium chloride with concentrated sulphuric acid, and the gas is passed through a guard tube containing calcium chloride. Anhydrous calcium chloride is a strong drying agent because it readily takes up water. Drying matters here: dry HCl gas does not change the colour of dry blue litmus, which shows that an acid releases H⁺ ions only in the presence of water. Calcium chloride does not react with HCl, so it is safe to use with this gas.
B is wrong: calcium bromide is not the drying agent used in a guard tube.
C is wrong: calcium iodide is not used for drying gases in this experiment.
D is wrong: calcium fluoride is insoluble and does not absorb water, so it cannot act as a drying agent.
Exam tip: guard tube = anhydrous CaCl₂; dry HCl gas does not turn dry blue litmus red, because H⁺ ions form only in water.
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