CDS (II) 2021 General Knowledge · Question 26 of 120

Correct — A, Calcium chloride. The guard tube in this arrangement has one job — to keep the hydrogen chloride gas dry — and the material packed into it is calcium chloride. The laboratory note that goes with the preparation says it plainly: if the climate is very humid, the gas produced has to be passed through a guard tube, also called a drying tube, containing calcium chloride to dry it. Anhydrous calcium chloride is chosen because it is a powerful drying agent, taking up water vapour readily and holding it, and because it does not react with hydrogen chloride, which rules out most of the alternatives a chemist might otherwise reach for. Drying matters here because the whole point of the experiment is to show that dry hydrogen chloride gas has no acidic character — it leaves dry litmus paper unchanged — and that acidity appears only when water is present, because the separation of the hydrogen ion from the hydrogen chloride molecule cannot happen without it.

• (b)Calcium bromide — Not a laboratory drying agent for this purpose. It is far less used than the chloride and does not appear in the standard preparation.
• (c)Calcium iodide — Also not used as a desiccant here. Iodides are prone to slow oxidation in air, which would contaminate rather than purify the gas stream.
• (d)Calcium fluoride — Calcium fluoride is the mineral fluorspar and is essentially insoluble in water, which makes it useless as a drying agent. A desiccant has to take up water eagerly.

A drying agent, or desiccant, is a substance that removes water vapour from a gas or from an enclosed space by absorbing or adsorbing it. Anhydrous calcium chloride, silica gel, concentrated sulphuric acid and phosphorus pentoxide are the common laboratory choices, and the right one for a given job is the one that dries effectively without reacting with the substance being dried. A guard tube packed with a desiccant sits in the gas line to catch moisture on the way through.

All four options are calcium halides, so the item cannot be settled by spotting the odd element out — the halogen has to be identified. Calcium chloride is the one that turns up throughout the syllabus as a drying agent, and its behaviour is familiar from everyday life in the moisture absorbers sold for damp cupboards. The chemistry underneath is straightforward: anhydrous calcium chloride binds water strongly to form hydrates, so it pulls water out of a passing gas stream. Knowing why the experiment needs drying at all is worth as much as knowing the name. Dry hydrogen chloride gas does not turn dry litmus red; only when water is available do hydrogen ions appear, and acidic behaviour with them. A humid day would let enough moisture into the tube to spoil that demonstration, which is exactly why the guard tube is specified as a precaution rather than as a fixed part of the apparatus.

• A guard tube, or drying tube, containing calcium chloride is used to dry the hydrogen chloride gas produced in the laboratory preparation.
• Anhydrous calcium chloride is a strong desiccant because it takes up water to form hydrates, and it does not react with hydrogen chloride.
• Dry hydrogen chloride gas does not show acidic behaviour; hydrogen ions are released only in the presence of water.
• Silica gel is the other common desiccant met in the syllabus, used in small sachets to protect tablets and powders from moisture.
• Calcium fluoride is the mineral fluorspar and is practically insoluble in water, so it cannot serve as a drying agent.

• Assuming any calcium salt will dry a gas; the halide matters, and fluoride is insoluble.
• Thinking the guard tube purifies the gas chemically; it removes only moisture.
• Forgetting that dry hydrogen chloride is not acidic — the acidity is a property of the aqueous solution.

As a name-the-desiccant item, as a reasoning question on why hydrogen chloride must be dried, or as an application question on where silica gel and calcium chloride are used.

• practice — not a real PYQ

  Dry hydrogen chloride gas does not turn dry blue litmus paper red because

  • (a)it is not an acid at all
  • (b)hydrogen ions are produced only in the presence of water
  • (c)litmus does not respond to any gas
  • (d)the gas is too dilute to be detected

  Answer(b) hydrogen ions are produced only in the presence of water — the hydrogen ion cannot separate from the hydrogen chloride molecule without it.

• practice — not a real PYQ

  A small sachet of silica gel is packed with medicine tablets because silica gel

  • (a)kills bacteria
  • (b)absorbs moisture
  • (c)absorbs all gases in the bottle
  • (d)prevents the tablets from breaking

  Answer(b) absorbs moisture — it is a desiccant with a very large surface area, used to keep the contents dry.