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Choosing a drying agent for a gas

Remove water vapour without removing the gas: compare calcium oxide, concentrated sulfuric acid and fused calcium chloride.

  • SEC G3 Pure Chemistry 2027

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Collecting a gas and drying it answer different questions. Collection depends on the task, density and water solubility; drying depends on chemical compatibility. A drying agent is useful only if it removes water vapour while leaving the gas you want unchanged.

Start with collecting and measuring gases if you need to choose the collection apparatus first.

Removing water without removing the gas

Pass the moist gas through a suitable drying agent before it enters a dry collection vessel. A liquid agent is held in a wash bottle: the inlet dips below the liquid so the gas bubbles through it, and the outlet is above the liquid. With a solid agent, the gas passes over granules in a drying tube.

Three gas drying agents and their apparatusConcentrated sulfuric acid is shown in a wash bottle with moist gas bubbling through the liquid. Quicklime and fused calcium chloride are shown as solid granules in drying tubes. Labels state which agents cannot be used for ammonia and that quicklime is suitable for ammonia.Concentrated H₂SO₄Liquid drying agentmoist gas indry gas outDo not use for NH₃: it reacts.Quicklime, CaOSolid drying agentmoist gas passes over solid granulesSuitable for drying NH₃.Avoid acidic gases such as CO₂ or SO₂.Fused CaCl₂Solid drying agentinoutmoist gas passes over solid granulesNH₃ forms a complex: unsuitable.
A drying agent must remove water without reacting with the gas. Concentrated sulfuric acid and fused calcium chloride are unsuitable for ammonia; quicklime is the standard drying agent for ammonia.

The three agents in this course are:

Drying agentWhat to checkImportant incompatibility
Concentrated sulfuric acid, H₂SO₄It absorbs water; use it only with a gas that does not react with it.Ammonia is basic and reacts with the acid to form an ammonium salt.
Quicklime (calcium oxide), CaOIt removes water and is suitable for ammonia.It reacts with acidic gases such as carbon dioxide and sulfur dioxide.
Fused (anhydrous) calcium chloride, CaCl₂It takes up water; check that it does not also take up the target gas.It forms a compound with ammonia, so it is unsuitable for drying ammonia.

“Fused” calcium chloride has been heated to remove its water. You need not learn a formula for its compound with ammonia here: the practical consequence is that ammonia would be removed too.

For quicklime, water removal can be represented by:

CaO(s) + H₂O(g) → Ca(OH)₂(s)

Contrast this useful water-removal reaction with the unwanted reaction when the target gas is carbon dioxide:

CaO(s) + CO₂(g) → CaCO₃(s)

Both equations involve the same agent, but only the first removes the impurity we want to remove.

Use the supervised apparatus

Concentrated sulfuric acid and calcium oxide can cause burns. Follow your teacher’s procedure, wear eye protection and avoid contact. Ammonia and other harmful gases require the instructed fume-cupboard precautions; do not inhale them.

Choosing and explaining an agent

Ask two questions: Does it remove water? Does it react with this gas? State both parts of the reason. A substance that absorbs water strongly can still be the wrong drying agent.

Dry does not mean pure

A dry sample contains little water vapour. It may still contain air or other gases. Drying alone does not establish the identity or purity of the sample.

Worked examples

Examiner practice 1

Choosing a Drying Agent

2 marks

Examination question

Choose a suitable drying agent for ammonia from concentrated sulfuric acid, quicklime and anhydrous calcium chloride. Justify the choice. [2 marks]

Name the agent and explain compatibility

View solution step by step

  1. Select the agent

    1 mark

    Method

    Choose quicklime, CaO.

    Reason

    It removes water vapour from ammonia.

    Working

    Drying agent: quicklime.

  2. Justify compatibility

    1 mark

    Method

    State that quicklime does not react with ammonia.

    Reason

    Concentrated sulfuric acid reacts with basic ammonia, while anhydrous calcium chloride forms a complex with it.

    Working

    CaO dries NH₃ without removing it chemically.

Common misconception 2

Error Analysis (Two Mistakes)

Find and correct the mistake

Learner method

A learner passes ammonia through concentrated sulfuric acid and then collects it by downward delivery to obtain a dry sample. Identify and correct both errors.

Check chemical compatibility and gas density

Suitable drying agent
Correct delivery method

View solution step by step

  1. Correct the drying agent

    Method

    Replace concentrated sulfuric acid with quicklime, CaO.

    Reason

    Ammonia is basic and reacts with the acid, so the original agent removes ammonia rather than merely water vapour.

    Working

    Dry NH₃ with quicklime.

  2. Correct the delivery direction

    Method

    Collect ammonia by upward delivery.

    Reason

    Ammonia is less dense than air and therefore rises into the vessel.

    Working

    Correct method: quicklime followed by upward delivery.

Try it independently

Mind stretcher 1: Dry carbon dioxide without consuming itExtension

A learner chooses quicklime to dry carbon dioxide because it is used to dry ammonia. Explain why the choice fails. What additional property must a replacement agent have?

Show answer

Quicklime reacts with carbon dioxide to form calcium carbonate, so it removes the gas as well as water. Choose an agent known to remove water without reacting with carbon dioxide. Suitability for ammonia does not establish suitability for a different gas.

Mind stretcher 2: Follow the gas through a wash bottleExtension

A drying bottle contains a compatible liquid agent. Its inlet tube ends above the liquid surface. Why might the gas remain moist, and how should the tubes be arranged?

Show answer

The gas can pass across the space above the liquid with insufficient contact. The inlet should dip below the liquid so the gas bubbles through it; the outlet remains above the liquid to carry gas onward without deliberately delivering the liquid into the collection vessel.

Practise and check

Topic check

See what you know across this topic, then go back to anything you got wrong.

Take the topic check

Syllabus and review details

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