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Collection of Gases and Measurement of their Volumes

Gas collection methods: over water, upward/downward delivery, gas syringe, and choosing drying agents using solubility and density rules.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • name appropriate apparatus for the measurement of time, temperature, mass and volume; including burettes, pipettes, measuring cylinders and gas syringes
  • suggest suitable apparatus, given relevant information, for a variety of simple experiments, including drying and collection of gases and measurement of rates of reaction (drying agents will be limited to calcium oxide, concentrated sulfuric acid and fused calcium chloride).

Collecting gases correctly is a scoring topic: exam questions often test whether you can choose the right collection method and the correct drying agent.

1. Definition

Gas collection is the process of transferring and keeping a gas produced in a reaction using apparatus suited to the gas and the required measurement.

2. Key Ideas

  • Solubility: how easily a gas dissolves in water.
  • Density (relative to air): whether a gas is less dense than air (rises) or more dense than air (sinks).
  • You must choose a method that prevents loss of the gas (e.g., dissolving in water, escaping from the jar).
  • If you need a dry gas, pass it through a drying agent that removes water vapour without reacting with the gas.

3. Detailed Explanations

Quick Recall (the 2 questions to ask)
  • Need to measure volume? Use a gas syringe when the gas must be collected and its volume measured.
  • Soluble in water? If yes, do not collect over water (you will lose gas).
  • Collecting in a gas jar? Use density: less dense → rises (upward delivery); more dense → sinks (downward delivery).

A. How to Choose a Collection Method

Use this decision table (mark-scheme logic):

MethodWhen to use itTypical gases (O-Level)Key phrase
Displacement of waterGas is insoluble / slightly soluble in water and does not react with waterH₂, O₂“Collected over water”
Gas syringeThe gas volume must be measured; the gas must not react with the syringeMany gases produced in rate or preparation experiments“Collect and measure the gas volume”
Upward delivery (displacement of air)Gas is soluble in water and less dense than airNH₃“Gas rises”
Downward delivery (displacement of air)Gas is soluble in water and more dense than airCl₂, HCl, SO₂, CO₂“Gas sinks”

Mini-example (how to use the table): NH₃ is soluble in water → not over water; it is less dense than air → collect by upward delivery.

Exam Trap: CO2 is not usually collected over water

When a gas jar of CO₂ is required, use downward delivery because CO₂ is soluble in water and denser than air. When its volume must be measured, a gas syringe is usually the appropriate apparatus.

Gas collection over water, by upward delivery and by downward deliveryThree labelled panels compare an inverted gas jar over a water trough, an inverted gas jar collecting a less-dense gas that rises, and an upright gas jar collecting a denser gas that sinks. Arrows also show displaced water or air.1. Over waterInsoluble or slightly soluble gasGas displaces water from the jar.2. Upward deliverySoluble gas less dense than airGas rises; air exits below.inverted gas jar3. Downward deliverySoluble gas denser than airGas sinks; air exits at the top.upright gas jar
Choose the method from the task and gas properties: collect insoluble gases over water; use upward delivery for soluble gases less dense than air and downward delivery for soluble gases denser than air.

B. Displacement of Water

This method collects gas in an inverted gas jar/measuring cylinder over a water trough. The gas is collected because it displaces water, so it must not dissolve significantly in water.

C. Upward Delivery (Gas Lighter Than Air)

This method is used when the gas is soluble in water (so you cannot collect it over water) and less dense than air, so it rises into an inverted gas jar.

Common example: ammonia, NH₃.

D. Downward Delivery (Gas Heavier Than Air)

This method is used when the gas is soluble in water and more dense than air, so it sinks into an upright gas jar.

Examples: chlorine (Cl₂), hydrogen chloride (HCl), sulphur dioxide (SO₂), carbon dioxide (CO₂).

Laboratory Warning

Cl₂, HCl, and SO₂ harm the respiratory system if inhaled. Keep the apparatus in a fume cupboard, avoid breathing the gases and follow your teacher’s instructions.

E. Drying a Gas Sample

To obtain a dry gas, pass the gas through a drying agent. The drying agent must remove water vapour without reacting with the gas.

Drying agentUsed forCannot be used forReason
Concentrated sulphuric acid (H₂SO₄)Many gasesNH₃NH₃ is a base and reacts with acids to form salts (see Acids).
Quicklime (calcium oxide, CaO)Basic gasesAcidic gases such as CO₂, SO₂It reacts with acidic gases.
Fused (anhydrous) calcium chloride (CaCl₂)Many gasesNH₃Forms a complex with NH₃.
Laboratory Warning

Concentrated sulphuric acid and quicklime can cause chemical burns, especially to the eyes and skin. Wear eye protection, avoid contact and rinse an affected area immediately with plenty of water while alerting your teacher.

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 agentmoist 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.

F. Measuring the Volume of a Gas

To measure gas volume accurately, a gas syringe is commonly used.

  • Record the initial syringe reading; if it is not zero, subtract it from the final reading.
  • Ensure all connections are airtight (otherwise gas escapes and volume is too low).
  • Read the scale at eye level.
Gas-syringe method for measuring reaction rateA conical flask containing marble chips and dilute hydrochloric acid is sealed with a bung and connected by a delivery tube to a graduated gas syringe. Carbon dioxide collected is recorded at regular time intervals.marble chips + dilute HClgraduated gas syringerecord CO₂ volume / cm³ at regular timesairtight delivery tube
Gas syringe method: keep the apparatus airtight, record the initial and final readings, then calculate the volume collected.

For other measurement rules (units, precision, reading scales), see Measurement of Time, Temperature, Mass and Volume.

G. Selecting Apparatus for Rate Measurements

Choose apparatus that measures a quantity which changes with time:

Observable changeSuitable apparatusMeasurement recorded
Gas is producedGas syringe + stopwatchGas volume at regular times
Gas escapes from the flaskElectronic balance + stopwatchDecrease in mass at regular times
Mass-loss method for measuring reaction rateA conical flask containing marble chips and dilute hydrochloric acid stands on an electronic balance. A loose cotton-wool plug reduces spray while allowing carbon dioxide to escape, so the decreasing mass is recorded at regular time intervals.marble chips + dilute HClloose cotton-wool plugreduces spray; does not seal flaskCO₂ escapesmass / gelectronic balancerecord decreasing mass at regular times
Mass-loss method: record the mass at regular time intervals. The cotton wool reduces spray but must not seal the flask, so gas can escape.

Use a gas syringe when the gas must be captured or its volume measured. Use mass loss only when a gaseous product can escape safely; keep the same apparatus and timing intervals in every run.

4. Common Mistakes

  • Choosing displacement of water for a gas that is soluble in water (especially NH₃ and CO₂).
  • Mixing up upward vs downward delivery by forgetting the density rule (“less dense than air rises”).
  • Using a drying agent that reacts with the gas (e.g., NH₃ through H₂SO₄).
  • Leaving gaps/leaks in the apparatus and then trusting the gas syringe reading.

5. Exam Tips

2-step selection for method

Step 1: Is the gas soluble in water? If yes, do not collect over water. Step 2: Compare density with air to choose upward (lighter) or downward (heavier) delivery.

Use mark-scheme verbs

Use “less dense than air so it rises” or “more dense than air so it sinks” and “soluble in water so cannot be collected over water”.

If the question expects gas tests (e.g., identifying CO₂, H₂), see Qualitative Analysis (QA).

6. Worked Examples

Modelled example 1

Choosing a Collection Method (Oxygen)

Core

Problem

Oxygen is produced in the laboratory. Choose the most suitable collection method from upward delivery, downward delivery and displacement of water, and justify it.
Study the worked solution

  1. Check water suitability

    Method

    Recognise that oxygen is only slightly soluble in water.

    Reason

    Little oxygen is lost by dissolving, and it does not react with the collection water under these conditions.

    Working

    O₂ can be collected over water.

  2. Choose the method

    Method

    Select displacement of water.

    Reason

    The incoming oxygen replaces water in an inverted vessel and remains as a collected sample.

    Working

    Method: displacement of water.

Guided practice 2

Choosing a Collection Method (Ammonia)

About 5 min

Problem

Choose the most suitable method for collecting ammonia, NH₃: displacement of water, upward delivery or downward delivery.

Apply solubility, then density

Collect over water?
Density relative to air
Collection method

Hints

Hint 1: solubility first
Ammonia dissolves readily in water, so eliminate one method.
Hint 2: density second
A gas less dense than air rises into the collection vessel.
View solution step by step

  1. Eliminate water displacement

    Method

    Do not collect ammonia over water.

    Reason

    Ammonia is soluble in water, so much of the gas would dissolve.

    Working

    Water displacement is unsuitable.

  2. Use density to choose delivery

    Method

    Select upward delivery.

    Reason

    Ammonia is less dense than air, so it rises and displaces air downwards.

    Working

    Collect NH₃ by upward delivery.

Common misconception 3

Error Analysis (Two Mistakes)

Find and correct the mistake

Learner method

A learner passes ammonia through concentrated sulphuric 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 sulphuric 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.

Examiner practice 4

Choosing a Drying Agent

2 marks

Examination question

Choose a suitable drying agent for ammonia from concentrated sulphuric 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 sulphuric acid reacts with basic ammonia, while anhydrous calcium chloride forms a complex with it.

    Working

    CaO dries NH₃ without removing it chemically.

Challenge 5

Choosing a Collection Method (Carbon Dioxide)

Minimal support

Method-selection transfer

A gas jar of carbon dioxide is required, but its volume does not need to be measured. Choose between displacement of water, a gas syringe, upward delivery and downward delivery, and justify the choice.

Use the collection purpose and gas properties

Need a measured volume?
Collection method

Hints

Hint 1: purpose
A gas syringe is useful when a volume must be measured; that is not required here.
Hint 2: properties
Carbon dioxide is soluble in water and more dense than air.
View solution step by step

  1. Apply the task constraint

    Method

    Choose a gas-jar delivery method rather than a measuring syringe.

    Reason

    The question requires a sample, not a numerical gas volume.

    Working

    Use displacement of air in a gas jar.

  2. Apply solubility and density

    Method

    Select downward delivery.

    Reason

    Carbon dioxide is soluble in water and more dense than air, so it sinks and displaces air upwards.

    Working

    Collect CO₂ by downward delivery.

7. Mind Stretchers

Mind stretcher 1: Technique JustificationExtension

Question: A student says: “I collected SO₂ over water because it is a gas.” Explain why this is wrong using two mark-scheme points.

Show Answer
  1. SO₂ is soluble in water, so it dissolves and you lose gas.
  2. It should be collected by downward delivery because it is more dense than air.

Mind stretcher 2: Leak Diagnosis (Gas Syringe)Extension

Question: A reaction produces gas, but the gas syringe reading stays at 0 cm³. Give two possible reasons related to the apparatus (not the chemistry).

Show Answer
  1. There is a leak in the connections so gas escapes to the air.
  2. The syringe/plunger is stuck or not connected properly, so gas cannot push the plunger out.

8. Quiz

Quiz Time!

Practise choosing a collection method, measuring gas volume and selecting a drying agent.

Start gas collection quiz