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Collecting gases and measuring their volumes

Choose gas collection apparatus from the task, water solubility and density, and measure gas volume without losing the product.

  • SEC G3 Pure Chemistry 2027

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Start with the purpose: do you need a gas-jar sample, a dry sample or a measured volume? Then use water solubility, density relative to air and compatibility with the apparatus to choose a method that avoids unnecessary gas loss.

Start with the sample and the goal

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.

What you need to know

  • 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.

Choosing and explaining a method

Choose from the task and properties
  • Need a measured volume? Consider a compatible gas syringe.
  • Would much of the gas dissolve in or react with water? Avoid water collection.
  • Collecting by air displacement? A less-dense gas rises into an inverted jar; a denser gas enters an upright jar.

How to choose a collection method

Use the task and gas properties together:

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 less dense than air; useful when water is unsuitableNH₃“Gas rises”
Downward delivery (displacement of air)Gas is more dense than air; useful when water is unsuitableCl₂, 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

Carbon dioxide has appreciable water solubility, so some is lost when collected over water. This does not make water collection impossible, but it can affect the amount recovered. For a gas-jar sample, downward delivery avoids that loss 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 with the delivery tube entering through its submerged open end, 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 deliveryGas less dense than airGas rises; air exits below.inverted gas jar3. Downward deliveryGas denser than airGas sinks; air exits at the top.upright gas jar
Choose the method from the task and gas properties: collect gases of low water solubility that do not react with water over water; air displacement uses upward delivery for gases less dense than air and downward delivery for gases denser than air, and is especially useful when water collection is unsuitable.

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.

Upward delivery (gas lighter than air)

Use an inverted gas jar for a gas less dense than air. It rises into the jar and displaces air downwards. This is especially useful when the gas dissolves readily in water, but high water solubility is not a requirement for air displacement.

Common example: ammonia, NH₃.

Downward delivery (gas heavier than air)

Use an upright gas jar for a gas denser than air. It moves down into the jar and displaces air upwards. Water solubility helps you decide whether water collection is suitable; density determines the direction of air displacement.

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

Laboratory Warning

Cl₂, HCl, NH₃ 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.

Drying a gas sample

A drying agent must remove water vapour without reacting with the gas. For ammonia, choose quicklime (calcium oxide), rather than concentrated sulfuric acid or fused calcium chloride. The acid reacts with ammonia, and calcium chloride forms a compound with it. Quicklime is unsuitable for acidic gases such as carbon dioxide because it reacts with them.

Continue to choosing a drying agent for the apparatus, compatibility table and worked examples.

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.
  • Check that the plunger moves freely and that the syringe can hold the expected volume. Start gas-producing rate experiments with reactants separated inside the sealed apparatus, then mix and start timing together; fitting a bung after mixing can lose the first gas produced.
Gas-syringe method for measuring reaction rateMarble chips and dilute hydrochloric acid react in an airtight conical flask. A delivery tube connects the flask headspace to the inlet of a clamped gas syringe. Collected gas pushes a sealing piston outwards; its rod is behind the piston, outside the gas chamber. Record the volume increase and time. The setup is schematic, not to scale.graduated gas syringeairtight delivery tubesyringe heldin a clamprecord gas volume / cm³at regular timesmarble chips + dilute HCl

Reaction: marble chips and dilute hydrochloric acid release carbon dioxide.

Gas path: gas passes from the flask headspace through an airtight tube into the syringe, pushing its piston outwards.

Measurement: clamp the syringe, allow its plunger to move freely and record volume changes at regular times.

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.

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 rateMarble chips and dilute hydrochloric acid react in a conical flask on an electronic balance. Carbon dioxide leaves through a loose cotton-wool plug, which reduces spray without sealing the neck. Record the decreasing mass of the flask and contents and the time. The setup is schematic, not to scale.loose cotton-wool plugreduces spray; does not seal flaskCO₂ escapesmarble chips+ dilute HClmass / gelectronic balancerecord total mass / g

Reaction: marble chips and dilute hydrochloric acid release carbon dioxide.

Gas path: carbon dioxide escapes through a loose cotton-wool plug. The flask is not sealed.

Measurement: record the total mass of the flask and contents at regular times. The decrease is the gas mass lost if other mass losses are negligible.

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.

Avoiding 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.

Using the evidence

2-step selection for method

Step 1: Would dissolving or reacting with water cause unacceptable gas loss? If so, use another method. Step 2: Compare density with air to choose upward (lighter) or downward (heavier) delivery.

Explain the choice using properties

Use “less dense than air so it rises” or “more dense than air so it sinks” and “dissolves readily in water, so collecting over water would lose much of the gas”.

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

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.

The drying-gases lesson now develops this example.

The drying-gases lesson now develops this example.

Challenge 3

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.

Try it independently

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.

Practise and check

Practise and check

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

Open the topic check
Syllabus and review details

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