Collection of Gases and Measurement of their Volumes
Key idea: Gas collection methods: over water, upward/downward delivery, gas syringe, and choosing drying agents using solubility and density rules.
Before you start: Name and select apparatus for measuring time, temperature, mass and volume
By the end, you can
- Select apparatus for drying and collecting gases and for measuring reaction rates
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
- 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):
| Method | When to use it | Typical gases (O-Level) | Key phrase |
|---|---|---|---|
| Displacement of water | Gas is insoluble / slightly soluble in water and does not react with water | H₂, O₂ | “Collected over water” |
| Gas syringe | The gas volume must be measured; the gas must not react with the syringe | Many 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 air | NH₃ | “Gas rises” |
| Downward delivery (displacement of air) | Gas is soluble in water and more dense than air | Cl₂, 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.
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.
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).
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 agent | Used for | Cannot be used for | Reason |
|---|---|---|---|
| Concentrated sulphuric acid (H₂SO₄) | Many gases | NH₃ | NH₃ is a base and reacts with acids to form salts (see Acids). |
| Quicklime (calcium oxide, CaO) | Basic gases | Acidic gases such as CO₂, SO₂ | It reacts with acidic gases. |
| Fused (anhydrous) calcium chloride (CaCl₂) | Many gases | NH₃ | Forms a complex with NH₃. |
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.
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.
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 change | Suitable apparatus | Measurement recorded |
|---|---|---|
| Gas is produced | Gas syringe + stopwatch | Gas volume at regular times |
| Gas escapes from the flask | Electronic balance + stopwatch | Decrease in mass at regular times |
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
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 “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
Example 1: Choosing a Collection Method (Oxygen)Core
Question: Oxygen gas is produced in the lab. Which method is most suitable to collect a sample of oxygen?
- (A) Upward delivery
- (B) Downward delivery
- (C) Displacement of water
Show Answer
Answer: (C)
Reason: O₂ is only slightly soluble in water, so it can be collected by displacement of water.
Example 2: Choosing a Collection Method (Ammonia)Core
Question: Which method is most suitable for collecting ammonia, NH₃?
- (A) Displacement of water
- (B) Upward delivery
- (C) Downward delivery
Show Answer
Answer: (B)
Reason (mark-scheme): NH₃ is soluble in water, so it cannot be collected over water. It is less dense than air, so it is collected by upward delivery.
Example 3: Choosing a Collection Method (Carbon Dioxide)Core
Question: A gas jar of carbon dioxide, CO₂, is required and its volume does not need to be measured. Which collection method is most suitable?
- (A) Displacement of water
- (B) Upward delivery
- (C) Downward delivery
Show Answer
Answer: (C)
Reason (mark-scheme): CO₂ is soluble in water, so it is not usually collected over water. It is more dense than air, so it is collected by downward delivery.
Example 4: Choosing a Drying AgentCore
Question: Which drying agent is suitable for drying ammonia, NH₃?
- (A) Concentrated sulphuric acid, H₂SO₄
- (B) Quicklime, CaO
- (C) Anhydrous calcium chloride, CaCl₂
Show Answer
Answer: (B)
Reason: NH₃ reacts with acids and forms a complex with CaCl₂. Quicklime (CaO) can dry NH₃ without reacting with it.
Example 5: Error Analysis (Two Mistakes)Core
Question: A student wants a dry sample of ammonia, NH₃. The student passes the gas through concentrated sulphuric acid and collects it by downward delivery. Identify two mistakes.
Show Answer
- Wrong drying agent: NH₃ reacts with H₂SO₄, so ammonia is removed instead of dried.
- Wrong collection method: NH₃ is less dense than air, so it should be collected by upward delivery, not 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
- SO₂ is soluble in water, so it dissolves and you lose gas.
- 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
- There is a leak in the connections so gas escapes to the air.
- The syringe/plunger is stuck or not connected properly, so gas cannot push the plunger out.
8. Quiz
Practise choosing a collection method, measuring gas volume and selecting a drying agent.
Start gas collection quiz