Topic 2 of 6
Collect a gas using its properties
Solubility and density determine a suitable setup.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Collect a gas using its properties
Solubility and density determine a suitable setup.
A gas syringe is convenient for measuring gas volume. The bung and connections must be airtight, the plunger must move freely, and the capacity must exceed the expected volume. Begin collection promptly: gas lost before sealing makes measured volume too small.
| Method | Suitable when | Example or limitation |
|---|---|---|
| Displacement of water | Gas is insoluble or only slightly soluble in water | Hydrogen or oxygen; unsuitable for very soluble ammonia. |
| Upright gas jar with gas entering near the bottom | Gas is denser than air and would dissolve in water | Carbon dioxide can displace air upwards. |
| Inverted gas jar with gas entering near the top | Gas is less dense than air | Ammonia displaces air downwards. |
| Gas syringe | Gas can be collected without significant leakage or reaction with apparatus | Useful for volume-time measurements; very soluble gases need an appropriate dry setup. |
Pure only
To obtain a dry gas, pass it through a drying agent that removes water without reacting with the gas. The drying stage goes after generation and before dry collection. Collecting a dried gas over water would make it wet again.
Pure only
| Agent | Selection principle |
|---|---|
| Calcium oxide | Suitable for ammonia; it is basic and would react with acidic gases such as carbon dioxide. |
| Concentrated sulfuric acid | Absorbs water, but reacts with ammonia, so cannot dry it. |
| Fused calcium chloride | Anhydrous salt absorbs water; unsuitable for ammonia, which it also absorbs. |
Check your understandingWhy is collection over water unsuitable for ammonia?Think it through, then reveal the answer
Ammonia is very soluble in water, so much of the gas would dissolve instead of being collected. Use its low density to collect it in an inverted dry jar.