Topic 2 of 6
Collect a gas using its properties
Solubility and density determine a suitable setup.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (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. |
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.