1. Water displacement: trap in water
For gases that do not dissolve in water, we can trap them in water using a method with the fancy name of displacement of water.
This method requires a delivery tube, a measuring cylinder, and a basin of water.
We first fill a measuring cylinder with water and invert it in a basin of water. We then use a delivery tube to bubble gas into the inverted measuring cylinder. As gas bubbles and fills up the inverted measuring cylinder, the water level drops.
2. Upward delivery: let it float
Air is a mixture of mainly nitrogen and oxygen gas.
Gases that are less dense than air rise, like helium in balloons and billowing hot smoke.
We use this property to collect gases less dense than air by delivering them into an inverted gas jar. This is the upward delivery method.
3. Downward delivery: sink and never leave
Conversely, we collect gases denser than air using downward delivery.
This is the laziest method. We simply deliver the gas into a gas jar and let it sink.
4. Cheat sheet: list of common gases
| gas | solubility | density |
|---|---|---|
| hydrogen, H2 | insoluble | less dense |
| oxygen, O2 | insoluble | similar |
| chlorine, Cl2 | insoluble | denser |
| carbon dioxide, CO2 | slightly soluble | denser |
| ammonia, NH3 | soluble | less dense |
| hydrogen chloride, HCl | soluble | denser |
| sulfur dioxide, SO2 | soluble | denser |
We cannot collect soluble gases by the displacement of water. Instead, we must use upward delivery for soluble gases less dense than air, or downward delivery for soluble gases denser than air.