CNC Illustration by Shuqi Tang

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.

The collection of carbon dioxide gas (not very soluble) by the displacement of water.

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.

The collection of chlorine by downward delivery

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.