Collecting by air displacement

There is a second way of collecting hydrogen — and it uses no water at all.

Figure to be added

Figure 16.3 — collecting hydrogen gas by downward displacement of air; the hydrogen arriving along the rubber tubing and the atmospheric air leaving from below.

The gas produced can also be collected by holding a gas jar inverted over the delivery tube, as shown in figure 16.3.

How it works

Hydrogen is less dense than ordinary air, so inside the gas jar it travels upwards.

The atmospheric air in the jar is then pushed downwards and out of it.

Downward displacement of air

This method of collecting a gas is called the "downward displacement of air" method.

The two methods compared

Downward displacement of water

Usable if the gas does not dissolve in water. The gas jar is inverted, standing in water.

Downward displacement of air

Usable if the gas is lighter than air. The gas jar is inverted, with no water at all.

So either method serves for hydrogen — it neither dissolves in water nor is heavier than air.