Making hydrogen

Making hydrogen calls for exactly the metal-and-dilute-acid reaction we met in section 16.3.

The reaction

In the laboratory, hydrogen (H₂) gas can be produced by reacting a metal such as zinc (Zn) or magnesium (Mg) with an acid such as dilute hydrochloric (HCl) or dilute sulphuric (H₂SO₄) acid.

Zn + 2HCl → ZnCl₂ + H₂

Figure to be added

Figure 16.2 — producing hydrogen gas in the laboratory; the thistle funnel, the conical flask, the rubber bung, the dilute hydrochloric acid, the pieces of zinc, the delivery tube, the rubber tubing, the gas jar and the trough.

Downward displacement of water

As the figure shows, a gas jar is filled with water and held inverted, and the gas is led into it. This method of collecting a gas is called the downward displacement of water.

Why that name?

Because as the gas enters the jar, the water in it is pushed down and driven out.

Why this works

Because hydrogen is only slightly soluble in water. A gas that dissolved readily in water could not be collected this way.