When acids react with some metals, they produce hydrogen gas. This is because the metal atoms in the metal react with the hydrogen ions in the acid to form hydrogen gas. The reaction can be represented by the following equation:

Metal + Acid → Salt + Hydrogen gas

For example, when iron reacts with hydrochloric acid, it produces iron chloride and hydrogen gas. The reaction can be represented by the following equation:

Fe + 2HCl → FeCl2 + H2

The amount of hydrogen gas produced depends on the amount of metal and acid used, as well as the temperature and pressure of the reaction.