Hydrogen gas is released when an acid reacts with a metal such as zinc. This is because the metal atoms in the zinc react with the hydrogen ions in the acid to form hydrogen gas and a metal salt. The reaction can be represented by the following equation:

Zn + 2HCl → ZnCl2 + H2

In this equation, Zn represents the zinc metal, HCl represents the hydrochloric acid, ZnCl2 represents the zinc chloride salt, and H2 represents the hydrogen gas.