When an metal reacts with an acid, hydrogen gas (H2) is usually produced. This is because most metals are more reactive than hydrogen, causing them to displace hydrogen ions (H+) from the acid. The reaction between a metal and an acid can be represented by the following general equation:

Metal + Acid → Salt + Hydrogen gas

For example, when zinc (Zn) metal is added to hydrochloric acid (HCl), the following reaction occurs:

Zn + 2HCl → ZnCl2 + H2

In this reaction, zinc atoms displace hydrogen ions from hydrochloric acid, resulting in the formation of zinc chloride (ZnCl2) and hydrogen gas (H2). The hydrogen gas produced can usually be observed as bubbles rising from the reaction mixture.