When an acid is reacted with metal [[1]] gas is liberated.

No matter how strong the acid is or what kind of metal is used, if there is a displacement reaction with these reactants, hydrogen gas is always produced. The reactivity of the metals affects how quickly the reaction proceeds. At the top of the reactivity range, highly reactive metals like potassium and sodium react with acids right away to produce hydrogen. On the other hand, reactions involving metals similar to zinc happen more gradually.

When metals are reacted with metal, the metal salt is formed and hydrogen gas is evolved. The example given below involves the reaction of HCl with Na;

$2\mathrm{HCl}+2\mathrm{Na}\to 2\mathrm{NaCl}+{\mathrm{H}}_2$