The chemical equation for the gas evolution reaction that occurs when magnesium metal reacts with hydrochloric acid is:Mg s + 2 HCl aq MgCl2 aq + H2 g In this reaction, magnesium Mg reacts with hydrochloric acid HCl to produce magnesium chloride MgCl2 and hydrogen gas H2 . This reaction demonstrates the relationship between acid strength and reactivity with metals because the stronger the acid, the more reactive it is with metals. Hydrochloric acid is a strong acid, and it readily reacts with magnesium to produce a significant amount of hydrogen gas. If a weaker acid were used, the reaction would be slower and produce less hydrogen gas.