$\mathrm{Mg}(\mathrm{s})+2\,\mathrm{HCl}(\mathrm{aq})\rightarrow\mathrm{MgCl}_2(\mathrm{aq})+\mathrm{H}_2(\mathrm{g})$Magnesium, being more reactive than hydrogen, displaces it in a chloride molecule, resulting in a single displacement reaction.$\mathrm{NaOH}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq})\rightarrow\mathrm{NaCl}(\mathrm{aq})+\mathrm{H}_2\mathrm{O}(\mathrm{l})$ In a doubledisplacement reaction, the reactants exchange ions and form new compounds. The reaction between hydrochloric acid $(\mathrm{HCl})$ and sodium hydroxide $(\mathrm{NaOH})$ to form a salt $(\mathrm{NaCl})$ and water $(\mathrm{HOH})$ is an example of a double displacement reaction, specifically a neutralisation reaction.