When hydrochloric acid (HCl) reacts with magnesium metal (Mg), magnesium chloride (MgCl₂) and hydrogen gas (H₂) are formed. The balanced chemical equation for this reaction is:
\[ \text{Mg (s)} + 2 \text{HCl (aq)} \rightarrow \text{MgCl}_2 \text{ (aq)} + \text{H}_2 \text{ (g)} \]
This equation indicates that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.