Look at the chemical reaction below
Magnesium + hydrochloride acid —> magnesium chloride + hydrogen
Mg + 2HCI —> MgCI2 + H2
In this reaction
Magnesium (Mg) is a reactant or product
Hydrogen (H2) is a reactant or product
Magnesium chloride (MhCI2) is a reactant or product
Hydrochloride acid (HCI) is a reactant or product