Here's why:

* Single Displacement: A single displacement reaction occurs when one element replaces another element in a compound.

* In this case: Magnesium (Mg) is more reactive than hydrogen (H), so it displaces hydrogen from hydrochloric acid (HCl).

* The reaction: Magnesium reacts with hydrochloric acid to form magnesium chloride (MgCl2) and hydrogen gas (H2).

Let's break it down further:

* Reactants: Magnesium (Mg) and hydrochloric acid (HCl)

* Products: Magnesium chloride (MgCl2) and hydrogen gas (H2)

Overall Reaction:

Mg + 2HCl → MgCl2 + H2