No, when magnesium (Mg) and hydrochloric acid (HCl) are mixed, they don't form a new compound. Instead, they undergo a chemical reaction called a single displacement reaction. Here's what happens:

* Magnesium (Mg) is a metal.

* Hydrochloric acid (HCl) is an acid containing hydrogen (H) and chlorine (Cl).

The reaction:

Magnesium (Mg) is more reactive than hydrogen (H). When they come into contact, the magnesium displaces the hydrogen from the hydrochloric acid. This results in the formation of:

* Magnesium chloride (MgCl2), a soluble salt.

* Hydrogen gas (H2), which is released as bubbles.

The chemical equation:

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

In summary:

The reaction between magnesium and hydrochloric acid produces a solution of magnesium chloride and hydrogen gas, not a new compound.