I'm not entirely sure how to answer your question, so I'll approach it from several angles.
When you add a strong acid to a metal, you produce hydrogen gas and a compound with the acid anion and metal ion.
Mg + 2HCl -----> MgCl2 + H2
Na + H2SO4 ----> Na2SO4 + H2
The reaction is a so called oxidation-reduction reaction.
Magnesium is oxidized (loses electrons) to go from being in elemental form to that of the Mg+2 ion. (in MgCl2)
Hydrogen is reduced (gains electrons) to go from being H+ (in HCl) to Hydrogen gas.
The reactivity of the metal is reduced when acid is added.
Elemental metals go from a more reactive uncombined form to a less
reactive ionic form, generally speaking. (More so for the alkali and
alkali earth metals than the transition metals)
(republished on 07/13/06)