Is the following equation balanced? If not, how would you balance it? # Mg (s) + HCI (aq) -> MgCl_2#
For every reactant particle there must be a corresponding product particle. This is formally a redox reaction that may be solved systematically by the use of oxidation numbers. You will have to hunt these down.
In the meantime, can you give the balanced equation for the oxidation of aluminum by hydrochloric acid?