Balancing of chemical equation means that the number of atoms of each element is equal on both the sides of the equation.
For example: 2H2 + O2 → 2H2O
The equation is already balanced, since there are two hydrogen atoms on both sides and two oxygen atoms on both sides.
Magnesium – Mg + 2HCl → MgCl2 + H2
Aluminium – Al + 3HCl → AlCl3 + H2 1