Note the equation below is not balanced. This is because the total number of atoms of each element in the reactants is not equal to the total number of atoms of each element in the products.
Ca + 2HCl CaCl 2 + underline ?
In order to balance the equation, how many H atoms must be present in the second product that is formed?
There must be 2 H atoms present in the second product in order to balance the equation.
Therefore, the balanced equation would be:
Ca + 2HCl -> CaCl2 + H2