This is an oxidation-reduction (redox) reaction

2 HI + 2 e- → 2 H0 (reduction) 

Ca0 - 2 e- → CaII (oxidation)

HClO4 is an oxidizing agent, while Ca is a reducing agent.

Word equation

 Calcium + perchloric acid → calcium perchlorate + hydrogen

Input interpretation

Ca + 2HClO4 → Ca(ClO4)2 + H2

Balanced equation

 Ca + 2HClO4 → Ca(ClO4)2 + H2

To balance the equation, stoichiometric coefficients are added to the reactants and products. The number of atoms of each element on both sides of the equation should be equal.

c1 Ca + c2 2HClO4 → c3 Ca(ClO4)2 + c4 H2

Ca: c1 = c3 Cl: c2 = 2c3 H: 2c4 = c2 O: 4c2 = 8c3

Choosing an arbitrary coefficient value of 1 for c1, the remaining coefficients are solved using the system of equations:

c1 = 1, c2 = 2, c3 = 1, c4 = 1

Substituting the coefficients back into the balanced equation yields:

Ca + 2HClO4 → Ca(ClO4)2 + H2

This reaction shows the reaction between calcium and perchloric acid, producing calcium perchlorate and hydrogen gas.