This is an **oxidation-reduction** (redox) reaction:

* 2 HI + 2 e- → 2 H0 (reduction)*

* Ca0 - 2 e- → CaII (oxidation)*

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

### Word equation

* calcium + perchloric acid → calcium perchlorate + hydrogen*

### Input interpretation

* Ca + HClO4 → Ca(ClO4)2 + H2*

* Calcium perchloric acid calcium perchlorate hydrogen*

### Balanced equation

Balance the chemical equation algebraically:

* Ca + HClO4 → Ca(ClO4)2 + H2*

Add stoichiometric coefficients, c, to the reactants and products:

* c1Ca + c2HClO4 → c3 Ca(ClO4)2 + c4 H2*

Set the number of atoms in the reactants equal to the number of atoms in the products for **Ca, Cl, H **and **O:**

*Ca: c1 = c3*

*Cl: c2 = 2c3 *

*H: c2 = 2c4*

*O: 4c2 = 8c3*

Since the coefficients are relative quantities and underdetermined, choose a coefficient to set arbitrarily. To keep the coefficients small, the arbitrary value is ordinarily one. For instance, set **c1 = 1** and solve the system of equations for the remaining coefficients:

* c1 = 1 c2=2 *

* c3 = 1 c4=1*

* *

Substitute the coefficients into the chemical reaction to obtain the balanced equation:

Answer:

* Ca + HClO4 → Ca(ClO4)2 + H2*

### Word equation

* calcium + perchloric acid → calcium perchlorate + hydrogen*