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

* 2 HI + 2e- → 2 H0 (reduction)*

* Ca0 - 2 e- → CaII (oxidation)*

** H2O** is an

**oxidizing**agent,

*Ca*is a

**reducing**agent.

### Word equation

* calcium + water → calcium hydroxide + hydrogen*

### Input interpretation

* Ca + H2O → Ca(OH)2 + H2*

*Calcium water calcium hydroxide hydrogen*

### Balanced equation

Balance the chemical equation algebraically:

* Ca + H2O → Ca(OH)2 + H2*

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

* Ca + H2O → Ca(OH)2 + H2*

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

*O:**Ca: c1 = c3*

*H: 2c2 = 2c3 + 2c4*

*O: c2 = 2c3*

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 + H2O → Ca(OH)2 + H2*