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: