Oxygen gas reacts with hydrogen gas to form liquid water.

Balancing The Chemical Equation for The Formation of liquid water  

The purpose of balancing a chemical equation is to find the relative quantities of the molecules and atom types in the equation.  When the equation is balanced there will be the same number of each atom type on opposite sides of the equation. Consider the unbalanced equation for hydrogen gas reacting with oxygen gas.

                                     H₂(g) + O₂(g)  H₂O(l)

                                     H=0                   H=0

                                     O=0                   O=0

The reactant side containing hydrogen gas and oxygen gas and the product side containing liquid water do not have equal numbers of each atom type.  There appears to be two hydrogen atoms on the left of the arrow and two on the right.  This makes the hydrogen balanced.  However, there are two oxygen atoms on the left and only one oxygen atom on the right.  All of the atom types must be equal before the equation is balanced. To make the number of each type of atom on opposite sides of the arrow equal, the coefficients must be adjusted until the equation is balanced.  Compare the following results. 

                                    2H₂(g) + 1O₂(g)  2H₂O(l)

                                     H=4                   H=4

                                     O=2                   O=2

The coefficient 2 in front of the H₂(g) results in 4 atoms of hydrogen on the left side of the arrow.  The coefficient 2 in front of the H₂O(l) results in 4 atoms of hydrogen and 2 atoms of oxygen on the right side of the arrow.  The coefficient 1 in front of the O₂(g) results in 2 atoms of oxygen on the left side of the arrow. The equation is balanced.  The ratio of coefficients 2:1:2 balances the reaction. The ratio of coefficients 4:2:4, 6:3:6 and 8:4:8, also, balance the reaction; however, the lowest ratio of coefficients 2:1:2 is the accepted answer.

© Pat Thayer 2014-2016