In balancing equations, addition of
In using the oxidation number method for balancing equations, we go through a series of steps in which we balance, in order, (for an acidic reaction)
The change in oxidation number for the oxidizing agent and the reducing agent.
The number of oxygen atoms in the equation or half-reaction
The charge on each side of the equation.
It is valid to add water molecules at step 2, for two reasons. First, these reactions often are occurring in aqueous solution and water is an available reactant or an acceptable product. Second, since the oxidation number of oxygen and hydrogen in
(In balancing a basic reaction, the steps are a bit different, but the idea is still the same.)