Next, you have to balance the oxygen by adding water and hydrogen by adding H plus ions. Now, this only works for acidic solutions. In basic solutions, you balance the H atoms, the hydrogen atoms by adding oh minus. So by adding hydroxide ions in the fifth step you balance the charge by adding electrons to one side. Remember, charge is concerned so whatever charge is lost must be gained. So that's why you need to balance the charge by adding electrons to one side of the reaction.