Consider adding a strong base (e.g. NaOH) to a solution of a strong acid (e.g. HCl).
Before any base is added, the pH is given by the strong acid solution. Therefore, pH < 7.
When base is added, before the equivalence point, the pH is given by the amount of strong acid in excess. Therefore, pH < 7.
At equivalence point, the amount of base added is stoichiometrically equivalent to the amount of acid originally present. Therefore, the pH is determined by the salt solution. Therefore, pH = 7.
