Acidic, Basic and Neutral Solutions

An acidic solution is defined as one in which the hydrogen ion (or hydronium ion) concentration is greater than the hydroxide ion concentration. A basic solution is one in which the reverse is true, that is, one in which [OH^-] exceeds [H₃O⁺] and a neutral solution is one in which [OH^-] equals [H₃O⁺].

Since the product [H3O⁺] [OH^-] is constant, if the concentration of either of these increases, the other one would decrease.