For any solution at #25 ^oC#, the product of the #["OH"^-]# and #["H"_3"O"^+]# always equals a defined constant:
#K_"w" = ["OH"^-]["H"_3"O"^+] = 1.00 xx 10^-14 M^2#
The constant #K_"w"# is the equilibrium constant for water, called the ion-product constant, and is equal to #1.00 xx 10^-14 M^2#
From this relationship, we can see that #["OH"^-]# and #["H"_3"O"^+]# are inversely proportional; as the concentration of hydroxide ions increases, the concentration of hydronium/hydrogen ions decreases.
An acidic solution is one in which the hydronium ion concentration is greater than that of the hydroxide ion concentration. Why? Because the acidity of a solution can be measured by the solution's #"pH"#; a lower #"pH"# indicates a higher #["H"_3"O"^+]#. If the #"pH" < 7#, the solution is said to be acidic, and #["OH"^-] < ["H"_3"O"^+]#. For a neutral solution, #"pH" = 7#, and the hydronium and hydroxide concentrations are equal. If the #"pH"# is higher than #7#, the solution is basic, and because of the equation
#K_"w" = ["OH"^-]["H"_3"O"^+] = 1.00 xx 10^-14 M^2#
the #["OH"^-]# is larger than the #["H"_3"O"^+]#. Thus, if #["OH"^-] > ["H"_3"O"^+]#, the solution is basic (#"pH" > 7#).
