Dilution drives the pH value of an acidic or basic solution toward 7. The pH values of acidic solutions increase with dilution, while the pH values of basic solutions decrease with dilution.
Diluting an acid reduces the concentration of the H+ ions that cause acidity, while diluting a base reduces the concentration of the OH- ions that cause basicity. Since a pH value is an expression of the concentration of these ions, changing these concentrations by introducing more solvent causes the pH value to change.
The pH scale covers a range between 0 and 14 pH. A pH of exactly 7 is neutral. pH values greater than 7 are basic, while pH values less than 7 are acidic. The pH scale is logarithmic with a base of 10, meaning each integer pH value is 10 times more acidic or basic than the one preceding or succeeding it respectively. For dilution to decrease the acidity or basicity of a solution, the solvent being used must be less acidic or basic than this solution.
Alternatively, an acidic diluent may be used to dilute a basic solution, or vice versa, in a neutralization reaction. Because of the logarithmic nature of the pH scale, diluting a strong acid or base even slightly causes its respective pH to rise or drop considerably. Weak acids and bases with pH values close to 7 are correspondingly not as affected by dilution.