What makes a solution acidic or alkaline?

Acidic solutions have a higher concentration of hydrogen ions (H^(+)) and a low concentration of hydroxide ions (OH^(-)).

Alkaline solutions have a higher concentration of hydroxide ions (OH^(-)) and a low concentration of hydrogen ions(H^(+)).

Water at pH = 7 has equal amounts of hydrogen ions and alkaline ions.

It naturally dissociates as H2O(l) < ====-> H^(+) + OH^(-).