This document discusses pH and how it is used to measure the acidity or basicity of solutions. It explains that pH is determined by measuring the concentration of hydrogen ions [H+] or hydronium ions [H3O+] in a solution. The pH scale ranges from 0 to 14, with acids having a pH below 7, bases above 7, and neutrals at 7. It also provides examples of calculating the pH of acid and base solutions given their concentrations.