The symbol pH stands for the "power of hydrogen." The pH of a solution measures how many hydrogen ions are present in a solution. The numbers on the pH scale tell whether a solution is acidic or basic.
Values for pH usually run from 0 to 14, with lower numbers representing extremely acidic substances and higher numbers delineating substances that are strong bases. Pure water has a neutral pH, which is 7. Concentrated hydrochloric acid is extremely acidic and registers at a pH of 0. A concentrated strong base, such as sodium hydroxide, or lye, has a pH of 14. The pH of substances is measured by using a pH meter or with red and blue litmus paper.