The centimeter is a unit measure of length. The gram is a unit measure of weight. So, pH is the unit measure we use to say how much acid is in a substance. The pH scale goes from 0 to 14. A pH of 0 means a very high acid activity. Substances such as lemon juice and vinegar are acidic with pH values of 2 to 3. Nitric acid and hydrochloric acid are very strong with pH values of 0, while stomach acid has a pH of 1. Addition of a strong acid, such as sulfuric acid ( H2SO4 ) to water makes the resulting solution very high in active acid concentration. This is called an acidic solution.

On the other end of the scale are the alkaline substances, which range from 8 to 14. Common alkalis are seawater (pH 8), household ammonia (pH 11), oven cleaners (pH 13), and the very strong alkali, sodium hydroxide (pH 14). The addition of a strong base or alkali material, such as sodium hydroxide ( NaOH ), to water makes the resulting solution very low in active acid concentration. This is called a very basic or alkali solution.

In between these two extremes is a pH of 7. This is the pH of pure water. Water, which is neither very acidic nor very alkali, is said to be neutral.

Technically, the scale actually refers to the concentration of positively-charged hydrogen (H+) ions and negatively-charged hydroxyl (OH-) ions in solution. More hydrogen ions than hydroxyl ions makes an acidic solution, while an alkaline solution contains more hydroxyl ions than hydrogen ions. The pH scale is a logarithmic one, meaning that each pH unit has 10 times as many hydrogen ions as the unit above it. So, at pH 4, there are 10 times more hydrogen ions than at pH 5 and 100 times more hydrogen ions than at pH 6.