Pure distilled water has a pH of 7. The pH scale ranges from 0 to 14, with pH levels less than 7 considered acids and pH levels over 7 considered bases. A pH level of 7 is neutral.
The pH scale measures the concentration of hydrogen and hydroxide ions present in a solution. Acids have more hydrogen ions, while bases have more hydroxide ions. Since distilled water is neutral, this means it contains an equivalent concentration of hydrogen and hydroxide ions. Upon exposure to the air, distilled water becomes acidic due to the absorption of carbon dioxide from the atmosphere.