The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. A pH of 7 is neutral. Values less than 7 are acidic, while those greater than 7 are basic. Each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5, and 100 times more acidic than a pH of 6.

Pure water is neutral, with a pH of 7.0. When chemicals are mixed with water, the mixture can become either acidic or basic. Alkaline is another word for basic.

Vinegar and lemon juice are acidic, while laundry detergents and ammonia are basic. Mixing acids and bases can cancel out their extreme effects, similar to the way mixing hot and cold water can even out the temperature.

Normal rain is slightly acidic, with a pH of approximately 5.6. Precipitation with pH less than 5.6 is considered acidic. In the year 2000, the most acidic rain falling in the United States had a pH of about 4.3.

pH values for some common substances are shown below: