pH scale

The pH scale measures how acidic or basic a water-based solution is, on a range that runs from 0 to 14. A value below 7 is acidic, and the lower the number the stronger the acid; exactly 7 is neutral, which is pure water; and a value above 7 is basic, or alkaline, with the highest numbers being the strongest bases. Lemon juice and vinegar sit around 2 to 3, black coffee around 5, water at 7, baking soda near 9, and household bleach or drain cleaner at 13. The scale applies to solutions in water, so it says nothing about a dry material.

The steps are multiplicative rather than additive: each whole step represents a tenfold change in acidity. A pH of 4 is ten times more acidic than a pH of 5 and one hundred times more acidic than a pH of 6, which is why small-looking differences on the scale mean large differences in behavior.

Acids and bases are told apart easily by testing. Litmus paper turns red in an acid and blue in a base, and other indicators change color across the range. When an acid and a base are brought together they neutralize each other, producing a salt and water — and mixing equal amounts strong enough to cancel out leaves a pH of about 7.