What is pH and what does it measure?

The pH is a measure of how acidic or basic (alkaline) an aqueous solution is.

Pure water is neutral, with a pH = 7. Things with a pH <7 are acidic, and things with a pH >7 are basic.

The theoretical pH of an aqueous solution is defined this way:

pH = -log10 [H3O]

Related Information:

The pH approximates the negative logarithm, base 10, of the Hydronium ion (H3O+) molar concentration of an aqueous solution. It's an indicator of the relative acidity or basicity (alkalinity) of the solution.

What makes a solution acidic is the amount of H3O+ in it. A highly acidic solution will have a high concentration of H3O+, and so it will have a low pH. A basic solution has a very low concentration of H3O+, and so it has a high pH. Note that H3O+ and H+ really refer to the same thing, but H+ doesn't actually exist by itself in water: it immediately combines with an H2O molecule to form H3O+ instead.

Substances with very high, or very low, pH are dangerous to human hair, eyes, and skin.

Examples: pH of common substances are:

Stomach acid -- 1

Lemon juice -- 2.2

Pure water -- 7

Milk of magnesia -- 10.5