Paul Kissel, age 14, of Santa Rosa, Calif. for his question:

WHAT DOES THE PH CHEMICAL SYMBOL STAND FOR?

Chemists use a number called the pH to indicate the strength of an acid or a base. The number is usually on a scale ranging from zero to 14. A pH below seven indicates that a solution is acidic and a pH above seven indicates that a solution is basic.

You write the symbol with a lower case "p" and an upper case "H" like this: pH.

Strong acids have lower pH's than weak acids and strong bases have higher pH's than weak bases. A neutral solution, such as pure water, is neither acidic nor basic. Its pH is seven.

A Danish biochemist named Soren Sorensen invented the pH system in 1909. The number indicates the concentration of hydrogen ions in a solution. A solution's pH is defined as the negative logarithm, to the base 10, of its hydrogen ion concentration. This concentration is expressed in moles of hydrogen ions per liter of solution.

An electronic pH meter or special dyes called acid base indicators are used to measure pH. The color of an indicator depends on the concentrator of hydrogen ions. pH paper contains several indicators that change color at different pH's. When dipped into a solution, the paper's color indicates the approximate pH of the solution.