pH
Definition
The 0 to 14 scale measuring acidity: below 7 is acidic, 7 is neutral, above 7 is basic. Each whole number is a 10x change in hydrogen ion concentration.
Symbol: ·
Technical definition
pH is the negative base-10 logarithm of hydrogen ion activity (pH = -log[H+]). The logarithmic scale means each unit change represents a tenfold change in [H+], and at 25 C, pH + pOH = 14.
Example
Lemon juice (pH about 2) has 10,000 times the [H+] of pure water (pH 7). A small pH difference hides a huge concentration gap.
Common confusion
pH can go below 0 or above 14 for very concentrated solutions. The 0-14 range is just the common dilute-solution window, not a hard limit.