Glossary
pH
pH measures how acidic or basic a solution is on a scale that usually runs 0–14: below 7 is acidic, 7 neutral, above 7 basic, in steps of ten.
pH is a number on a scale that usually runs from 0 to 14, saying how acidic or basic a water-based solution is. Below 7 is acidic, exactly 7 is neutral (pure water), and above 7 is basic (also called alkaline). The scale counts hydrogen ions (H⁺): an acidic solution has more of them than water, and a basic solution fewer. Each step of one on the scale is a factor of ten, so a pH of 3 has ten times as many hydrogen ions as a pH of 4, and two steps (pH 3 to pH 5) is 10 × 10 = 100 times. The direction trips people up: because the scale measures a "backwards" quantity, up means down — a higher pH means fewer hydrogen ions, so it is the less acidic solution, and a lower pH is the more acidic one.
Taught in
Related terms
- acid — An acid releases hydrogen ions (H⁺) in water, giving a solution with a pH below 7.
- percent concentration — Concentration is how much solute is in a solution: percent concentration is the solute over the whole solution, times 100.
Source: OpenStax Chemistry 1e and Chemistry: Atoms First 1e, first editions (CC BY 4.0). Each definition restates the Chemistry guide in this exam, which carries the per-source citations on its own page. (OpenStax 1e accessed 2026-10-08)