Glossary
acid
An acid is a substance that gives up hydrogen ions in water, which lowers pH below 7 and lets it neutralize a base.
An acid is a substance that releases hydrogen ions when it dissolves in water — a proton donor, in the classic definition. Those free hydrogen ions are what every property follows from: a sour taste, the ability to turn blue litmus red, the ability to conduct electricity, and the ability to react with a metal or a carbonate and give off gas. The stronger the acid, the more completely it gives up its hydrogen ions. Hydrochloric acid in stomach fluid is strong and gives nearly all of them up; acetic acid in vinegar is weak and gives up only a small fraction, even though it is concentrated enough to taste. In water the hydrogen ions do not stay bare — each attaches to a water molecule — but the effect on pH is the same. An acid's opposite number is a base, and the two react to produce a salt and water.
Taught in
Related terms
- pH — pH is a measure of how acidic a solution is — how many hydrogen ions it holds — on a scale that usually runs from 0 to 14, with 7 neutral.
- buffer — A buffer is a solution that resists a change in pH when acid or base is added, usually by pairing a weak acid with its partner base.
- covalent bond — A covalent bond is a shared pair of electrons between two atoms, the usual bond between nonmetals and the bond that holds molecules together.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)