Short answer
In the Brønsted–Lowry model, an acid donates a proton and a base accepts a proton. Acetic acid can donate a proton to water; ammonia can accept a proton from water. Whether a species acts as acid or base can depend on its reaction partner. 1 2
An acid donates a proton to a base, which accepts it; the roles are paired in a reaction.
On this page
At a glance
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| Question | Acid | Base |
|---|---|---|
| Proton role | Donor | Acceptor |
| Aqueous tendency | Often raises hydronium | Often lowers hydronium or raises hydroxide |
| Example | Acetic acid | Ammonia |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Acid. A substance or species that donates a proton in a Brønsted–Lowry reaction. 1
Base. A substance or species that accepts a proton in a Brønsted–Lowry reaction. 2
Key differences
OpenStax presents proton transfer as the organizing rule for this comparison. Arrhenius definitions focus on hydronium and hydroxide formation in water, while the Brønsted–Lowry framework also handles reactions where no hydroxide-containing base is present. 1 2
How to tell them apart
In a reaction equation, follow the proton: the species losing H⁺ acts as acid, and the species gaining it acts as base. For an aqueous solution, pH can be a clue but does not alone define every chemical species. 1 2
Where they overlap
Acids and bases occur as conjugate pairs. Water can act as either a proton donor or acceptor in different reactions, so the categories are roles rather than permanent opposing substances. 1 2
Edge cases
Lewis acid/base theory uses electron-pair acceptance/donation and is broader; this article deliberately uses proton transfer. Strength and concentration are separate questions: a dilute strong acid and a concentrated weak acid should not be conflated. 1 2
Why the distinction exists
The distinction predicts how substances exchange protons, form conjugate partners and affect solution chemistry. 1 2
Common misconceptions
A base is not simply a substance with OH in its formula. Ammonia is a base under the proton-acceptor definition. 1 2
Examples
When HCl transfers H⁺ to water, HCl is the acid and water is the base; water becomes hydronium. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Brønsted-Lowry Acids and Bases. Proton donors and acceptors.
- OpenStax — Classifying Chemical Reactions. Acid-base; Oxidation-reduction.