In the Brønsted description, an acid donates a proton and a base accepts it.
Key knowledge
A Brønsted acid is a proton donor and a Brønsted base is a proton acceptor. After donating a proton, an acid becomes its conjugate base; after accepting one, a base becomes its conjugate acid.
Conditions and scope
This is the Brønsted proton-transfer description, not the broader Lewis acid/base description.
Acid or base behaviour depends on the reaction partner and medium; water can take either role.
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References
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Published proton-transfer and conjugate-pair definitions, including water/ammonia examples.
2026-10-10 · English donor/acceptor roles and conjugate pairing checked against section 14.1; no pH or strength value is claimed.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Agreement within this scope
Proton donation/acceptance and conjugate pairs.
Acid and base roles depend on the reaction partner.
A solution containing suitable amounts of a weak acid and its conjugate base, or the corresponding weak-base pair, can resist pH changes from added acid or base. This response follows coupled acid–base equilibria and has finite capacity.
Buffer behaviour involves a weak acid/conjugate-base or corresponding base pair.
For HA ⇌ H⁺ + A⁻ in a stated solvent convention, Ka relates the equilibrium activities. For a conjugate acid–base pair in water, KaKb=Kw under consistent definitions. Acid strength describes the equilibrium tendency to transfer a proton, while the fraction ionized also depends on composition.
Ka quantifies a specified Brønsted proton-transfer equilibrium.
Hydrogen in an ionic hydride is formally H− and can act as a strong base or reducing species; proton transfer with water produces hydrogen gas and hydroxide. Molecular hydrogen compounds instead have bonding and acid/base properties set by their partner elements and structure. A formula containing hydrogen does not imply hydride-like reactivity.