What it is
In the Brønsted–Lowry definition, an acid is a substance that can donate a hydrogen ion (H⁺, a proton) and a base is a substance that can accept one. An acid–base reaction is a hydrogen ion changing places.
- Hydrochloric acid, HCl. HCl + H₂O → H₃O⁺ + Cl⁻. The acid hands the hydrogen ion to water.
- Ammonia, NH₃. NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The base takes a hydrogen ion from water. Ammonia contains no hydroxide, yet the solution ends up with hydroxide ions.
- Acetic acid, CH₃COOH. Donates a hydrogen ion, but only to a small extent (see below).
- Water. H₂O + H₂O ⇌ H₃O⁺ + OH⁻. Water can both donate and accept a hydrogen ion, so it acts as an acid or a base depending on its partner.
Acids and bases come in pairs. When HA donates a hydrogen ion, A⁻ is left, which can take it up again: A⁻ is HA’s conjugate base. Acetic acid and acetate, CH₃COO⁻, are such a pair.
Strong or weak is about how much of the acid gives up its hydrogen ion, not about how much acid there is. HCl gives up practically all of it in water. Acetic acid has pKa 4.756 (see the acid constants table), and in a 0.10 mol/dm³ solution only about 1.3 % of the molecules have donated their hydrogen ion.
How acidic a solution is is measured by its pH. For 0.010 mol/dm³ HCl the hydrogen ion concentration is 0.010 mol/dm³ and pH = 2; for 0.0010 mol/dm³ it is pH = 3. Ten times fewer ions is one pH unit higher. For 0.10 mol/dm³ acetic acid the weak-acid formula gives pH = ½ · (4.756 + 1) ≈ 2.88, although a 0.10 mol/dm³ HCl solution has pH 1.
Neither an acid nor a base: table salt, NaCl. Na⁺ donates no hydrogen ion and Cl⁻ practically accepts none, so an NaCl solution is neutral, with a pH of about 7 at 25 °C. Sugar (sucrose) dissolves without forming ions and is neither an acid nor a base either.
Why it matters
- Neutralisation. HCl + NaOH → NaCl + H₂O. A hydrogen ion from the acid meets a hydroxide ion from the base and water forms. Hydrochloric acid and sodium hydroxide of the same concentration, 0.10 mol/dm³, neutralise each other volume for volume: 25 mL of each, 0.0025 mol each. Titration is built on it.
- The pH scale and the ion product of water say how hydrogen and hydroxide ions relate in water.
- Strength. The acid dissociation constant Ka and the base dissociation constant Kb measure how readily an acid donates and a base accepts a hydrogen ion. Indicators change colour at different pH values.
- Buffers are an acid and its conjugate base together, see buffer solutions.
Common misconceptions
- “A strong acid is a concentrated acid.” Strong describes what share donates a hydrogen ion; concentrated describes how much is there. 0.0010 mol/dm³ HCl is strong but dilute (pH 3).
- “Bases contain hydroxide.” Sodium hydroxide does, but ammonia (NH₃) and acetate do not. What they share is that they can accept a hydrogen ion.
- “Neutral means pH 7.” That holds for pure water at 25 °C. At other temperatures the ion product of water changes, see the table.
- “Every acid is dangerous.” Vinegar, citric acid and carbonic acid are acids. The hazard depends on strength and concentration.
Common questions
What is the difference between an acid and a base?
An acid can donate a hydrogen ion, a base can accept one. In water, at 25 °C, an acid gives a pH below 7
and a base a pH above 7.
Is water an acid or a base?
Both, depending on what it meets. Against a strong acid like HCl, water takes up a hydrogen ion and acts as
a base; against ammonia it donates one and acts as an acid.
What is a conjugate base?
What is left when an acid has donated its hydrogen ion. The conjugate base of HCl is Cl⁻, of CH₃COOH it is
CH₃COO⁻. The weaker the acid, the stronger its conjugate base.