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Arrhenius: what changes in water?
Identify acids and bases by the ions they produce in aqueous solution.
A-Level 9476 (2026-2027)
The Arrhenius definition is about aqueous solution
Acids increase hydrogen-ion concentration; bases increase hydroxide-ion concentration.
An Arrhenius acid increases the concentration of hydrogen ions in water. An Arrhenius base increases the concentration of hydroxide ions. The H+(aq) notation is shorthand for a proton solvated by water; H3O+ is a useful explicit representation.
| Substance | What happens in water | Classification |
|---|---|---|
| HCl | HCl + H2O → H3O+ + Cl-: the water accepts a proton. | Acid |
| CH3COOH | CH3COOH + H2O ⇌ H3O+ + CH3COO-: only a fraction ionises. | Weak acid |
| NaOH | NaOH(s) → Na+(aq) + OH-(aq): dissolution releases hydroxide ions. | Base and soluble alkali |
| NH3 | NH3 + H2O ⇌ NH4+ + OH-: reaction with water generates hydroxide. | Aqueous basic behaviour, despite no OH group in its formula |
Neutralisation between a strong aqueous acid and a strong aqueous alkali has the net ionic equation H+(aq) + OH-(aq) → H2O(l). Counter-ions such as Na+ and Cl- remain in solution and do not belong in the net ionic equation.
This definition is convenient for aqueous behaviour but is not a general explanation of every acid-base reaction. A reaction between gaseous ammonia and hydrogen chloride can produce ammonium chloride without water being present. Following proton transfer explains it more broadly.