Lesson 1 of 3
Arrhenius: what changes in water?
Identify acids and bases by the ions they produce in aqueous solution.
A-Level 9476 (2026-2027)
I can understand and apply the Arrhenius theory.Syllabus
Syllabus 9476, 4(a). Understand and apply the Arrhenius theory.
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.
Check your understanding
Ammonia has no OH group. Why is it still an Arrhenius base in water?
Show the answer
It reacts with water to form OH- ions, raising [OH-]
NH3 + H2O <=> NH4+ + OH-. An Arrhenius base increases the hydroxide-ion concentration in water, however the OH- is produced.