Arrhenius was first to define acids and bases in aqueous solution. An acid is a substance that dissociate in aqueous solution to give hydrogen ions. All mineral acids like HCl, HNO₃ and H₂SO₄ containing hydrogen when dissolved in water dissociates completely into H+ ions as:
HCl → H++ Cl⁻
HNO₃ → H+ + NO3–
H₂SO₄ → 2 H+ + SO₄²-
Arrhenius name these acids which dissociates completely in water as strong acids.
Other acids like acetic acid CH₃COOH, carbonic acid H2CO3 and phosphoric acid H3PO4 when dissolved in water does not dissociate completely into ions. As a result an equilibrium is setup between ions and and undissociated acid molecules as:
CH₃COOH ⇔ H+ + CH₃COO-
H2CO3 ⇔ 2 H+ + CO2−
H3PO4 ⇔ 3 H+ + PO4 3−
Arrhenius named these as weak acids.
According to Arrhenius a base is a substance which contains hydroxyl group and which when dissolved in water gives hydroxyl ion OH- as
NaOH →Na+ + OH⁻
KOH → K+ + OH⁻
Arrhenius named these bases which dissociate completely as strong bases.
The substances which do not completely dissociate into hydroxide ions named as weak bases. For example- NH4OH, Mg(OH)2 , Al(OH)3 do not dissociate completely to give OH⁻ ions.
NH4OH ⇔ NH₄⁺ + OH⁻
Mg(OH)2 ⇔ Mg2+ + OH⁻
Al(OH)3 ⇔ Al3+ + OH⁻
Arrhenius named these as weak bases.
H+ obtained from acid and OH⁻ from bases combine together to form unionized water. Arrhenius described this combination of H+ and OH⁻ ions as neutralization reaction.