Arrhenious Concept Of Acids And Bases-Overview

In 1884, Swedish chemist Svante Arrhenius introduced a concept to define acids and bases based on their behavior in aqueous solutions.

An acid is a substance that dissociates in water to produce hydrogen ions (H⁺):

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

Examples: HCl, HNO₃, CH₃COOH, HCN, HBr, HI, H₂SO₄, H₃PO₄

In these compounds, hydrogen is bonded to an electronegative atom. Upon dissociation in water, the bond breaks to release H⁺.


  HCl ⇌ H⁺ + Cl⁻
  H₃PO₄ ⇌ H⁺ + H₂PO₄⁻
  HClO₄ ⇌ H⁺ + ClO₄⁻

A base is a substance that dissociates in water to produce hydroxide ions (OH⁻):

BOH(aq) ⇌ B⁺(aq) + OH⁻(aq)

Examples: NaOH, NH₄OH, Ca(OH)₂, KOH

HCl is an acid because it produces H⁺:

HCl ⇌ H⁺ + Cl⁻

NaOH is a base because it produces OH⁻:

NaOH ⇌ Na⁺ + OH⁻

Properties of acidic solutions (e.g., sour taste) are due to H⁺(aq), while basic properties (e.g., slippery feel) are due to OH⁻(aq).

Modern Statement of Arrhenius Concept

A substance that increases the concentration of H⁺ ions in water.

A substance that increases the concentration of OH⁻ ions in water.

Only applicable in aqueous solutions; doesn't explain behavior in non-aqueous solvents.
Free H⁺ and OH⁻ do not exist independently; they form complex ions like H₃O⁺.
Cannot explain acid-base behavior in the gas phase (no solvent present).
Cannot define substances like CO₂ (acid) and NH₃ (base) which don’t contain H⁺ or OH⁻.
Not all OH-containing compounds are bases (e.g., alcohols).