Strong and weak electrolytes

The degree of dissociation α is the ratio of the number of molecules that have broken into ions to the total number of dissolved molecules; it is given from 0 to 1 or from 0 to 100 %. α depends on the nature of the solute and solvent, on the concentration and on temperature, so it is quoted together with concentration and temperature. Substances with a large α even in dilute solution are strong electrolytes (most salts, HCl, HNO₃, H₂SO₄, HClO₄, NaOH, KOH, Ca(OH)₂, Ba(OH)₂); those with a small α are weak electrolytes (H₂CO₃, H₂SiO₃, H₂S, H₂SO₃, HNO₂, water, NH₄OH, insoluble hydroxides). Writing an oxygen acid as E(OH)ₘOₙ and calling n the number of oxygens outside OH groups, the acid is judged strong if n ≥ 2 and weak if n < 2 (by this rule H₃PO₄, n = 1, falls among the weak acids; in fact it is of medium strength). In a weak electrolyte molecules and ions are in equilibrium, e.g. CH₃COOH ⇄ H⁺ + CH₃COO⁻, with K = [H⁺][CH₃COO⁻] / [CH₃COOH]. K does not depend on concentration, only on the nature of the electrolyte and solvent and on the temperature: the larger K, the more strongly the electrolyte ionises.