Strong and weak electrolytes

In 1887 Arrhenius proposed the theory of electrolytic dissociation: when electrolytes dissolve in water or melt they split into ions. Under an electric current positive ions move toward the cathode — they are cations — and negative ions toward the anode — they are anions; for example NaCl → Na⁺ + Cl⁻, Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄²⁻. Dissociation is reversible: oppositely charged ions can collide and recombine into molecules (association). Substances whose solution or melt conducts a current are electrolytes (acids, alkalis, salts); those that do not are non-electrolytes (sugar, alcohols, distilled water, nonpolar compounds). Those that dissociate almost completely in aqueous solution are strong electrolytes: strong acids (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄), alkalis (hydroxides of groups IA and IIA except Be and Mg) and well-soluble salts. Those that dissociate only partly are weak electrolytes: H₂CO₃, H₂S, H₂SO₃, HF, HCN, HNO₂, CH₃COOH, NH₄OH and insoluble bases. For oxygen acids HₙEOₘ there is an approximate rule: if m – n ≥ 2 the acid is strong (HNO₃: 3 – 1 = 2), if m – n < 2 it is weak (HNO₂: 2 – 1 = 1). The conductivity experiment is demonstrated only by the teacher, observing safety precautions.