11.4.1—And interpret

In a disproportionation reaction, the same element is both oxidised and reduced. Chlorine reacts with cold dilute sodium hydroxide to form chloride and chlorate(I), while hot concentrated alkali forms chloride and chlorate(V).

Use oxidation numbers to show the two pathways. In cold solution chlorine goes from 0 to −1 and +1; in hot solution it goes from 0 to −1 and +5. Balance atoms and charge before naming the reaction.

Cold: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. Hot: 3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O. The conditions determine the product.

Do not use one equation for every sodium-hydroxide condition, and do not call the reaction simple oxidation or reduction.