The reactions of chlorine

With cold, dilute sodium hydroxide, chlorine reacts to form sodium chloride and sodium chlorate(I) (NaClO, also written NaOCl): $\text{Cl}_2 + 2\text{NaOH} \rightarrow \text{NaCl} + \text{NaClO} + \text{H}_2\text{O}$

This is disproportionation. Chlorine starts at oxidation number 0 and ends as:

• −1 in NaCl (reduced), and
• +1 in NaClO (oxidised).

The same element is simultaneously oxidised and reduced. The product, sodium chlorate(I), is the active ingredient in household bleach.

With hot, concentrated sodium hydroxide, the disproportionation goes further, forming sodium chloride and sodium chlorate(V) (NaClO₃): $3\text{Cl}_2 + 6\text{NaOH} \rightarrow 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}$

Here chlorine goes from 0 to:

• −1 in NaCl (reduced), and
• +5 in NaClO₃ (oxidised).

Again this is disproportionation — just to a higher oxidation state because of the hotter, more concentrated conditions.

When chlorine is added to water for purification, it disproportionates: $\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HCl} + \text{HClO}$ Chlorine goes from 0 to −1 (in HCl) and +1 (in HClO, chloric(I) acid).

The active species are HClO (HOCl) and the chlorate(I) ion ClO⁻, which are powerful disinfectants — they kill bacteria and make the water safe to drink.

Benefits vs risks (a common discussion point):

• Benefits: kills bacteria/pathogens, preventing diseases such as cholera and typhoid; a residual amount keeps water safe through the pipes.
• Risks: chlorine is toxic; it can react with organic matter to form small amounts of potentially harmful chlorinated compounds. On balance, the benefit of killing pathogens far outweighs the risks.