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Why does chlorine water lose its oxidizing power over time?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

Chlorine water loses its oxidizing power over time because the hypochlorous acid (HClO) formed in the solution is unstable and gradually decomposes.12

When chlorine dissolves in water, it undergoes a disproportionation reaction, producing both hydrochloric acid and hypochlorous acid according to the following equation:

${Cl}_{2} + H_{2} O ⇌ HCl + HClO$

The hypochlorous acid is responsible for the solution's strong oxidizing properties, which are utilized for bleaching and disinfection. As the hypochlorous acid breaks down over time, the solution's effectiveness as an oxidizing agent diminishes.21