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Why does chlorine water become acidic when it stands?

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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 becomes acidic when it stands because of a process called disproportionation, in which the chlorine atoms in the dissolved molecules are simultaneously oxidized and reduced.1

Specifically, when chlorine water stands, the chlorine reacts with water to form hydrochloric acid (HCl) and hypochlorous acid (HOCl) according to the following equation:

${Cl}_{2} + H_{2} O ⇌ HCl + HOCl$

In this reaction, the formation of hydrochloric acid—a strong acid—increases the concentration of hydrogen ions in the solution, thereby lowering the pH and making the water acidic. Additionally, chlorine water loses its effectiveness as an oxidizing agent over time because the hypochlorous acid gradually decomposes.231