What happens when `Cl_(2)` reacts with cold dilute solution of sodium hydroxide?
(b) Why does chlorine water lose its yellow colour on standing?

Explanation

(a) With cold dilute solution of sodium hydroxide , `Cl_(2)` undergoes disproportionation to give a mixture of sodium chloride, `NaCl` and sodium hypochlorite `(NaOCl)`.
`Cl_(2)+2NaOH rarr NaOCl+H_(2)O`
(b) Cholorine water is yellow due to the presence of hypochlorous acid, `HOCl` on standing, `HOCl` being unstable decomposes to form `HCl`. Hence the yellow colour disappears.
`Cl_(2)+H_(2)IO rarr HCl+underset("yellow")(HOCl) rarr underset("colourless")(2HCl)+[O]`.