When $\mathrm{Cl}_2$ gas reacts with hot and concentrated sodium hydroxide solution, the oxidation number of chlorine changes from

When chlorine gas reacts with hot and concentrated $\mathrm{NaOH}$ solution, it disproportionates into chloride $\left(\mathrm{Cl}^{-}\right)$and chlorate $\left(\mathrm{ClO}_3^{-}\right)$ions.
In this process, oxidation number of chlorine changes from 0 to -1 and 0 to +5 . Note In disproportionation reactions, the same element undergoes oxidation as well as reduction.