Reaction of chlorine with hot/cold solution of hydroxide
The chlorate ($\ce{ClO3-}$) is formed by disproportionation of hypochlorite and hypochlorous acid:

$$\ce{ClO- + 2 HClO -&gt; ClO3- + 2 HCl}$$

Higher temperature promotes the formation of hypochlorous acid through hydrolysis of hypochlorite and therefore speeds up the reaction.