The answer is :
2Hâ‚‚O (l) ------>2Hâ‚‚ (g) + Oâ‚‚ (g) Â
That is ,
Oxidation number of oxygen increases in reaction :
2Hâ‚‚O (l) ------>2Hâ‚‚ (g) + Oâ‚‚ (g) Â
In Hâ‚‚O, oxidation number of oxygen is -2 but in Oâ‚‚ molecule it is 0.
So, oxidation number of oxygen increases in this reaction that is from -2 to 0.
So in the reaction that is 2Hâ‚‚O (l) ------>2Hâ‚‚ (g) + Oâ‚‚ (g) , Â oxidation number of oxygen increases .