When chlorine is passed into water and the resulting solution is exposed to sunlight, the products formed are hydrochloric acid and oxygen gas.

Firstly, Chlorine reacts with water to form hypochlorous acid.

 Cl\(_2\)  +  H\(_2\)O → HClO 

Secondly, hypochlorous acid breaks down into Hydrochloric acid and Oxygen gas. 

  2HClO → 2HCl + O\(_2\)

Ultraviolet light from the sun provides the energy needed to break the chlorine molecule and starts the reaction.