In water with good pH levels, the chlorine added generates hypochlorous (HOCl) and a Hypochlorite ion (OCl). The Hypochlorous acid is the part that does the disinfecting. Hypochlorous acid is fairly unstable. High heat drives the chlorine from hypochlorous acide in the form its elemental gas (very small quantities). The same thing happens when the hypochlorous acid is exposed to UV rays from the sun.

That is why pool owners are careful not to let the water get too hot (one problem with hot tubs) and they make sure to add cyanuric acid, which binds with the chlorine to form chlorimide, which is much more stable (not as easily driven off by heat & sun) and at the right PPM levels is still a very effective disinfectant.

The suseptability of chlorine to high temperatures is also why it is recommended that when washing dishes you use lukewarm water for the chlorine rinse rather than hot water. You don't want to use cold water either, since that reduces the chlorine's disinfectant properties.