Does letting water sit remove chlorine?

Yes, chlorine is fairly unstable and will gradually evaporate from water. However, many water treatment plants have begun using chloramine instead of chlorine because it is a more stable disinfectant formed by combining ammonia and chlorine. Chloramine cannot be easily removed from water via evaporation and must be neutralized using a dechlorinator. If you are sure your tap water contains chlorine and not chloramine, you can let the water sit for 1-5 days to allow all the chlorine to evaporate. To speed up the evaporation process, aerate the water with an air stone for 12-24 hours or boil the water for 15-20 minutes. Use multi-test strips to measure the water and make sure no chlorine is left.

For more information, read the full article on dechlorinators.