Naturally chlorine is a gas. But it is in liquid form when we get it in the tap water. And leaving it in a bucket over night may or may not get rid of it. People keep swimming pools and add chlorine to them every now and then cause only some of it will gas out. It matters how much chlorine content is in this persons tap water source. Since we do not know what the ppm of chlorine present in the tap water in this case, It is bad info to tell someone that leaving it out over night will take care of it.