Chlorine is a very active element. It seeks to bind with something, and easily does with water. Chlorine gas is more dense than air, and doesn;t "eaporate" (technically), but yeah, the concentration would dissipate into other forms and be roughly harmless in fairly short order. No comfort if you happen to be in a plume of it though. Makes acid inside your lungs and quickly perforates organs and blood vessels.
Someone on twitter was laughing at the CNN woman smelling the back pack.
I think things would go faster if you just told us what you DON'T know.
:)