Working on my world, I'm reminded of ice mages that can instantly freeze their opponent using an *Ice Ray*. 

That got me thinking if this is possible in the real world. The most promising method of instant freezing is using an ice pack, which immediately absorbs heat when the chemicals are mixed.

However, I'm skeptic of the freezing part. The best I guess such mixture can do is sort of frostbite in an area around the wound. That is fine to me. 

What I'm asking is whether utilizing an instant cold pack **as a melee weapon** to achieve that effect is possible or not? 

If yes, in what form can such a weapon can be effectively used?

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Right now I'm thinking about something similar to a poisoned dagger, but using a dagger with ammonium nitrate wrapped in plastic. On a successful full stab (the blade fully penetrates the flesh), the ammonium nitrate will dissolve and cause endothermic reaction, hopefully enough to cause the intended effect of frostbite to surrounding tissues.