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Originally Posted by mdntranger
Not being a scientist, I'm going to take a stab at this....
The amount of heat needed to change a given amount of liquid to a gas without a change in temperature is its HEAT OF VAPORIZATION, sometimes referred to as latent (hidden) heat. The heat of vaporization for water is 540 calories per gram (9720 cal/mole) and the heat of vaporization for ammonia is 327 calories per gram (5559 cal/mole).
As we all know, it is the evaporation process that makes you feel cool as heat is required for evaporation. Since it takes less heat to make ammonia evaporate (327 c/g), more ammonia can evaporate faster thus creating a cooler sensation than just water alone (540 c/g). Using an ammonia/water solution lowers the heat of vaporization of water (I'll leave that to the scientists to tell you the exact numbers) thereby making this solution 'cooler' than just water alone.
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Spirit of Ammonia is 35% alcohol, but even so, such a tiny amount is mixed with the water, I seriously doubt the lower evaporation temperature has much to do with the effect of Florida water as a refreshing / cooling agent. I suspect its main effect over plain ice water is just like smelling salts (which has the same active ingredient in a capsule).