[norma0]

Does there exist a substance which is solid at temperatures below 45 C and becomes cooler when mixed with water or salty water? I want such a substance to rub it into my sweaty skin in hot weather. Thanks!

[discodermolide]

Does there exist a substance which is solid at temperatures below 45 C and becomes cooler when mixed with water or salty water? I want such a substance to rub it into my sweaty skin in hot weather. Thanks!

Ammonium chloride forms a freezing mixture with water ice, as does conc. sulfuric acid.
Neither would I rub into my skin.

The best for you is water spray yourself with water, the heat of evaporation will cool you down (the principle behind sweating).

[Enthalpy]

Erythritol does that, among others. It's used in some sweets that are to taste fresh: they actually get cold in contact with water in the mouth.
http://en.wikipedia.org/wiki/Erythritol

But do you really want to cover you skin with a polyol or a sugar when you're sweating?

[opsomath]

Nearly all ammonium salts do this. (The poster who said sulfuric acid has it backwards, that gives off heat.) However, it seems unlikely that you will get enough cooling effect to help from a chemical system.

Maybe splash yourself with water?

[discodermolide]

Nearly all ammonium salts do this. (The poster who said sulfuric acid has it backwards, that gives off heat.) However, it seems unlikely that you will get enough cooling effect to help from a chemical system.

Maybe splash yourself with water?

Sorry but water ice and conc. Sulphuric acid is a freezing mixture. I broke a rather expensive stirrer in a 2.5 cubic meter reactor finding that out.
Sorry for the long link, here is a mention of this:
http://books.google.ch/books?id=C2k3AAAAMAAJ&pg=PA421&lpg=PA421&dq=freezing+mixture+with+sulphuric+acid&source=bl&ots=thOvUPLUTi&sig=xG6MQfYBng742ynrozBlEsIsYDk&hl=en&sa=X&ei=WsEaUM-rO8zO4QTMqoD4Cw&ved=0CEoQ6AEwBw#v=onepage&q=freezing%20mixture%20with%20sulphuric%20acid&f=false