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+ Inorganic chemistry
+ Nitrate
+ Water
+ Chemistry
Posted by
Nick Dayton

Does ammonium nitrate dissolve in water?

Bob Given  Follow

Yep.

The solubility[1] of ammonium nitrate (formula NH₄NO₃) varies roughly between 100 grams to 400 grams per 100 ml of water in the 0 to 80ºC temperature range. In other words, a whole lot of NH₄NO₃ dissolves even in nearly-freezing water.

This process has an important consequence. Dissolution of ammonium nitrate in water can be schematised as follows:

NH₄NO₃(s) → NH₄⁺(aq) + NO₃¯(aq)

Such a process is employed in instant cold packs when we press hard on them - think of, say, a twisted ankle that requires immediate relief with something cold. Inside instant cold packs there are ammonium nitrate and a small pouch of water: when we press the pack, such a pouch gets broken and its water comes into contact with NH₄NO₃, thus dissolving it. The dissolution is strongly endothermic (i.e. it occurs with absorption of heat from the surroundings):[2]therefore, heat is absorbed as ammonium nitrate dissolves in water.

It turns out, you can place the instant cold pack on an injured limb (a twisted ankle, a swollen wrist, and so forth). This causes vasoconstriction[3] - a local decrease of the thickness of blood vessels, thus reducing the amount of blood and soothing the pain in the affected area.

Cool chemistry (pun intended).

Footnotes

[1] Ammonium nitrate - Wikipedia

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