Wikipedia:Reference desk/Archives/Science/2021 May 19

= May 19 =

re-using instant cold pack?
I bought a box of them for a sore muscle a while back. They are convenient but seemed kind of wasteful. The article ice pack says:
 * An instant cold pack is a device that consists of two bags; one containing water, inside a bag containing ammonium nitrate, calcium ammonium nitrate or urea. When the inner bag of water is broken by squeezing the package, it dissolves the solid in an endothermic reaction. This reaction absorbs heat from the surroundings, quickly lowering the pack's temperature.[3]

Does that mean the after use, the outer bag now contains some kind of granules soaked with water? Is there some way to dry them out for re-use with more water, i.e. to undo the chemical reaction? I know there is something like that for sodium acetate heat packs but obviously the chemistry is different. Thanks. 2602:24A:DE47:BA60:8FCB:EA4E:7FBD:4814 (talk) 04:11, 19 May 2021 (UTC)
 * The reactant dissolves in the water. It will be a solution or slurry of varying consistency depending on the proportion of water to reactant. Here's a video on YouTube of a guy taking one apart. You would have to add energy to separate the solute back out. I guess heating it to evaporate the water would do the trick? But you can just use reusable ice packs instead. The only difference is they're not "instant"; you have to heat or cool them before use. --47.155.96.47 (talk) 05:34, 19 May 2021 (UTC)


 * I'm not sure I'd call it a chemical reaction. Dissolving is more like a physical phase transition, similar to melting. It should be quite reversible. Now note that ammonium nitrate isn't only a great fertiliser, it's also a great explosive. I expect they added some impurities to make it less likely to explode, but still, I'd be a bit hesitant to heat it on an open flame to dry. Urea is, after water, the main ingredient of urine, which used to be an important feedstock for the chemical industry. Nowadays it's mostly produced industrially. You could try making your own cold packs from your own dried-in urine. PiusImpavidus (talk) 09:02, 19 May 2021 (UTC)


 * Urea is pretty cheap to buy as a fertilizer, and I expect it will be lower cost for you to do that instead of trying to extract it yourself. Graeme Bartlett (talk) 12:28, 22 May 2021 (UTC)