Is there a crystal compound or chemical that can change to a cold temperature when heat is applied?

Question

Example: If I have a gel compound inside of an enclosed sac and want this compound to reverse it's temperature when heat is detected such as by a warm hand.

Please source your answer, thanks!

Answer 1

Well well, if you have a supersaturated solution in your sac, with a solute that have an endothermic crystallization, well, it should cool down when you touch it, or you just perturbate the metastable solution.

The crystallization of a supersaturated sodium acetate solution is an exothermic process that is available commercially is the form of hand warmers

To demonstrate an endothermic process, Barium hydroxide (octahydrate) and Ammonium nitrate are mixed (in a 2:1 ratio) in a small beaker. This reaction displays the endothermic process and illustrated the interaction between changes in enthalpy and entropy is spontaneous chemical reactions. For a process to take place spontaneously at constant temperature and pressure, the change in free energy must be negative. An endothermic reaction may thus be spontaneous at constant pressure if the positive value of the heat absorbed is offset by a sufficient increase in entropy (randomness). In the reaction between barium hydroxide octahydrate and ammonium nitrate, the large increase in entropy is related to the increase in the number of particles present and their states (remember that two solids are combining to form a solid product Barium Nitrate and some ammonia).
With this reaction, temperatures of -20o C can be achieved.

So in conclusion, if you have a little sac with 2 chambers with that 2 compounds, and you brake the separaion of the 2 chambers, the pack will freeze !

However there are already on the market small sacks called "instantaneus ice" and you can buy them from a Pharmacy, they contain just a supersaturated solution of ammonium nitrate (in the small button that you have to push inside) and water.