Reversible endothermic reaction

[HansVh]

hi,

In the past there were "toys" containing a fluid and a clip. When pressing the clip, the fluid became hard and it gave warmth. When the toy was boiled in water, it became fluid again and one could start over.

Now I was wondering of there is a chemical reaction that gives cold instead of warmth. If there is no such reversible reaction, what is a good endotherm reaction that creates cold quick and with à low temperature.

 

[Ygggdrasil]

Instant ice packs generally use the dissolution of a substance like ammonium nitrate or urea as an endothermic reaction. Not sure if you could get this to happen reversibly, however.