[jackymac11]

I am looking for an endothermic reaction that will get very cold, and sustain the coldness for a longer period of time(hopefully at least half and hour). Please let me know any compounds that would accomplish this.

[Arkcon]

You're question isn't very specific -- you use words like "very cold" without a reference point, so it's hard to give you a useful answer.  Let me take a wild guess -- you're aware of ammonium nitrate dissolution in water being cold, and want something ... more.  Looking at this favorite science webpage of mine, we see a list:  http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermic-examples.shtml  Right at the top is the endothermic "reaction" produced by melting ice.  The change of state is certainly endothermic, fairly inexpensive, and pretty easy to deliver.  Do you need something colder, or longer lasting?  You can try other changes of state, like dry ice, or others.

[jackymac11]

Yes, I see how general my original question was. My apoligies. I have tried ammonium nitrate before. I am looking for something that is more than that. What im hoping for is a compound that uses chemicals that are available at any time. meaning not using ice or anything that needs to be frozen or prepared beforehand. I have heard about the reaction of Ammonium Nitrate and Barium Hydroxide Octahydrate, however i know nothing about this or how long the reaction lasts. Thanks for the help