Expert: Mike Fulcher - 8/19/2009

Question
Hey Mike,
I was thinking about the military's MRE's the other day and was wondering a few things.
1. What chemical or otherwise do they use to react with the water to cause it to heat up the meals?
2. Is there a chemical/compound/substance that would have the reverse reaction and make it cooler?
3. If such a substance exists is it toxic to ingest in any portions?

Random I know but I was just wondering

Answer
I have a vague recollection that it is a calcium compound, perhaps calcium oxide.  When mixed with water, an exothermic reaction occurs to transform the CaO to Ca(OH)2.  Unfortunately I am unaware of any commercially available products that do the reverse, but chemically speaking it should be possible.  These items would use heat from the environment to support an endothermic reaction, resulting in a cooling of the surroundings.  My recommendation would be that you do not consume either item.  CaO isn't toxuc per say, but you might get some nasty burns in your mounth.