This question really has nothing to do with clandestine chemistry directly, but I've always wondered why ammonium nitrate cools down when it's added to water. See, you have to heat NH4NO3 to drive off the water, but when you do this to other hygrosopic compounds, when water is added to them, heat is given off, which equals the amount of heat needed to drive off the water, in accordance to the law of conservation of energy. Compouds in question, NaOH, and Copper(II) Sulfate(all I can think of off the top of my head). So why does NH4NO3 absorb heat when it's added to water? Wouldn't this mean that it is capable of absorbing an infinite amount of heat?
Anyway, just a bored bee's musings.
Zorro