Igor Chudov @ home wrote:

What I still do not understand though is what happens between the water and ammonium nitrate that consumes so much energy.

I mean, okay, you need to spend energy to mix these two things. Then, logically, they should not "want" to mix, right? But empirically, ammonuim nitrate literally sucks water vapors from the air. How come?

For the same reason that water evaporates and the remainder gets colder. The energy is consumed by breaking the chemical bonds in the ammonium nitrate. Temperature is the average amount of kinetic energy of the molecules, and so some molecules are moving faster than average and some are moving more slowly. The fast-moving molecules collide with the ammonium nitrate and their energy is consumed by breaking the ionic bonds. Since the fast-moving molecules get slowed down in the process, the average temperature drops. Eric Cordian wrote:

And for those who may think that endothermic reactions violate some basic law about entropy always increasing, I should point out that the increase in entropy from the uniform mixing of two different materials can more than compensate for the decrease in temperature. Ain't science wonderful?

Here's something to ponder: Consider two objects initially at the same temperature. One is at the focus of a hemispherical mirror. An elliptical mirror with both objects at its foci encloses the remaining space. Because of the spherical mirror, the first object reabsorbs most of its heat lost by radiation, but most of the second object's radiated heat is reflected upon the first. Hence the first object becomes warmer relative to the second. The entropy here appears to decrease, but according to thermodynamics that is impossible. Can anyone explain how it is that the total entropy would not decrease?