Question

Why is ammonium nitrate so water-soluble?

Answer 1

Do you ask why ammonium nitrate has a high water solubility......?

Explanation:

You should check the literature (or the webz) on the solubility of ammonium nitrate in water. It is VERY large, and as a physical scientist you should always interrogate data. Ammonium nitrate is a substance with a large enthalpy of hydration, but a fairly low lattice enthalpy; certainly its lattice enthalpy compared to say sodium chloride is low.

So why should the lattice enthalpy be low? Both ammonium, `NH_4^+`, and nitrate ion, `NO_3^-` are compound ions, and thus fairly large as ions go, certainly so compared to single metal cations, and even (comparably) larger anions. The force of electrostatic attraction, which in simple terms is Coulomb's Law between point charges, i.e. an `1/r^2` law, should be smaller because `r`, the distance between ions, is larger.....and this detracts from the lattice enthalpy of the salt.....