Question

Why is the boiling point of `NaCl` so high?

Answer 1

Consider the interparticle interaction in `NaCl(s)`; it is demonstrably strong.

Explanation:

In the `NaCl(s)` lattice, every `Na^+` ion is electrostatically bound to every `Cl^-` ion in the lattice, and vice versa.

Of course, every sodium ion is electrostatically repelled by every other sodium ion (and the same for each chloride anion), but if you sum up the attractive forces and the repulsive forces (which can certainly be done quantitatively!) a net attractive force results.

This attractive force is responsible for the high melting and boiling points of the sodium chloride lattice, and also its fragility when enough force is applied to break the structure.

Sodium chloride is thus a non-molecular species, with large forces of interaction between ionic particles.